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Date: Sat, 27 Apr 2019 11:31:53 -0300
From: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
To: Changbin Du <changbin.du@...il.com>
Cc: Jonathan Corbet <corbet@....net>, tglx@...utronix.de,
mingo@...hat.com, bp@...en8.de, x86@...nel.org,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/27] Documentation: x86: convert boot.txt to reST
Em Fri, 26 Apr 2019 23:31:25 +0800
Changbin Du <changbin.du@...il.com> escreveu:
> This converts the plain text documentation to reStructuredText format and
> add it to Sphinx TOC tree. No essential content change.
>
> Signed-off-by: Changbin Du <changbin.du@...il.com>
> Cc: Mauro Carvalho Chehab <mchehab+samsung@...nel.org>
> ---
> Documentation/x86/boot.rst | 1238 +++++++++++++++++++++++++++++++++++
> Documentation/x86/boot.txt | 1130 --------------------------------
See, one of the problems of not having a git tree where we could
pull and a changeset that was not detected as a rename is that
conflicts happen:
patching file Documentation/x86/boot.rst
patching file Documentation/x86/boot.txt
Hunk #1 FAILED at 1.
Not deleting file Documentation/x86/boot.txt as content differs from patch
1 out of 1 hunk FAILED -- saving rejects to file Documentation/x86/boot.txt.rej
(tried to apply this on the top of docs-next and linux-next - didn't try to
apply directly on the top of x86 tree)
This indicates that you should probably rebase your tree.
This was not detected as a rename because of this:
diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.rst
similarity index 47%
If you use "-M40" on git send-email, it will properly detect
this one as a rename.
> Documentation/x86/index.rst | 2 +
> 3 files changed, 1240 insertions(+), 1130 deletions(-)
> create mode 100644 Documentation/x86/boot.rst
> delete mode 100644 Documentation/x86/boot.txt
>
> diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
> new file mode 100644
> index 000000000000..9b289a72b53f
> --- /dev/null
> +++ b/Documentation/x86/boot.rst
> @@ -0,0 +1,1238 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +===========================
> +The Linux/x86 Boot Protocol
> +===========================
> +
> +On the x86 platform, the Linux kernel uses a rather complicated boot
> +convention. This has evolved partially due to historical aspects, as
> +well as the desire in the early days to have the kernel itself be a
> +bootable image, the complicated PC memory model and due to changed
> +expectations in the PC industry caused by the effective demise of
> +real-mode DOS as a mainstream operating system.
> +
> +Currently, the following versions of the Linux/x86 boot protocol exist.
> +
> +============= ============================================================
> +Old kernels zImage/Image support only. Some very early kernels
> + may not even support a command line.
> +
> +Protocol 2.00 (Kernel 1.3.73) Added bzImage and initrd support, as
> + well as a formalized way to communicate between the
> + boot loader and the kernel. setup.S made relocatable,
> + although the traditional setup area still assumed
> + writable.
> +
> +Protocol 2.01 (Kernel 1.3.76) Added a heap overrun warning.
> +
> +Protocol 2.02 (Kernel 2.4.0-test3-pre3) New command line protocol.
> + Lower the conventional memory ceiling. No overwrite
> + of the traditional setup area, thus making booting
> + safe for systems which use the EBDA from SMM or 32-bit
> + BIOS entry points. zImage deprecated but still
> + supported.
> +
> +Protocol 2.03 (Kernel 2.4.18-pre1) Explicitly makes the highest possible
> + initrd address available to the bootloader.
> +
> +Protocol 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.
> +
> +Protocol 2.05 (Kernel 2.6.20) Make protected mode kernel relocatable.
> + Introduce relocatable_kernel and kernel_alignment fields.
> +
> +Protocol 2.06 (Kernel 2.6.22) Added a field that contains the size of
> + the boot command line.
> +
> +Protocol 2.07 (Kernel 2.6.24) Added paravirtualised boot protocol.
> + Introduced hardware_subarch and hardware_subarch_data
> + and KEEP_SEGMENTS flag in load_flags.
> +
> +Protocol 2.08 (Kernel 2.6.26) Added crc32 checksum and ELF format
> + payload. Introduced payload_offset and payload_length
> + fields to aid in locating the payload.
> +
> +Protocol 2.09 (Kernel 2.6.26) Added a field of 64-bit physical
> + pointer to single linked list of struct setup_data.
> +
> +Protocol 2.10 (Kernel 2.6.31) Added a protocol for relaxed alignment
> + beyond the kernel_alignment added, new init_size and
> + pref_address fields. Added extended boot loader IDs.
> +
> +Protocol 2.11 (Kernel 3.6) Added a field for offset of EFI handover
> + protocol entry point.
> +
> +Protocol 2.12 (Kernel 3.8) Added the xloadflags field and extension fields
> + to struct boot_params for loading bzImage and ramdisk
> + above 4G in 64bit.
> +============= ============================================================
> +
> +MEMORY LAYOUT
> +=============
I would place all the titles in camel case, in order to standardize
to the style we're using on other documents. Same applies to other
titles on this document (and along this series, if any).
> +
> +The traditional memory map for the kernel loader, used for Image or
> +zImage kernels, typically looks like::
> +
> + | |
> + 0A0000 +------------------------+
> + | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
> + 09A000 +------------------------+
> + | Command line |
> + | Stack/heap | For use by the kernel real-mode code.
> + 098000 +------------------------+
> + | Kernel setup | The kernel real-mode code.
> + 090200 +------------------------+
> + | Kernel boot sector | The kernel legacy boot sector.
> + 090000 +------------------------+
> + | Protected-mode kernel | The bulk of the kernel image.
> + 010000 +------------------------+
> + | Boot loader | <- Boot sector entry point 0000:7C00
> + 001000 +------------------------+
> + | Reserved for MBR/BIOS |
> + 000800 +------------------------+
> + | Typically used by MBR |
> + 000600 +------------------------+
> + | BIOS use only |
> + 000000 +------------------------+
> +
> +When using bzImage, the protected-mode kernel was relocated to
> +0x100000 ("high memory"), and the kernel real-mode block (boot sector,
> +setup, and stack/heap) was made relocatable to any address between
> +0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
> +2.01 the 0x90000+ memory range is still used internally by the kernel;
> +the 2.02 protocol resolves that problem.
> +
> +It is desirable to keep the "memory ceiling" -- the highest point in
> +low memory touched by the boot loader -- as low as possible, since
> +some newer BIOSes have begun to allocate some rather large amounts of
> +memory, called the Extended BIOS Data Area, near the top of low
> +memory. The boot loader should use the "INT 12h" BIOS call to verify
> +how much low memory is available.
> +
> +Unfortunately, if INT 12h reports that the amount of memory is too
> +low, there is usually nothing the boot loader can do but to report an
> +error to the user. The boot loader should therefore be designed to
> +take up as little space in low memory as it reasonably can. For
> +zImage or old bzImage kernels, which need data written into the
> +0x90000 segment, the boot loader should make sure not to use memory
> +above the 0x9A000 point; too many BIOSes will break above that point.
> +
> +For a modern bzImage kernel with boot protocol version >= 2.02, a
> +memory layout like the following is suggested::
> +
> + ~ ~
> + | Protected-mode kernel |
> + 100000 +------------------------+
> + | I/O memory hole |
> + 0A0000 +------------------------+
> + | Reserved for BIOS | Leave as much as possible unused
> + ~ ~
> + | Command line | (Can also be below the X+10000 mark)
> + X+10000 +------------------------+
> + | Stack/heap | For use by the kernel real-mode code.
> + X+08000 +------------------------+
> + | Kernel setup | The kernel real-mode code.
> + | Kernel boot sector | The kernel legacy boot sector.
> + X +------------------------+
> + | Boot loader | <- Boot sector entry point 0000:7C00
> + 001000 +------------------------+
> + | Reserved for MBR/BIOS |
> + 000800 +------------------------+
> + | Typically used by MBR |
> + 000600 +------------------------+
There's an extra tail whitespace on the above line.
> + | BIOS use only |
> + 000000 +------------------------+
> +
> + ... where the address X is as low as the design of the boot loader permits.
> +
> +
> +THE REAL-MODE KERNEL HEADER
> +===========================
> +
> +In the following text, and anywhere in the kernel boot sequence, "a
> +sector" refers to 512 bytes. It is independent of the actual sector
> +size of the underlying medium.
> +
> +The first step in loading a Linux kernel should be to load the
> +real-mode code (boot sector and setup code) and then examine the
> +following header at offset 0x01f1. The real-mode code can total up to
> +32K, although the boot loader may choose to load only the first two
> +sectors (1K) and then examine the bootup sector size.
> +
> +The header looks like::
> + =========== ======= ===================== ================================
> + Offset/Size Proto Name Meaning
> + =========== ======= ===================== ================================
> + 01F1/1 ALL(1 setup_sects The size of the setup in sectors
> + 01F2/2 ALL root_flags If set, the root is mounted readonly
> + 01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
While I have no idea about the origin of this table, at least on my eyes,
it sounds to me that this table got truncated. That (1 and (2 really seems
to be, instead (1) and (2), matching the footnotes after this table. The
same applies to (3 below. So, I would add an extra space at the Proto column
and add the close parenthesis.
Those patches:
f8eeaaf41803 ("[PATCH] Make the bzImage format self-terminating")
5031296c5702 ("x86: add extension fields for bootloader type and version")
Seem to indicate that.
> + 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
> + 01FA/2 ALL vid_mode Video mode control
> + 01FC/2 ALL root_dev Default root device number
> + 01FE/2 ALL boot_flag 0xAA55 magic number
> + 0200/2 2.00+ jump Jump instruction
> + 0202/4 2.00+ header Magic signature "HdrS"
> + 0206/2 2.00+ version Boot protocol version supported
> + 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
> + 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
> + 020E/2 2.00+ kernel_version Pointer to kernel version string
> + 0210/1 2.00+ type_of_loader Boot loader identifier
> + 0211/1 2.00+ loadflags Boot protocol option flags
> + 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
> + 0214/4 2.00+ code32_start Boot loader hook (see below)
> + 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
> + 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
> + 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
> + 0224/2 2.01+ heap_end_ptr Free memory after setup end
> + 0226/1 2.02+(3 ext_loader_ver Extended boot loader version
> + 0227/1 2.02+(3 ext_loader_type Extended boot loader ID
> + 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
> + 022C/4 2.03+ initrd_addr_max Highest legal initrd address
> + 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
> + 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
> + 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
> + 0236/2 2.12+ xloadflags Boot protocol option flags
> + 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
> + 023C/4 2.07+ hardware_subarch Hardware subarchitecture
> + 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
> + 0248/4 2.08+ payload_offset Offset of kernel payload
> + 024C/4 2.08+ payload_length Length of kernel payload
> + 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
> + of struct setup_data
> + 0258/8 2.10+ pref_address Preferred loading address
> + 0260/4 2.10+ init_size Linear memory required during initialization
> + 0264/4 2.11+ handover_offset Offset of handover entry point
> + =========== ======= ===================== ================================
> +
> +.. note::
> + (1) For backwards compatibility, if the setup_sects field contains 0, the
> + real value is 4.
> +
> + (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
> + field are unusable, which means the size of a bzImage kernel
> + cannot be determined.
> +
> + (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
Perhaps a footnote markup here would work better - if ok for x86
maintainers.
> +
> +If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
> +the boot protocol version is "old". Loading an old kernel, the
> +following parameters should be assumed::
> +
> + Image type = zImage
> + initrd not supported
> + Real-mode kernel must be located at 0x90000.
> +
> +Otherwise, the "version" field contains the protocol version,
> +e.g. protocol version 2.01 will contain 0x0201 in this field. When
> +setting fields in the header, you must make sure only to set fields
> +supported by the protocol version in use.
> +
> +
> +DETAILS OF HEADER FIELDS
> +========================
> +
> +For each field, some are information from the kernel to the bootloader
> +("read"), some are expected to be filled out by the bootloader
> +("write"), and some are expected to be read and modified by the
> +bootloader ("modify").
> +
> +All general purpose boot loaders should write the fields marked
> +(obligatory). Boot loaders who want to load the kernel at a
> +nonstandard address should fill in the fields marked (reloc); other
> +boot loaders can ignore those fields.
> +
> +The byte order of all fields is littleendian (this is x86, after all.)
> + ============ =======
> + Field name: setup_sects
> + Type: read
> + Offset/size: 0x1f1/1
> + Protocol: ALL
> + ============ =======
Hmm... Sphinx 1.7 doesn't seem to be complaining[1], but this is actually a
violation of ReST. as the markup line is too short. It should longer,
like:
============ ===========
Field name: setup_sects
Type: read
Offset/size: 0x1f1/1
Protocol: ALL
============ ===========
For people reading this in plain text, this also makes it a lot better
to read.
Same applies to all other tables here.
[1] Btw, I guess this is new on Sphinx. I'm pretty sure I had warnings on
older versions on the tail table markup. You should remember that the
minimal Sphinx version we support is 1.3, as stated at
Documentation/conf.py (see needs_sphinx var there).
> +
> +The size of the setup code in 512-byte sectors. If this field is
> +0, the real value is 4. The real-mode code consists of the boot
> +sector (always one 512-byte sector) plus the setup code.
> +
> + ============ =======
> + Field name: root_flags
> + Type: modify (optional)
> + Offset/size: 0x1f2/2
> + Protocol: ALL
> + ============ =======
> +
> +If this field is nonzero, the root defaults to readonly. The use of
> +this field is deprecated; use the "ro" or "rw" options on the
> +command line instead.
This comment is general for all bootloader fields. I would, instead,
do:
============ =================
Field name: root_flags
Type: modify (optional)
Offset/size: 0x1f2/2
Protocol: ALL
============ =================
If this field is nonzero, the root defaults to readonly. The use of
this field is deprecated; use the "ro" or "rw" options on the
command line instead.
E.g. keep the original indentation of both the table and the text
below. There are some good reasons for that:
1) The explanation below each table belongs to the table. By doing
such indentation, it becomes clearer;
2) The number of changes will be a lot less, making easier for reviewers
to notice if you changed something;
3) the "similarity" used at git rename algorithm will increase a lot,
making this to show as a rename at the git stats;
4) it would avoid miss-formatting issues (line the one for "loadflags"
below), as a new table will end any previous literal block.
> +
> + ============ =======
> + Field name: syssize
> + Type: read
> + Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
> + Protocol: 2.04+
> + ============ =======
> +
> +The size of the protected-mode code in units of 16-byte paragraphs.
> +For protocol versions older than 2.04 this field is only two bytes
> +wide, and therefore cannot be trusted for the size of a kernel if
> +the LOAD_HIGH flag is set.
> +
> + ============ =======
> + Field name: ram_size
> + Type: kernel internal
> + Offset/size: 0x1f8/2
> + Protocol: ALL
> + ============ =======
> +
> +This field is obsolete.
> +
> + ============ =======
> + Field name: vid_mode
> + Type: modify (obligatory)
> + Offset/size: 0x1fa/2
> + ============ =======
> +
> +Please see the section on SPECIAL COMMAND LINE OPTIONS.
> +
> + ============ =======
> + Field name: root_dev
> + Type: modify (optional)
> + Offset/size: 0x1fc/2
> + Protocol: ALL
> + ============ =======
> +
> +The default root device device number. The use of this field is
> +deprecated, use the "root=" option on the command line instead.
> +
> + ============ =======
> + Field name: boot_flag
> + Type: read
> + Offset/size: 0x1fe/2
> + Protocol: ALL
> + ============ =======
> +
> +Contains 0xAA55. This is the closest thing old Linux kernels have
> +to a magic number.
> +
> + ============ =======
> + Field name: jump
> + Type: read
> + Offset/size: 0x200/2
> + Protocol: 2.00+
> + ============ =======
> +
> +Contains an x86 jump instruction, 0xEB followed by a signed offset
> +relative to byte 0x202. This can be used to determine the size of
> +the header.
> +
> + ============ =======
> + Field name: header
> + Type: read
> + Offset/size: 0x202/4
> + Protocol: 2.00+
> + ============ =======
> +
> +Contains the magic number "HdrS" (0x53726448).
> +
> + ============ =======
> + Field name: version
> + Type: read
> + Offset/size: 0x206/2
> + Protocol: 2.00+
> + ============ =======
> +
> +Contains the boot protocol version, in (major << 8)+minor format,
> +e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
> +10.17.
> +
> + ============ =======
> + Field name: realmode_swtch
> + Type: modify (optional)
> + Offset/size: 0x208/4
> + Protocol: 2.00+
> + ============ =======
> +
> +Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
Hmm... this book have some references based on chapter names.
> +
> + ============ =======
> + Field name: start_sys_seg
> + Type: read
> + Offset/size: 0x20c/2
> + Protocol: 2.00+
> + ============ =======
> +
> +The load low segment (0x1000). Obsolete.
> +
> + ============ =======
> + Field name: kernel_version
> + Type: read
> + Offset/size: 0x20e/2
> + Protocol: 2.00+
> + ============ =======
> +
> +If set to a nonzero value, contains a pointer to a NUL-terminated
> +human-readable kernel version number string, less 0x200. This can
> +be used to display the kernel version to the user. This value
> +should be less than (0x200*setup_sects).
> +
> +For example, if this value is set to 0x1c00, the kernel version
> +number string can be found at offset 0x1e00 in the kernel file.
> +This is a valid value if and only if the "setup_sects" field
> +contains the value 15 or higher, as::
> +
> + 0x1c00 < 15*0x200 (= 0x1e00) but
> + 0x1c00 >= 14*0x200 (= 0x1c00)
> +
> + 0x1c00 >> 9 = 14
> +
> +So the minimum value for setup_secs is 15.
I would keep it the way it was before, e. g.:
0x1c00 < 15*0x200 (= 0x1e00) but
0x1c00 >= 14*0x200 (= 0x1c00)
0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
As moving the last line out of the literal block makes it harder to
understand.
> +
> + ============ =======
> + Field name: type_of_loader
> + Type: write (obligatory)
> + Offset/size: 0x210/1
> + Protocol: 2.00+
> + ============ =======
> +
> +If your boot loader has an assigned id (see table below), enter
> +0xTV here, where T is an identifier for the boot loader and V is
> +a version number. Otherwise, enter 0xFF here.
> +
> +For boot loader IDs above T = 0xD, write T = 0xE to this field and
> +write the extended ID minus 0x10 to the ext_loader_type field.
> +Similarly, the ext_loader_ver field can be used to provide more than
> +four bits for the bootloader version.
> +
> +For example, for T = 0x15, V = 0x234, write::
> +
> + type_of_loader <- 0xE4
> + ext_loader_type <- 0x05
> + ext_loader_ver <- 0x23
> +
> +Assigned boot loader ids (hexadecimal):
> +
> + == ==============================
Same here: markup too short
> + 0 LILO
> + (0x00 reserved for pre-2.00 bootloader)
> + 1 Loadlin
> + 2 bootsect-loader
> + (0x20, all other values reserved)
> + 3 Syslinux
> + 4 Etherboot/gPXE/iPXE
> + 5 ELILO
> + 7 GRUB
> + 8 U-Boot
> + 9 Xen
> + A Gujin
> + B Qemu
> + C Arcturus Networks uCbootloader
> + D kexec-tools
> + E Extended (see ext_loader_type)
> + F Special (0xFF = undefined)
> + 10 Reserved
> + 11 Minimal Linux Bootloader
> + <http://sebastian-plotz.blogspot.de>
> + 12 OVMF UEFI virtualization stack
> + == ==============================
> +
> +Please contact <hpa@...or.com> if you need a bootloader ID value assigned.
> +
> + ============ =======
> + Field name: loadflags
> + Type: modify (obligatory)
> + Offset/size: 0x211/1
> + Protocol: 2.00+
> + ============ =======
> +
> +This field is a bitmask.
> +::
I don't see any reason why converting the stuff below into a literal block.
Just removing the "::" and add a few blank lines in order to avoid Sphinx
to change the font of the previous line to bold would likely produce a
better output (IMHO).
> +
> + Bit 0 (read): LOADED_HIGH
> + - If 0, the protected-mode code is loaded at 0x10000.
> + - If 1, the protected-mode code is loaded at 0x100000.
> +
> + Bit 1 (kernel internal): KASLR_FLAG
> + - Used internally by the compressed kernel to communicate
> + KASLR status to kernel proper.
> + - If 1, KASLR enabled.
> + - If 0, KASLR disabled.
> +
> + Bit 5 (write): QUIET_FLAG
> + - If 0, print early messages.
> + - If 1, suppress early messages.
> + This requests to the kernel (decompressor and early
> + kernel) to not write early messages that require
> + accessing the display hardware directly.
> +
> + Bit 6 (write): KEEP_SEGMENTS
> + Protocol: 2.07+
> + - If 0, reload the segment registers in the 32bit entry point.
> + - If 1, do not reload the segment registers in the 32bit entry point.
> + Assume that %cs %ds %ss %es are all set to flat segments with
> + a base of 0 (or the equivalent for their environment).
> +
> + Bit 7 (write): CAN_USE_HEAP
> + Set this bit to 1 to indicate that the value entered in the
> + heap_end_ptr is valid. If this field is clear, some setup code
> + functionality will be disabled.
> +
> +
> + ============ =======
> + Field name: setup_move_size
> + Type: modify (obligatory)
> + Offset/size: 0x212/2
> + Protocol: 2.00-2.01
> + ============ =======
Due to its alignment, this table will be displayed as part of the
previous code-block.
> +
> +When using protocol 2.00 or 2.01, if the real mode kernel is not
> +loaded at 0x90000, it gets moved there later in the loading
> +sequence. Fill in this field if you want additional data (such as
> +the kernel command line) moved in addition to the real-mode kernel
> +itself.
> +
> +The unit is bytes starting with the beginning of the boot sector.
> +
> +This field is can be ignored when the protocol is 2.02 or higher, or
> +if the real-mode code is loaded at 0x90000.
> +
> + ============ =======
> + Field name: code32_start
> + Type: modify (optional, reloc)
> + Offset/size: 0x214/4
> + Protocol: 2.00+
> + ============ =======
> +
> +The address to jump to in protected mode. This defaults to the load
> +address of the kernel, and can be used by the boot loader to
> +determine the proper load address.
> +
> +This field can be modified for two purposes:
> +
> + 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
> +
> + 2. if a bootloader which does not install a hook loads a
> + relocatable kernel at a nonstandard address it will have to modify
> + this field to point to the load address.
> +
> + ============ =======
> + Field name: ramdisk_image
> + Type: write (obligatory)
> + Offset/size: 0x218/4
> + Protocol: 2.00+
> + ============ =======
> +
As I mentioned before, the identation for those tables make it
confusing to read.
In this specific case, both in HTML and in text mode, it sounds
that ramdisk_image field has something related to the code32_start
field.
> +The 32-bit linear address of the initial ramdisk or ramfs. Leave at
> +zero if there is no initial ramdisk/ramfs.
> +
> + ============ =======
> + Field name: ramdisk_size
> + Type: write (obligatory)
> + Offset/size: 0x21c/4
> + Protocol: 2.00+
> + ============ =======
> +
> +Size of the initial ramdisk or ramfs. Leave at zero if there is no
> +initial ramdisk/ramfs.
> +
> + ============ =======
> + Field name: bootsect_kludge
> + Type: kernel internal
> + Offset/size: 0x220/4
> + Protocol: 2.00+
> + ============ =======
> +
> +This field is obsolete.
> +
> + ============ =======
> + Field name: heap_end_ptr
> + Type: write (obligatory)
> + Offset/size: 0x224/2
> + Protocol: 2.01+
> + ============ =======
> +
> +Set this field to the offset (from the beginning of the real-mode
> +code) of the end of the setup stack/heap, minus 0x0200.
> +
> + ============ =======
> + Field name: ext_loader_ver
> + Type: write (optional)
> + Offset/size: 0x226/1
> + Protocol: 2.02+
> + ============ =======
> +
> +This field is used as an extension of the version number in the
> +type_of_loader field. The total version number is considered to be
> +(type_of_loader & 0x0f) + (ext_loader_ver << 4).
> +
> +The use of this field is boot loader specific. If not written, it
> +is zero.
> +
> +Kernels prior to 2.6.31 did not recognize this field, but it is safe
> +to write for protocol version 2.02 or higher.
> +
> + ============ =======
> + Field name: ext_loader_type
> + Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
> + Offset/size: 0x227/1
> + Protocol: 2.02+
> + ============ =======
> +
> +This field is used as an extension of the type number in
> +type_of_loader field. If the type in type_of_loader is 0xE, then
> +the actual type is (ext_loader_type + 0x10).
> +
> +This field is ignored if the type in type_of_loader is not 0xE.
> +
> +Kernels prior to 2.6.31 did not recognize this field, but it is safe
> +to write for protocol version 2.02 or higher.
> +
> + ============ =======
> + Field name: cmd_line_ptr
> + Type: write (obligatory)
> + Offset/size: 0x228/4
> + Protocol: 2.02+
> + ============ =======
> +
> +Set this field to the linear address of the kernel command line.
> +The kernel command line can be located anywhere between the end of
> +the setup heap and 0xA0000; it does not have to be located in the
> +same 64K segment as the real-mode code itself.
> +
> +Fill in this field even if your boot loader does not support a
> +command line, in which case you can point this to an empty string
> +(or better yet, to the string "auto".) If this field is left at
> +zero, the kernel will assume that your boot loader does not support
> +the 2.02+ protocol.
> +
> + ============ =======
> + Field name: initrd_addr_max
> + Type: read
> + Offset/size: 0x22c/4
> + Protocol: 2.03+
> + ============ =======
> +
> +The maximum address that may be occupied by the initial
> +ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
> +field is not present, and the maximum address is 0x37FFFFFF. (This
> +address is defined as the address of the highest safe byte, so if
> +your ramdisk is exactly 131072 bytes long and this field is
> +0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
> +
> + ============ =======
> + Field name: kernel_alignment
> + Type: read/modify (reloc)
> + Offset/size: 0x230/4
> + Protocol: 2.05+ (read), 2.10+ (modify)
> + ============ =======
> +
> +Alignment unit required by the kernel (if relocatable_kernel is
> +true.) A relocatable kernel that is loaded at an alignment
> +incompatible with the value in this field will be realigned during
> +kernel initialization.
> +
> +Starting with protocol version 2.10, this reflects the kernel
> +alignment preferred for optimal performance; it is possible for the
> +loader to modify this field to permit a lesser alignment. See the
> +min_alignment and pref_address field below.
> +
> + ============ =======
> + Field name: relocatable_kernel
> + Type: read (reloc)
> + Offset/size: 0x234/1
> + Protocol: 2.05+
> + ============ =======
> +
> +If this field is nonzero, the protected-mode part of the kernel can
> +be loaded at any address that satisfies the kernel_alignment field.
> +After loading, the boot loader must set the code32_start field to
> +point to the loaded code, or to a boot loader hook.
> +
> + ============ =======
> + Field name: min_alignment
> + Type: read (reloc)
> + Offset/size: 0x235/1
> + Protocol: 2.10+
> + ============ =======
> +
> +This field, if nonzero, indicates as a power of two the minimum
> +alignment required, as opposed to preferred, by the kernel to boot.
> +If a boot loader makes use of this field, it should update the
> +kernel_alignment field with the alignment unit desired; typically::
> +
> + kernel_alignment = 1 << min_alignment
> +
> +There may be a considerable performance cost with an excessively
> +misaligned kernel. Therefore, a loader should typically try each
> +power-of-two alignment from kernel_alignment down to this alignment.
> +
> + ============ =======
> + Field name: xloadflags
> + Type: read
> + Offset/size: 0x236/2
> + Protocol: 2.12+
> + ============ =======
> +
> +This field is a bitmask.
> +::
Again, no reason why converting this to a literal block...
> +
> + Bit 0 (read): XLF_KERNEL_64
> + - If 1, this kernel has the legacy 64-bit entry point at 0x200.
> +
> + Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
> + - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
> +
> + Bit 2 (read): XLF_EFI_HANDOVER_32
> + - If 1, the kernel supports the 32-bit EFI handoff entry point
> + given at handover_offset.
> +
> + Bit 3 (read): XLF_EFI_HANDOVER_64
> + - If 1, the kernel supports the 64-bit EFI handoff entry point
> + given at handover_offset + 0x200.
> +
> + Bit 4 (read): XLF_EFI_KEXEC
> + - If 1, the kernel supports kexec EFI boot with EFI runtime support.
> +
> + ============ =======
> + Field name: cmdline_size
> + Type: read
> + Offset/size: 0x238/4
> + Protocol: 2.06+
> + ============ =======
... and this table will now be part of the literal block.
> +
> +The maximum size of the command line without the terminating
> +zero. This means that the command line can contain at most
> +cmdline_size characters. With protocol version 2.05 and earlier, the
> +maximum size was 255.
> +
> + ============ =======
> + Field name: hardware_subarch
> + Type: write (optional, defaults to x86/PC)
> + Offset/size: 0x23c/4
> + Protocol: 2.07+
> + ============ =======
> +
> +In a paravirtualized environment the hardware low level architectural
> +pieces such as interrupt handling, page table handling, and
> +accessing process control registers needs to be done differently.
> +
> +This field allows the bootloader to inform the kernel we are in one
> +one of those environments.
> +
> + ========== =======
> + 0x00000000 The default x86/PC environment
> + 0x00000001 lguest
> + 0x00000002 Xen
> + 0x00000003 Moorestown MID
> + 0x00000004 CE4100 TV Platform
> + ========== =======
> +
> + ============ =======
> + Field name: hardware_subarch_data
> + Type: write (subarch-dependent)
> + Offset/size: 0x240/8
> + Protocol: 2.07+
> + ============ =======
> +
> +A pointer to data that is specific to hardware subarch
> +This field is currently unused for the default x86/PC environment,
> +do not modify.
> +
> + ============ =======
> + Field name: payload_offset
> + Type: read
> + Offset/size: 0x248/4
> + Protocol: 2.08+
> + ============ =======
> +
> +If non-zero then this field contains the offset from the beginning
> +of the protected-mode code to the payload.
> +
> +The payload may be compressed. The format of both the compressed and
> +uncompressed data should be determined using the standard magic
> +numbers. The currently supported compression formats are gzip
> +(magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
> +(magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
> +02 21). The uncompressed payload is currently always ELF (magic
> +number 7F 45 4C 46).
> +
> + ============ =======
> + Field name: payload_length
> + Type: read
> + Offset/size: 0x24c/4
> + Protocol: 2.08+
> + ============ =======
> +
> +The length of the payload.
> +
> + ============ =======
> + Field name: setup_data
> + Type: write (special)
> + Offset/size: 0x250/8
> + Protocol: 2.09+
> + ============ =======
> +
> +The 64-bit physical pointer to NULL terminated single linked list of
> +struct setup_data. This is used to define a more extensible boot
> +parameters passing mechanism. The definition of struct setup_data is
> +as follow::
> +
> + struct setup_data {
> + u64 next;
> + u32 type;
> + u32 len;
> + u8 data[0];
> + };
> +
> +Where, the next is a 64-bit physical pointer to the next node of
> +linked list, the next field of the last node is 0; the type is used
> +to identify the contents of data; the len is the length of data
> +field; the data holds the real payload.
> +
> +This list may be modified at a number of points during the bootup
> +process. Therefore, when modifying this list one should always make
> +sure to consider the case where the linked list already contains
> +entries.
> +
> + ============ =======
> + Field name: pref_address
> + Type: read (reloc)
> + Offset/size: 0x258/8
> + Protocol: 2.10+
> + ============ =======
> +
> +This field, if nonzero, represents a preferred load address for the
> +kernel. A relocating bootloader should attempt to load at this
> +address if possible.
> +
> +A non-relocatable kernel will unconditionally move itself and to run
> +at this address.
> +
> + ============ =======
> + Field name: init_size
> + Type: read
> + Offset/size: 0x260/4
> + ============ =======
> +
> +This field indicates the amount of linear contiguous memory starting
> +at the kernel runtime start address that the kernel needs before it
> +is capable of examining its memory map. This is not the same thing
> +as the total amount of memory the kernel needs to boot, but it can
> +be used by a relocating boot loader to help select a safe load
> +address for the kernel.
> +
> +The kernel runtime start address is determined by the following algorithm::
> +
> + if (relocatable_kernel)
> + runtime_start = align_up(load_address, kernel_alignment)
> + else
> + runtime_start = pref_address
> +
> + ============ =======
> + Field name: handover_offset
> + Type: read
> + Offset/size: 0x264/4
> + ============ =======
This table is also part of the preceding literal block, due to its
indent.
> +
> +This field is the offset from the beginning of the kernel image to
> +the EFI handover protocol entry point. Boot loaders using the EFI
> +handover protocol to boot the kernel should jump to this offset.
> +
> +See EFI HANDOVER PROTOCOL below for more details.
> +
> +
> +THE IMAGE CHECKSUM
> +==================
> +
> +From boot protocol version 2.08 onwards the CRC-32 is calculated over
> +the entire file using the characteristic polynomial 0x04C11DB7 and an
> +initial remainder of 0xffffffff. The checksum is appended to the
> +file; therefore the CRC of the file up to the limit specified in the
> +syssize field of the header is always 0.
> +
> +
> +THE KERNEL COMMAND LINE
> +=======================
> +
> +The kernel command line has become an important way for the boot
> +loader to communicate with the kernel. Some of its options are also
> +relevant to the boot loader itself, see "special command line options"
> +below.
> +
> +The kernel command line is a null-terminated string. The maximum
> +length can be retrieved from the field cmdline_size. Before protocol
> +version 2.06, the maximum was 255 characters. A string that is too
> +long will be automatically truncated by the kernel.
> +
> +If the boot protocol version is 2.02 or later, the address of the
> +kernel command line is given by the header field cmd_line_ptr (see
> +above.) This address can be anywhere between the end of the setup
> +heap and 0xA0000.
> +
> +If the protocol version is *not* 2.02 or higher, the kernel
> +command line is entered using the following protocol:
> +
> + - At offset 0x0020 (word), "cmd_line_magic", enter the magic
> + number 0xA33F.
> +
> + - At offset 0x0022 (word), "cmd_line_offset", enter the offset
> + of the kernel command line (relative to the start of the
> + real-mode kernel).
> +
> + - The kernel command line *must* be within the memory region
> + covered by setup_move_size, so you may need to adjust this
> + field.
> +
> +
> +MEMORY LAYOUT OF THE REAL-MODE CODE
> +===================================
> +
> +The real-mode code requires a stack/heap to be set up, as well as
> +memory allocated for the kernel command line. This needs to be done
> +in the real-mode accessible memory in bottom megabyte.
> +
> +It should be noted that modern machines often have a sizable Extended
> +BIOS Data Area (EBDA). As a result, it is advisable to use as little
> +of the low megabyte as possible.
> +
> +Unfortunately, under the following circumstances the 0x90000 memory
> +segment has to be used:
> +
> + - When loading a zImage kernel ((loadflags & 0x01) == 0).
> + - When loading a 2.01 or earlier boot protocol kernel.
> +
> + For the 2.00 and 2.01 boot protocols, the real-mode code
The actual diff here is:
- -> For the 2.00 and 2.01 boot protocols, the real-mode code
+ For the 2.00 and 2.01 boot protocols, the real-mode code
I suspect that the reason for the "->" that used to be here is to
mark this comment as a note.
You should probably be doing either:
Note:
For the 2.00 and 2.01 boot protocols, the real-mode code
Or use some note markup here.
> + can be loaded at another address, but it is internally
> + relocated to 0x90000. For the "old" protocol, the
> + real-mode code must be loaded at 0x90000.
> +
> +When loading at 0x90000, avoid using memory above 0x9a000.
> +
> +For boot protocol 2.02 or higher, the command line does not have to be
> +located in the same 64K segment as the real-mode setup code; it is
> +thus permitted to give the stack/heap the full 64K segment and locate
> +the command line above it.
> +
> +The kernel command line should not be located below the real-mode
> +code, nor should it be located in high memory.
> +
> +
> +SAMPLE BOOT CONFIGURATION
> +=========================
> +
> +As a sample configuration, assume the following layout of the real
> +mode segment.
> +
> +When loading below 0x90000, use the entire segment:
> +
> + ============= =======
> + 0x0000-0x7fff Real mode kernel
> + 0x8000-0xdfff Stack and heap
> + 0xe000-0xffff Kernel command line
> + ============= =======
> +
> +When loading at 0x90000 OR the protocol version is 2.01 or earlier:
> +
> + ============= =======
> + 0x0000-0x7fff Real mode kernel
> + 0x8000-0x97ff Stack and heap
> + 0x9800-0x9fff Kernel command line
> + ============= =======
> +
> +Such a boot loader should enter the following fields in the header::
> +
> + unsigned long base_ptr; /* base address for real-mode segment */
> +
> + if ( setup_sects == 0 ) {
> + setup_sects = 4;
> + }
> +
> + if ( protocol >= 0x0200 ) {
> + type_of_loader = <type code>;
> + if ( loading_initrd ) {
> + ramdisk_image = <initrd_address>;
> + ramdisk_size = <initrd_size>;
> + }
> +
> + if ( protocol >= 0x0202 && loadflags & 0x01 )
> + heap_end = 0xe000;
> + else
> + heap_end = 0x9800;
> +
> + if ( protocol >= 0x0201 ) {
> + heap_end_ptr = heap_end - 0x200;
> + loadflags |= 0x80; /* CAN_USE_HEAP */
> + }
> +
> + if ( protocol >= 0x0202 ) {
> + cmd_line_ptr = base_ptr + heap_end;
> + strcpy(cmd_line_ptr, cmdline);
> + } else {
> + cmd_line_magic = 0xA33F;
> + cmd_line_offset = heap_end;
> + setup_move_size = heap_end + strlen(cmdline)+1;
> + strcpy(base_ptr+cmd_line_offset, cmdline);
> + }
> + } else {
> + /* Very old kernel */
> +
> + heap_end = 0x9800;
> +
> + cmd_line_magic = 0xA33F;
> + cmd_line_offset = heap_end;
> +
> + /* A very old kernel MUST have its real-mode code
> + loaded at 0x90000 */
> +
> + if ( base_ptr != 0x90000 ) {
> + /* Copy the real-mode kernel */
> + memcpy(0x90000, base_ptr, (setup_sects+1)*512);
> + base_ptr = 0x90000; /* Relocated */
> + }
> +
> + strcpy(0x90000+cmd_line_offset, cmdline);
> +
> + /* It is recommended to clear memory up to the 32K mark */
> + memset(0x90000 + (setup_sects+1)*512, 0,
> + (64-(setup_sects+1))*512);
> + }
> +
> +
> +LOADING THE REST OF THE KERNEL
> +==============================
> +
> +The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
> +in the kernel file (again, if setup_sects == 0 the real value is 4.)
> +It should be loaded at address 0x10000 for Image/zImage kernels and
> +0x100000 for bzImage kernels.
> +
> +The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
> +bit (LOAD_HIGH) in the loadflags field is set::
> +
> + is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
> + load_address = is_bzImage ? 0x100000 : 0x10000;
> +
> +Note that Image/zImage kernels can be up to 512K in size, and thus use
> +the entire 0x10000-0x90000 range of memory. This means it is pretty
> +much a requirement for these kernels to load the real-mode part at
> +0x90000. bzImage kernels allow much more flexibility.
> +
> +
> +SPECIAL COMMAND LINE OPTIONS
> +============================
> +
> +If the command line provided by the boot loader is entered by the
> +user, the user may expect the following command line options to work.
> +They should normally not be deleted from the kernel command line even
> +though not all of them are actually meaningful to the kernel. Boot
> +loader authors who need additional command line options for the boot
> +loader itself should get them registered in
> +Documentation/admin-guide/kernel-parameters.rst to make sure they will not
> +conflict with actual kernel options now or in the future.
> +
> + vga=<mode>
> + <mode> here is either an integer (in C notation, either
> + decimal, octal, or hexadecimal) or one of the strings
> + "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
> + (meaning 0xFFFD). This value should be entered into the
> + vid_mode field, as it is used by the kernel before the command
> + line is parsed.
> +
> + mem=<size>
> + <size> is an integer in C notation optionally followed by
> + (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
> + << 30, << 40, << 50 or << 60). This specifies the end of
> + memory to the kernel. This affects the possible placement of
> + an initrd, since an initrd should be placed near end of
> + memory. Note that this is an option to *both* the kernel and
> + the bootloader!
> +
> + initrd=<file>
> + An initrd should be loaded. The meaning of <file> is
> + obviously bootloader-dependent, and some boot loaders
> + (e.g. LILO) do not have such a command.
> +
> +In addition, some boot loaders add the following options to the
> +user-specified command line:
> +
> + BOOT_IMAGE=<file>
> + The boot image which was loaded. Again, the meaning of <file>
> + is obviously bootloader-dependent.
> +
> + auto
> + The kernel was booted without explicit user intervention.
> +
> +If these options are added by the boot loader, it is highly
> +recommended that they are located *first*, before the user-specified
> +or configuration-specified command line. Otherwise, "init=/bin/sh"
> +gets confused by the "auto" option.
> +
> +
> +RUNNING THE KERNEL
> +==================
> +
> +The kernel is started by jumping to the kernel entry point, which is
> +located at *segment* offset 0x20 from the start of the real mode
> +kernel. This means that if you loaded your real-mode kernel code at
> +0x90000, the kernel entry point is 9020:0000.
> +
> +At entry, ds = es = ss should point to the start of the real-mode
> +kernel code (0x9000 if the code is loaded at 0x90000), sp should be
> +set up properly, normally pointing to the top of the heap, and
> +interrupts should be disabled. Furthermore, to guard against bugs in
> +the kernel, it is recommended that the boot loader sets fs = gs = ds =
> +es = ss.
> +
> +In our example from above, we would do::
> +
> + /* Note: in the case of the "old" kernel protocol, base_ptr must
> + be == 0x90000 at this point; see the previous sample code */
> +
> + seg = base_ptr >> 4;
> +
> + cli(); /* Enter with interrupts disabled! */
> +
> + /* Set up the real-mode kernel stack */
> + _SS = seg;
> + _SP = heap_end;
> +
> + _DS = _ES = _FS = _GS = seg;
> + jmp_far(seg+0x20, 0); /* Run the kernel */
> +
> +If your boot sector accesses a floppy drive, it is recommended to
> +switch off the floppy motor before running the kernel, since the
> +kernel boot leaves interrupts off and thus the motor will not be
> +switched off, especially if the loaded kernel has the floppy driver as
> +a demand-loaded module!
> +
> +
> +ADVANCED BOOT LOADER HOOKS
> +==========================
> +
> +If the boot loader runs in a particularly hostile environment (such as
> +LOADLIN, which runs under DOS) it may be impossible to follow the
> +standard memory location requirements. Such a boot loader may use the
> +following hooks that, if set, are invoked by the kernel at the
> +appropriate time. The use of these hooks should probably be
> +considered an absolutely last resort!
> +
> +IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
> +%edi across invocation.
> +
> + realmode_swtch:
> + A 16-bit real mode far subroutine invoked immediately before
> + entering protected mode. The default routine disables NMI, so
> + your routine should probably do so, too.
> +
> + code32_start:
> + A 32-bit flat-mode routine *jumped* to immediately after the
> + transition to protected mode, but before the kernel is
> + uncompressed. No segments, except CS, are guaranteed to be
> + set up (current kernels do, but older ones do not); you should
> + set them up to BOOT_DS (0x18) yourself.
> +
> + After completing your hook, you should jump to the address
> + that was in this field before your boot loader overwrote it
> + (relocated, if appropriate.)
> +
> +
> +32-bit BOOT PROTOCOL
> +====================
> +
> +For machine with some new BIOS other than legacy BIOS, such as EFI,
> +LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
> +based on legacy BIOS can not be used, so a 32-bit boot protocol needs
> +to be defined.
> +
> +In 32-bit boot protocol, the first step in loading a Linux kernel
> +should be to setup the boot parameters (struct boot_params,
> +traditionally known as "zero page"). The memory for struct boot_params
> +should be allocated and initialized to all zero. Then the setup header
> +from offset 0x01f1 of kernel image on should be loaded into struct
> +boot_params and examined. The end of setup header can be calculated as
> +follow::
> +
> + 0x0202 + byte value at offset 0x0201
> +
> +In addition to read/modify/write the setup header of the struct
> +boot_params as that of 16-bit boot protocol, the boot loader should
> +also fill the additional fields of the struct boot_params as that
> +described in zero-page.txt.
> +
> +After setting up the struct boot_params, the boot loader can load the
> +32/64-bit kernel in the same way as that of 16-bit boot protocol.
> +
> +In 32-bit boot protocol, the kernel is started by jumping to the
> +32-bit kernel entry point, which is the start address of loaded
> +32/64-bit kernel.
> +
> +At entry, the CPU must be in 32-bit protected mode with paging
> +disabled; a GDT must be loaded with the descriptors for selectors
> +__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
> +segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
> +must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> +must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
> +address of the struct boot_params; %ebp, %edi and %ebx must be zero.
> +
> +64-bit BOOT PROTOCOL
> +====================
> +
> +For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
> +and we need a 64-bit boot protocol.
> +
> +In 64-bit boot protocol, the first step in loading a Linux kernel
> +should be to setup the boot parameters (struct boot_params,
> +traditionally known as "zero page"). The memory for struct boot_params
> +could be allocated anywhere (even above 4G) and initialized to all zero.
> +Then, the setup header at offset 0x01f1 of kernel image on should be
> +loaded into struct boot_params and examined. The end of setup header
> +can be calculated as follows::
> +
> + 0x0202 + byte value at offset 0x0201
> +
> +In addition to read/modify/write the setup header of the struct
> +boot_params as that of 16-bit boot protocol, the boot loader should
> +also fill the additional fields of the struct boot_params as described
> +in zero-page.txt.
> +
> +After setting up the struct boot_params, the boot loader can load
> +64-bit kernel in the same way as that of 16-bit boot protocol, but
> +kernel could be loaded above 4G.
> +
> +In 64-bit boot protocol, the kernel is started by jumping to the
> +64-bit kernel entry point, which is the start address of loaded
> +64-bit kernel plus 0x200.
> +
> +At entry, the CPU must be in 64-bit mode with paging enabled.
> +The range with setup_header.init_size from start address of loaded
> +kernel and zero page and command line buffer get ident mapping;
> +a GDT must be loaded with the descriptors for selectors
> +__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
> +segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
> +must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> +must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
> +address of the struct boot_params.
> +
> +EFI HANDOVER PROTOCOL
> +=====================
> +
> +This protocol allows boot loaders to defer initialisation to the EFI
> +boot stub. The boot loader is required to load the kernel/initrd(s)
> +from the boot media and jump to the EFI handover protocol entry point
> +which is hdr->handover_offset bytes from the beginning of
> +startup_{32,64}.
> +
> +The function prototype for the handover entry point looks like this::
> +
> + efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
> +
> +'handle' is the EFI image handle passed to the boot loader by the EFI
> +firmware, 'table' is the EFI system table - these are the first two
> +arguments of the "handoff state" as described in section 2.3 of the
> +UEFI specification. 'bp' is the boot loader-allocated boot params.
> +
> +The boot loader *must* fill out the following fields in bp::
> +
> + - hdr.code32_start
> + - hdr.cmd_line_ptr
> + - hdr.ramdisk_image (if applicable)
> + - hdr.ramdisk_size (if applicable)
> +
> +All other fields should be zero.
> diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
> deleted file mode 100644
> index f4c2a97bfdbd..000000000000
> --- a/Documentation/x86/boot.txt
> +++ /dev/null
> @@ -1,1130 +0,0 @@
> - THE LINUX/x86 BOOT PROTOCOL
> - ---------------------------
> -
> -On the x86 platform, the Linux kernel uses a rather complicated boot
> -convention. This has evolved partially due to historical aspects, as
> -well as the desire in the early days to have the kernel itself be a
> -bootable image, the complicated PC memory model and due to changed
> -expectations in the PC industry caused by the effective demise of
> -real-mode DOS as a mainstream operating system.
> -
> -Currently, the following versions of the Linux/x86 boot protocol exist.
> -
> -Old kernels: zImage/Image support only. Some very early kernels
> - may not even support a command line.
> -
> -Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
> - well as a formalized way to communicate between the
> - boot loader and the kernel. setup.S made relocatable,
> - although the traditional setup area still assumed
> - writable.
> -
> -Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
> -
> -Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
> - Lower the conventional memory ceiling. No overwrite
> - of the traditional setup area, thus making booting
> - safe for systems which use the EBDA from SMM or 32-bit
> - BIOS entry points. zImage deprecated but still
> - supported.
> -
> -Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
> - initrd address available to the bootloader.
> -
> -Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
> -
> -Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
> - Introduce relocatable_kernel and kernel_alignment fields.
> -
> -Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
> - the boot command line.
> -
> -Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
> - Introduced hardware_subarch and hardware_subarch_data
> - and KEEP_SEGMENTS flag in load_flags.
> -
> -Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
> - payload. Introduced payload_offset and payload_length
> - fields to aid in locating the payload.
> -
> -Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
> - pointer to single linked list of struct setup_data.
> -
> -Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment
> - beyond the kernel_alignment added, new init_size and
> - pref_address fields. Added extended boot loader IDs.
> -
> -Protocol 2.11: (Kernel 3.6) Added a field for offset of EFI handover
> - protocol entry point.
> -
> -Protocol 2.12: (Kernel 3.8) Added the xloadflags field and extension fields
> - to struct boot_params for loading bzImage and ramdisk
> - above 4G in 64bit.
> -
> -**** MEMORY LAYOUT
> -
> -The traditional memory map for the kernel loader, used for Image or
> -zImage kernels, typically looks like:
> -
> - | |
> -0A0000 +------------------------+
> - | Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
> -09A000 +------------------------+
> - | Command line |
> - | Stack/heap | For use by the kernel real-mode code.
> -098000 +------------------------+
> - | Kernel setup | The kernel real-mode code.
> -090200 +------------------------+
> - | Kernel boot sector | The kernel legacy boot sector.
> -090000 +------------------------+
> - | Protected-mode kernel | The bulk of the kernel image.
> -010000 +------------------------+
> - | Boot loader | <- Boot sector entry point 0000:7C00
> -001000 +------------------------+
> - | Reserved for MBR/BIOS |
> -000800 +------------------------+
> - | Typically used by MBR |
> -000600 +------------------------+
> - | BIOS use only |
> -000000 +------------------------+
> -
> -
> -When using bzImage, the protected-mode kernel was relocated to
> -0x100000 ("high memory"), and the kernel real-mode block (boot sector,
> -setup, and stack/heap) was made relocatable to any address between
> -0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
> -2.01 the 0x90000+ memory range is still used internally by the kernel;
> -the 2.02 protocol resolves that problem.
> -
> -It is desirable to keep the "memory ceiling" -- the highest point in
> -low memory touched by the boot loader -- as low as possible, since
> -some newer BIOSes have begun to allocate some rather large amounts of
> -memory, called the Extended BIOS Data Area, near the top of low
> -memory. The boot loader should use the "INT 12h" BIOS call to verify
> -how much low memory is available.
> -
> -Unfortunately, if INT 12h reports that the amount of memory is too
> -low, there is usually nothing the boot loader can do but to report an
> -error to the user. The boot loader should therefore be designed to
> -take up as little space in low memory as it reasonably can. For
> -zImage or old bzImage kernels, which need data written into the
> -0x90000 segment, the boot loader should make sure not to use memory
> -above the 0x9A000 point; too many BIOSes will break above that point.
> -
> -For a modern bzImage kernel with boot protocol version >= 2.02, a
> -memory layout like the following is suggested:
> -
> - ~ ~
> - | Protected-mode kernel |
> -100000 +------------------------+
> - | I/O memory hole |
> -0A0000 +------------------------+
> - | Reserved for BIOS | Leave as much as possible unused
> - ~ ~
> - | Command line | (Can also be below the X+10000 mark)
> -X+10000 +------------------------+
> - | Stack/heap | For use by the kernel real-mode code.
> -X+08000 +------------------------+
> - | Kernel setup | The kernel real-mode code.
> - | Kernel boot sector | The kernel legacy boot sector.
> -X +------------------------+
> - | Boot loader | <- Boot sector entry point 0000:7C00
> -001000 +------------------------+
> - | Reserved for MBR/BIOS |
> -000800 +------------------------+
> - | Typically used by MBR |
> -000600 +------------------------+
> - | BIOS use only |
> -000000 +------------------------+
> -
> -... where the address X is as low as the design of the boot loader
> -permits.
> -
> -
> -**** THE REAL-MODE KERNEL HEADER
> -
> -In the following text, and anywhere in the kernel boot sequence, "a
> -sector" refers to 512 bytes. It is independent of the actual sector
> -size of the underlying medium.
> -
> -The first step in loading a Linux kernel should be to load the
> -real-mode code (boot sector and setup code) and then examine the
> -following header at offset 0x01f1. The real-mode code can total up to
> -32K, although the boot loader may choose to load only the first two
> -sectors (1K) and then examine the bootup sector size.
> -
> -The header looks like:
> -
> -Offset Proto Name Meaning
> -/Size
> -
> -01F1/1 ALL(1 setup_sects The size of the setup in sectors
> -01F2/2 ALL root_flags If set, the root is mounted readonly
> -01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
> -01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
> -01FA/2 ALL vid_mode Video mode control
> -01FC/2 ALL root_dev Default root device number
> -01FE/2 ALL boot_flag 0xAA55 magic number
> -0200/2 2.00+ jump Jump instruction
> -0202/4 2.00+ header Magic signature "HdrS"
> -0206/2 2.00+ version Boot protocol version supported
> -0208/4 2.00+ realmode_swtch Boot loader hook (see below)
> -020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
> -020E/2 2.00+ kernel_version Pointer to kernel version string
> -0210/1 2.00+ type_of_loader Boot loader identifier
> -0211/1 2.00+ loadflags Boot protocol option flags
> -0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
> -0214/4 2.00+ code32_start Boot loader hook (see below)
> -0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
> -021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
> -0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
> -0224/2 2.01+ heap_end_ptr Free memory after setup end
> -0226/1 2.02+(3 ext_loader_ver Extended boot loader version
> -0227/1 2.02+(3 ext_loader_type Extended boot loader ID
> -0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
> -022C/4 2.03+ initrd_addr_max Highest legal initrd address
> -0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
> -0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
> -0235/1 2.10+ min_alignment Minimum alignment, as a power of two
> -0236/2 2.12+ xloadflags Boot protocol option flags
> -0238/4 2.06+ cmdline_size Maximum size of the kernel command line
> -023C/4 2.07+ hardware_subarch Hardware subarchitecture
> -0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
> -0248/4 2.08+ payload_offset Offset of kernel payload
> -024C/4 2.08+ payload_length Length of kernel payload
> -0250/8 2.09+ setup_data 64-bit physical pointer to linked list
> - of struct setup_data
> -0258/8 2.10+ pref_address Preferred loading address
> -0260/4 2.10+ init_size Linear memory required during initialization
> -0264/4 2.11+ handover_offset Offset of handover entry point
> -
> -(1) For backwards compatibility, if the setup_sects field contains 0, the
> - real value is 4.
> -
> -(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
> - field are unusable, which means the size of a bzImage kernel
> - cannot be determined.
> -
> -(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
> -
> -If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
> -the boot protocol version is "old". Loading an old kernel, the
> -following parameters should be assumed:
> -
> - Image type = zImage
> - initrd not supported
> - Real-mode kernel must be located at 0x90000.
> -
> -Otherwise, the "version" field contains the protocol version,
> -e.g. protocol version 2.01 will contain 0x0201 in this field. When
> -setting fields in the header, you must make sure only to set fields
> -supported by the protocol version in use.
> -
> -
> -**** DETAILS OF HEADER FIELDS
> -
> -For each field, some are information from the kernel to the bootloader
> -("read"), some are expected to be filled out by the bootloader
> -("write"), and some are expected to be read and modified by the
> -bootloader ("modify").
> -
> -All general purpose boot loaders should write the fields marked
> -(obligatory). Boot loaders who want to load the kernel at a
> -nonstandard address should fill in the fields marked (reloc); other
> -boot loaders can ignore those fields.
> -
> -The byte order of all fields is littleendian (this is x86, after all.)
> -
> -Field name: setup_sects
> -Type: read
> -Offset/size: 0x1f1/1
> -Protocol: ALL
> -
> - The size of the setup code in 512-byte sectors. If this field is
> - 0, the real value is 4. The real-mode code consists of the boot
> - sector (always one 512-byte sector) plus the setup code.
> -
> -Field name: root_flags
> -Type: modify (optional)
> -Offset/size: 0x1f2/2
> -Protocol: ALL
> -
> - If this field is nonzero, the root defaults to readonly. The use of
> - this field is deprecated; use the "ro" or "rw" options on the
> - command line instead.
> -
> -Field name: syssize
> -Type: read
> -Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
> -Protocol: 2.04+
> -
> - The size of the protected-mode code in units of 16-byte paragraphs.
> - For protocol versions older than 2.04 this field is only two bytes
> - wide, and therefore cannot be trusted for the size of a kernel if
> - the LOAD_HIGH flag is set.
> -
> -Field name: ram_size
> -Type: kernel internal
> -Offset/size: 0x1f8/2
> -Protocol: ALL
> -
> - This field is obsolete.
> -
> -Field name: vid_mode
> -Type: modify (obligatory)
> -Offset/size: 0x1fa/2
> -
> - Please see the section on SPECIAL COMMAND LINE OPTIONS.
> -
> -Field name: root_dev
> -Type: modify (optional)
> -Offset/size: 0x1fc/2
> -Protocol: ALL
> -
> - The default root device device number. The use of this field is
> - deprecated, use the "root=" option on the command line instead.
> -
> -Field name: boot_flag
> -Type: read
> -Offset/size: 0x1fe/2
> -Protocol: ALL
> -
> - Contains 0xAA55. This is the closest thing old Linux kernels have
> - to a magic number.
> -
> -Field name: jump
> -Type: read
> -Offset/size: 0x200/2
> -Protocol: 2.00+
> -
> - Contains an x86 jump instruction, 0xEB followed by a signed offset
> - relative to byte 0x202. This can be used to determine the size of
> - the header.
> -
> -Field name: header
> -Type: read
> -Offset/size: 0x202/4
> -Protocol: 2.00+
> -
> - Contains the magic number "HdrS" (0x53726448).
> -
> -Field name: version
> -Type: read
> -Offset/size: 0x206/2
> -Protocol: 2.00+
> -
> - Contains the boot protocol version, in (major << 8)+minor format,
> - e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
> - 10.17.
> -
> -Field name: realmode_swtch
> -Type: modify (optional)
> -Offset/size: 0x208/4
> -Protocol: 2.00+
> -
> - Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
> -
> -Field name: start_sys_seg
> -Type: read
> -Offset/size: 0x20c/2
> -Protocol: 2.00+
> -
> - The load low segment (0x1000). Obsolete.
> -
> -Field name: kernel_version
> -Type: read
> -Offset/size: 0x20e/2
> -Protocol: 2.00+
> -
> - If set to a nonzero value, contains a pointer to a NUL-terminated
> - human-readable kernel version number string, less 0x200. This can
> - be used to display the kernel version to the user. This value
> - should be less than (0x200*setup_sects).
> -
> - For example, if this value is set to 0x1c00, the kernel version
> - number string can be found at offset 0x1e00 in the kernel file.
> - This is a valid value if and only if the "setup_sects" field
> - contains the value 15 or higher, as:
> -
> - 0x1c00 < 15*0x200 (= 0x1e00) but
> - 0x1c00 >= 14*0x200 (= 0x1c00)
> -
> - 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
> -
> -Field name: type_of_loader
> -Type: write (obligatory)
> -Offset/size: 0x210/1
> -Protocol: 2.00+
> -
> - If your boot loader has an assigned id (see table below), enter
> - 0xTV here, where T is an identifier for the boot loader and V is
> - a version number. Otherwise, enter 0xFF here.
> -
> - For boot loader IDs above T = 0xD, write T = 0xE to this field and
> - write the extended ID minus 0x10 to the ext_loader_type field.
> - Similarly, the ext_loader_ver field can be used to provide more than
> - four bits for the bootloader version.
> -
> - For example, for T = 0x15, V = 0x234, write:
> -
> - type_of_loader <- 0xE4
> - ext_loader_type <- 0x05
> - ext_loader_ver <- 0x23
> -
> - Assigned boot loader ids (hexadecimal):
> -
> - 0 LILO (0x00 reserved for pre-2.00 bootloader)
> - 1 Loadlin
> - 2 bootsect-loader (0x20, all other values reserved)
> - 3 Syslinux
> - 4 Etherboot/gPXE/iPXE
> - 5 ELILO
> - 7 GRUB
> - 8 U-Boot
> - 9 Xen
> - A Gujin
> - B Qemu
> - C Arcturus Networks uCbootloader
> - D kexec-tools
> - E Extended (see ext_loader_type)
> - F Special (0xFF = undefined)
> - 10 Reserved
> - 11 Minimal Linux Bootloader <http://sebastian-plotz.blogspot.de>
> - 12 OVMF UEFI virtualization stack
> -
> - Please contact <hpa@...or.com> if you need a bootloader ID
> - value assigned.
> -
> -Field name: loadflags
> -Type: modify (obligatory)
> -Offset/size: 0x211/1
> -Protocol: 2.00+
> -
> - This field is a bitmask.
> -
> - Bit 0 (read): LOADED_HIGH
> - - If 0, the protected-mode code is loaded at 0x10000.
> - - If 1, the protected-mode code is loaded at 0x100000.
> -
> - Bit 1 (kernel internal): KASLR_FLAG
> - - Used internally by the compressed kernel to communicate
> - KASLR status to kernel proper.
> - If 1, KASLR enabled.
> - If 0, KASLR disabled.
> -
> - Bit 5 (write): QUIET_FLAG
> - - If 0, print early messages.
> - - If 1, suppress early messages.
> - This requests to the kernel (decompressor and early
> - kernel) to not write early messages that require
> - accessing the display hardware directly.
> -
> - Bit 6 (write): KEEP_SEGMENTS
> - Protocol: 2.07+
> - - If 0, reload the segment registers in the 32bit entry point.
> - - If 1, do not reload the segment registers in the 32bit entry point.
> - Assume that %cs %ds %ss %es are all set to flat segments with
> - a base of 0 (or the equivalent for their environment).
> -
> - Bit 7 (write): CAN_USE_HEAP
> - Set this bit to 1 to indicate that the value entered in the
> - heap_end_ptr is valid. If this field is clear, some setup code
> - functionality will be disabled.
> -
> -Field name: setup_move_size
> -Type: modify (obligatory)
> -Offset/size: 0x212/2
> -Protocol: 2.00-2.01
> -
> - When using protocol 2.00 or 2.01, if the real mode kernel is not
> - loaded at 0x90000, it gets moved there later in the loading
> - sequence. Fill in this field if you want additional data (such as
> - the kernel command line) moved in addition to the real-mode kernel
> - itself.
> -
> - The unit is bytes starting with the beginning of the boot sector.
> -
> - This field is can be ignored when the protocol is 2.02 or higher, or
> - if the real-mode code is loaded at 0x90000.
> -
> -Field name: code32_start
> -Type: modify (optional, reloc)
> -Offset/size: 0x214/4
> -Protocol: 2.00+
> -
> - The address to jump to in protected mode. This defaults to the load
> - address of the kernel, and can be used by the boot loader to
> - determine the proper load address.
> -
> - This field can be modified for two purposes:
> -
> - 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
> -
> - 2. if a bootloader which does not install a hook loads a
> - relocatable kernel at a nonstandard address it will have to modify
> - this field to point to the load address.
> -
> -Field name: ramdisk_image
> -Type: write (obligatory)
> -Offset/size: 0x218/4
> -Protocol: 2.00+
> -
> - The 32-bit linear address of the initial ramdisk or ramfs. Leave at
> - zero if there is no initial ramdisk/ramfs.
> -
> -Field name: ramdisk_size
> -Type: write (obligatory)
> -Offset/size: 0x21c/4
> -Protocol: 2.00+
> -
> - Size of the initial ramdisk or ramfs. Leave at zero if there is no
> - initial ramdisk/ramfs.
> -
> -Field name: bootsect_kludge
> -Type: kernel internal
> -Offset/size: 0x220/4
> -Protocol: 2.00+
> -
> - This field is obsolete.
> -
> -Field name: heap_end_ptr
> -Type: write (obligatory)
> -Offset/size: 0x224/2
> -Protocol: 2.01+
> -
> - Set this field to the offset (from the beginning of the real-mode
> - code) of the end of the setup stack/heap, minus 0x0200.
> -
> -Field name: ext_loader_ver
> -Type: write (optional)
> -Offset/size: 0x226/1
> -Protocol: 2.02+
> -
> - This field is used as an extension of the version number in the
> - type_of_loader field. The total version number is considered to be
> - (type_of_loader & 0x0f) + (ext_loader_ver << 4).
> -
> - The use of this field is boot loader specific. If not written, it
> - is zero.
> -
> - Kernels prior to 2.6.31 did not recognize this field, but it is safe
> - to write for protocol version 2.02 or higher.
> -
> -Field name: ext_loader_type
> -Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
> -Offset/size: 0x227/1
> -Protocol: 2.02+
> -
> - This field is used as an extension of the type number in
> - type_of_loader field. If the type in type_of_loader is 0xE, then
> - the actual type is (ext_loader_type + 0x10).
> -
> - This field is ignored if the type in type_of_loader is not 0xE.
> -
> - Kernels prior to 2.6.31 did not recognize this field, but it is safe
> - to write for protocol version 2.02 or higher.
> -
> -Field name: cmd_line_ptr
> -Type: write (obligatory)
> -Offset/size: 0x228/4
> -Protocol: 2.02+
> -
> - Set this field to the linear address of the kernel command line.
> - The kernel command line can be located anywhere between the end of
> - the setup heap and 0xA0000; it does not have to be located in the
> - same 64K segment as the real-mode code itself.
> -
> - Fill in this field even if your boot loader does not support a
> - command line, in which case you can point this to an empty string
> - (or better yet, to the string "auto".) If this field is left at
> - zero, the kernel will assume that your boot loader does not support
> - the 2.02+ protocol.
> -
> -Field name: initrd_addr_max
> -Type: read
> -Offset/size: 0x22c/4
> -Protocol: 2.03+
> -
> - The maximum address that may be occupied by the initial
> - ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
> - field is not present, and the maximum address is 0x37FFFFFF. (This
> - address is defined as the address of the highest safe byte, so if
> - your ramdisk is exactly 131072 bytes long and this field is
> - 0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
> -
> -Field name: kernel_alignment
> -Type: read/modify (reloc)
> -Offset/size: 0x230/4
> -Protocol: 2.05+ (read), 2.10+ (modify)
> -
> - Alignment unit required by the kernel (if relocatable_kernel is
> - true.) A relocatable kernel that is loaded at an alignment
> - incompatible with the value in this field will be realigned during
> - kernel initialization.
> -
> - Starting with protocol version 2.10, this reflects the kernel
> - alignment preferred for optimal performance; it is possible for the
> - loader to modify this field to permit a lesser alignment. See the
> - min_alignment and pref_address field below.
> -
> -Field name: relocatable_kernel
> -Type: read (reloc)
> -Offset/size: 0x234/1
> -Protocol: 2.05+
> -
> - If this field is nonzero, the protected-mode part of the kernel can
> - be loaded at any address that satisfies the kernel_alignment field.
> - After loading, the boot loader must set the code32_start field to
> - point to the loaded code, or to a boot loader hook.
> -
> -Field name: min_alignment
> -Type: read (reloc)
> -Offset/size: 0x235/1
> -Protocol: 2.10+
> -
> - This field, if nonzero, indicates as a power of two the minimum
> - alignment required, as opposed to preferred, by the kernel to boot.
> - If a boot loader makes use of this field, it should update the
> - kernel_alignment field with the alignment unit desired; typically:
> -
> - kernel_alignment = 1 << min_alignment
> -
> - There may be a considerable performance cost with an excessively
> - misaligned kernel. Therefore, a loader should typically try each
> - power-of-two alignment from kernel_alignment down to this alignment.
> -
> -Field name: xloadflags
> -Type: read
> -Offset/size: 0x236/2
> -Protocol: 2.12+
> -
> - This field is a bitmask.
> -
> - Bit 0 (read): XLF_KERNEL_64
> - - If 1, this kernel has the legacy 64-bit entry point at 0x200.
> -
> - Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
> - - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
> -
> - Bit 2 (read): XLF_EFI_HANDOVER_32
> - - If 1, the kernel supports the 32-bit EFI handoff entry point
> - given at handover_offset.
> -
> - Bit 3 (read): XLF_EFI_HANDOVER_64
> - - If 1, the kernel supports the 64-bit EFI handoff entry point
> - given at handover_offset + 0x200.
> -
> - Bit 4 (read): XLF_EFI_KEXEC
> - - If 1, the kernel supports kexec EFI boot with EFI runtime support.
> -
> -Field name: cmdline_size
> -Type: read
> -Offset/size: 0x238/4
> -Protocol: 2.06+
> -
> - The maximum size of the command line without the terminating
> - zero. This means that the command line can contain at most
> - cmdline_size characters. With protocol version 2.05 and earlier, the
> - maximum size was 255.
> -
> -Field name: hardware_subarch
> -Type: write (optional, defaults to x86/PC)
> -Offset/size: 0x23c/4
> -Protocol: 2.07+
> -
> - In a paravirtualized environment the hardware low level architectural
> - pieces such as interrupt handling, page table handling, and
> - accessing process control registers needs to be done differently.
> -
> - This field allows the bootloader to inform the kernel we are in one
> - one of those environments.
> -
> - 0x00000000 The default x86/PC environment
> - 0x00000001 lguest
> - 0x00000002 Xen
> - 0x00000003 Moorestown MID
> - 0x00000004 CE4100 TV Platform
> -
> -Field name: hardware_subarch_data
> -Type: write (subarch-dependent)
> -Offset/size: 0x240/8
> -Protocol: 2.07+
> -
> - A pointer to data that is specific to hardware subarch
> - This field is currently unused for the default x86/PC environment,
> - do not modify.
> -
> -Field name: payload_offset
> -Type: read
> -Offset/size: 0x248/4
> -Protocol: 2.08+
> -
> - If non-zero then this field contains the offset from the beginning
> - of the protected-mode code to the payload.
> -
> - The payload may be compressed. The format of both the compressed and
> - uncompressed data should be determined using the standard magic
> - numbers. The currently supported compression formats are gzip
> - (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
> - (magic number 5D 00), XZ (magic number FD 37), and LZ4 (magic number
> - 02 21). The uncompressed payload is currently always ELF (magic
> - number 7F 45 4C 46).
> -
> -Field name: payload_length
> -Type: read
> -Offset/size: 0x24c/4
> -Protocol: 2.08+
> -
> - The length of the payload.
> -
> -Field name: setup_data
> -Type: write (special)
> -Offset/size: 0x250/8
> -Protocol: 2.09+
> -
> - The 64-bit physical pointer to NULL terminated single linked list of
> - struct setup_data. This is used to define a more extensible boot
> - parameters passing mechanism. The definition of struct setup_data is
> - as follow:
> -
> - struct setup_data {
> - u64 next;
> - u32 type;
> - u32 len;
> - u8 data[0];
> - };
> -
> - Where, the next is a 64-bit physical pointer to the next node of
> - linked list, the next field of the last node is 0; the type is used
> - to identify the contents of data; the len is the length of data
> - field; the data holds the real payload.
> -
> - This list may be modified at a number of points during the bootup
> - process. Therefore, when modifying this list one should always make
> - sure to consider the case where the linked list already contains
> - entries.
> -
> -Field name: pref_address
> -Type: read (reloc)
> -Offset/size: 0x258/8
> -Protocol: 2.10+
> -
> - This field, if nonzero, represents a preferred load address for the
> - kernel. A relocating bootloader should attempt to load at this
> - address if possible.
> -
> - A non-relocatable kernel will unconditionally move itself and to run
> - at this address.
> -
> -Field name: init_size
> -Type: read
> -Offset/size: 0x260/4
> -
> - This field indicates the amount of linear contiguous memory starting
> - at the kernel runtime start address that the kernel needs before it
> - is capable of examining its memory map. This is not the same thing
> - as the total amount of memory the kernel needs to boot, but it can
> - be used by a relocating boot loader to help select a safe load
> - address for the kernel.
> -
> - The kernel runtime start address is determined by the following algorithm:
> -
> - if (relocatable_kernel)
> - runtime_start = align_up(load_address, kernel_alignment)
> - else
> - runtime_start = pref_address
> -
> -Field name: handover_offset
> -Type: read
> -Offset/size: 0x264/4
> -
> - This field is the offset from the beginning of the kernel image to
> - the EFI handover protocol entry point. Boot loaders using the EFI
> - handover protocol to boot the kernel should jump to this offset.
> -
> - See EFI HANDOVER PROTOCOL below for more details.
> -
> -
> -**** THE IMAGE CHECKSUM
> -
> -From boot protocol version 2.08 onwards the CRC-32 is calculated over
> -the entire file using the characteristic polynomial 0x04C11DB7 and an
> -initial remainder of 0xffffffff. The checksum is appended to the
> -file; therefore the CRC of the file up to the limit specified in the
> -syssize field of the header is always 0.
> -
> -
> -**** THE KERNEL COMMAND LINE
> -
> -The kernel command line has become an important way for the boot
> -loader to communicate with the kernel. Some of its options are also
> -relevant to the boot loader itself, see "special command line options"
> -below.
> -
> -The kernel command line is a null-terminated string. The maximum
> -length can be retrieved from the field cmdline_size. Before protocol
> -version 2.06, the maximum was 255 characters. A string that is too
> -long will be automatically truncated by the kernel.
> -
> -If the boot protocol version is 2.02 or later, the address of the
> -kernel command line is given by the header field cmd_line_ptr (see
> -above.) This address can be anywhere between the end of the setup
> -heap and 0xA0000.
> -
> -If the protocol version is *not* 2.02 or higher, the kernel
> -command line is entered using the following protocol:
> -
> - At offset 0x0020 (word), "cmd_line_magic", enter the magic
> - number 0xA33F.
> -
> - At offset 0x0022 (word), "cmd_line_offset", enter the offset
> - of the kernel command line (relative to the start of the
> - real-mode kernel).
> -
> - The kernel command line *must* be within the memory region
> - covered by setup_move_size, so you may need to adjust this
> - field.
> -
> -
> -**** MEMORY LAYOUT OF THE REAL-MODE CODE
> -
> -The real-mode code requires a stack/heap to be set up, as well as
> -memory allocated for the kernel command line. This needs to be done
> -in the real-mode accessible memory in bottom megabyte.
> -
> -It should be noted that modern machines often have a sizable Extended
> -BIOS Data Area (EBDA). As a result, it is advisable to use as little
> -of the low megabyte as possible.
> -
> -Unfortunately, under the following circumstances the 0x90000 memory
> -segment has to be used:
> -
> - - When loading a zImage kernel ((loadflags & 0x01) == 0).
> - - When loading a 2.01 or earlier boot protocol kernel.
> -
> - -> For the 2.00 and 2.01 boot protocols, the real-mode code
> - can be loaded at another address, but it is internally
> - relocated to 0x90000. For the "old" protocol, the
> - real-mode code must be loaded at 0x90000.
> -
> -When loading at 0x90000, avoid using memory above 0x9a000.
> -
> -For boot protocol 2.02 or higher, the command line does not have to be
> -located in the same 64K segment as the real-mode setup code; it is
> -thus permitted to give the stack/heap the full 64K segment and locate
> -the command line above it.
> -
> -The kernel command line should not be located below the real-mode
> -code, nor should it be located in high memory.
> -
> -
> -**** SAMPLE BOOT CONFIGURATION
> -
> -As a sample configuration, assume the following layout of the real
> -mode segment:
> -
> - When loading below 0x90000, use the entire segment:
> -
> - 0x0000-0x7fff Real mode kernel
> - 0x8000-0xdfff Stack and heap
> - 0xe000-0xffff Kernel command line
> -
> - When loading at 0x90000 OR the protocol version is 2.01 or earlier:
> -
> - 0x0000-0x7fff Real mode kernel
> - 0x8000-0x97ff Stack and heap
> - 0x9800-0x9fff Kernel command line
> -
> -Such a boot loader should enter the following fields in the header:
> -
> - unsigned long base_ptr; /* base address for real-mode segment */
> -
> - if ( setup_sects == 0 ) {
> - setup_sects = 4;
> - }
> -
> - if ( protocol >= 0x0200 ) {
> - type_of_loader = <type code>;
> - if ( loading_initrd ) {
> - ramdisk_image = <initrd_address>;
> - ramdisk_size = <initrd_size>;
> - }
> -
> - if ( protocol >= 0x0202 && loadflags & 0x01 )
> - heap_end = 0xe000;
> - else
> - heap_end = 0x9800;
> -
> - if ( protocol >= 0x0201 ) {
> - heap_end_ptr = heap_end - 0x200;
> - loadflags |= 0x80; /* CAN_USE_HEAP */
> - }
> -
> - if ( protocol >= 0x0202 ) {
> - cmd_line_ptr = base_ptr + heap_end;
> - strcpy(cmd_line_ptr, cmdline);
> - } else {
> - cmd_line_magic = 0xA33F;
> - cmd_line_offset = heap_end;
> - setup_move_size = heap_end + strlen(cmdline)+1;
> - strcpy(base_ptr+cmd_line_offset, cmdline);
> - }
> - } else {
> - /* Very old kernel */
> -
> - heap_end = 0x9800;
> -
> - cmd_line_magic = 0xA33F;
> - cmd_line_offset = heap_end;
> -
> - /* A very old kernel MUST have its real-mode code
> - loaded at 0x90000 */
> -
> - if ( base_ptr != 0x90000 ) {
> - /* Copy the real-mode kernel */
> - memcpy(0x90000, base_ptr, (setup_sects+1)*512);
> - base_ptr = 0x90000; /* Relocated */
> - }
> -
> - strcpy(0x90000+cmd_line_offset, cmdline);
> -
> - /* It is recommended to clear memory up to the 32K mark */
> - memset(0x90000 + (setup_sects+1)*512, 0,
> - (64-(setup_sects+1))*512);
> - }
> -
> -
> -**** LOADING THE REST OF THE KERNEL
> -
> -The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
> -in the kernel file (again, if setup_sects == 0 the real value is 4.)
> -It should be loaded at address 0x10000 for Image/zImage kernels and
> -0x100000 for bzImage kernels.
> -
> -The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
> -bit (LOAD_HIGH) in the loadflags field is set:
> -
> - is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
> - load_address = is_bzImage ? 0x100000 : 0x10000;
> -
> -Note that Image/zImage kernels can be up to 512K in size, and thus use
> -the entire 0x10000-0x90000 range of memory. This means it is pretty
> -much a requirement for these kernels to load the real-mode part at
> -0x90000. bzImage kernels allow much more flexibility.
> -
> -
> -**** SPECIAL COMMAND LINE OPTIONS
> -
> -If the command line provided by the boot loader is entered by the
> -user, the user may expect the following command line options to work.
> -They should normally not be deleted from the kernel command line even
> -though not all of them are actually meaningful to the kernel. Boot
> -loader authors who need additional command line options for the boot
> -loader itself should get them registered in
> -Documentation/admin-guide/kernel-parameters.rst to make sure they will not
> -conflict with actual kernel options now or in the future.
> -
> - vga=<mode>
> - <mode> here is either an integer (in C notation, either
> - decimal, octal, or hexadecimal) or one of the strings
> - "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
> - (meaning 0xFFFD). This value should be entered into the
> - vid_mode field, as it is used by the kernel before the command
> - line is parsed.
> -
> - mem=<size>
> - <size> is an integer in C notation optionally followed by
> - (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
> - << 30, << 40, << 50 or << 60). This specifies the end of
> - memory to the kernel. This affects the possible placement of
> - an initrd, since an initrd should be placed near end of
> - memory. Note that this is an option to *both* the kernel and
> - the bootloader!
> -
> - initrd=<file>
> - An initrd should be loaded. The meaning of <file> is
> - obviously bootloader-dependent, and some boot loaders
> - (e.g. LILO) do not have such a command.
> -
> -In addition, some boot loaders add the following options to the
> -user-specified command line:
> -
> - BOOT_IMAGE=<file>
> - The boot image which was loaded. Again, the meaning of <file>
> - is obviously bootloader-dependent.
> -
> - auto
> - The kernel was booted without explicit user intervention.
> -
> -If these options are added by the boot loader, it is highly
> -recommended that they are located *first*, before the user-specified
> -or configuration-specified command line. Otherwise, "init=/bin/sh"
> -gets confused by the "auto" option.
> -
> -
> -**** RUNNING THE KERNEL
> -
> -The kernel is started by jumping to the kernel entry point, which is
> -located at *segment* offset 0x20 from the start of the real mode
> -kernel. This means that if you loaded your real-mode kernel code at
> -0x90000, the kernel entry point is 9020:0000.
> -
> -At entry, ds = es = ss should point to the start of the real-mode
> -kernel code (0x9000 if the code is loaded at 0x90000), sp should be
> -set up properly, normally pointing to the top of the heap, and
> -interrupts should be disabled. Furthermore, to guard against bugs in
> -the kernel, it is recommended that the boot loader sets fs = gs = ds =
> -es = ss.
> -
> -In our example from above, we would do:
> -
> - /* Note: in the case of the "old" kernel protocol, base_ptr must
> - be == 0x90000 at this point; see the previous sample code */
> -
> - seg = base_ptr >> 4;
> -
> - cli(); /* Enter with interrupts disabled! */
> -
> - /* Set up the real-mode kernel stack */
> - _SS = seg;
> - _SP = heap_end;
> -
> - _DS = _ES = _FS = _GS = seg;
> - jmp_far(seg+0x20, 0); /* Run the kernel */
> -
> -If your boot sector accesses a floppy drive, it is recommended to
> -switch off the floppy motor before running the kernel, since the
> -kernel boot leaves interrupts off and thus the motor will not be
> -switched off, especially if the loaded kernel has the floppy driver as
> -a demand-loaded module!
> -
> -
> -**** ADVANCED BOOT LOADER HOOKS
> -
> -If the boot loader runs in a particularly hostile environment (such as
> -LOADLIN, which runs under DOS) it may be impossible to follow the
> -standard memory location requirements. Such a boot loader may use the
> -following hooks that, if set, are invoked by the kernel at the
> -appropriate time. The use of these hooks should probably be
> -considered an absolutely last resort!
> -
> -IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
> -%edi across invocation.
> -
> - realmode_swtch:
> - A 16-bit real mode far subroutine invoked immediately before
> - entering protected mode. The default routine disables NMI, so
> - your routine should probably do so, too.
> -
> - code32_start:
> - A 32-bit flat-mode routine *jumped* to immediately after the
> - transition to protected mode, but before the kernel is
> - uncompressed. No segments, except CS, are guaranteed to be
> - set up (current kernels do, but older ones do not); you should
> - set them up to BOOT_DS (0x18) yourself.
> -
> - After completing your hook, you should jump to the address
> - that was in this field before your boot loader overwrote it
> - (relocated, if appropriate.)
> -
> -
> -**** 32-bit BOOT PROTOCOL
> -
> -For machine with some new BIOS other than legacy BIOS, such as EFI,
> -LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
> -based on legacy BIOS can not be used, so a 32-bit boot protocol needs
> -to be defined.
> -
> -In 32-bit boot protocol, the first step in loading a Linux kernel
> -should be to setup the boot parameters (struct boot_params,
> -traditionally known as "zero page"). The memory for struct boot_params
> -should be allocated and initialized to all zero. Then the setup header
> -from offset 0x01f1 of kernel image on should be loaded into struct
> -boot_params and examined. The end of setup header can be calculated as
> -follow:
> -
> - 0x0202 + byte value at offset 0x0201
> -
> -In addition to read/modify/write the setup header of the struct
> -boot_params as that of 16-bit boot protocol, the boot loader should
> -also fill the additional fields of the struct boot_params as that
> -described in zero-page.txt.
> -
> -After setting up the struct boot_params, the boot loader can load the
> -32/64-bit kernel in the same way as that of 16-bit boot protocol.
> -
> -In 32-bit boot protocol, the kernel is started by jumping to the
> -32-bit kernel entry point, which is the start address of loaded
> -32/64-bit kernel.
> -
> -At entry, the CPU must be in 32-bit protected mode with paging
> -disabled; a GDT must be loaded with the descriptors for selectors
> -__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
> -segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
> -must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> -must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
> -address of the struct boot_params; %ebp, %edi and %ebx must be zero.
> -
> -**** 64-bit BOOT PROTOCOL
> -
> -For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
> -and we need a 64-bit boot protocol.
> -
> -In 64-bit boot protocol, the first step in loading a Linux kernel
> -should be to setup the boot parameters (struct boot_params,
> -traditionally known as "zero page"). The memory for struct boot_params
> -could be allocated anywhere (even above 4G) and initialized to all zero.
> -Then, the setup header at offset 0x01f1 of kernel image on should be
> -loaded into struct boot_params and examined. The end of setup header
> -can be calculated as follows:
> -
> - 0x0202 + byte value at offset 0x0201
> -
> -In addition to read/modify/write the setup header of the struct
> -boot_params as that of 16-bit boot protocol, the boot loader should
> -also fill the additional fields of the struct boot_params as described
> -in zero-page.txt.
> -
> -After setting up the struct boot_params, the boot loader can load
> -64-bit kernel in the same way as that of 16-bit boot protocol, but
> -kernel could be loaded above 4G.
> -
> -In 64-bit boot protocol, the kernel is started by jumping to the
> -64-bit kernel entry point, which is the start address of loaded
> -64-bit kernel plus 0x200.
> -
> -At entry, the CPU must be in 64-bit mode with paging enabled.
> -The range with setup_header.init_size from start address of loaded
> -kernel and zero page and command line buffer get ident mapping;
> -a GDT must be loaded with the descriptors for selectors
> -__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
> -segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
> -must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> -must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
> -address of the struct boot_params.
> -
> -**** EFI HANDOVER PROTOCOL
> -
> -This protocol allows boot loaders to defer initialisation to the EFI
> -boot stub. The boot loader is required to load the kernel/initrd(s)
> -from the boot media and jump to the EFI handover protocol entry point
> -which is hdr->handover_offset bytes from the beginning of
> -startup_{32,64}.
> -
> -The function prototype for the handover entry point looks like this,
> -
> - efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)
> -
> -'handle' is the EFI image handle passed to the boot loader by the EFI
> -firmware, 'table' is the EFI system table - these are the first two
> -arguments of the "handoff state" as described in section 2.3 of the
> -UEFI specification. 'bp' is the boot loader-allocated boot params.
> -
> -The boot loader *must* fill out the following fields in bp,
> -
> - o hdr.code32_start
> - o hdr.cmd_line_ptr
> - o hdr.ramdisk_image (if applicable)
> - o hdr.ramdisk_size (if applicable)
> -
> -All other fields should be zero.
> diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> index 7612d3142b2a..8f08caf4fbbb 100644
> --- a/Documentation/x86/index.rst
> +++ b/Documentation/x86/index.rst
> @@ -7,3 +7,5 @@ Linux x86 Support
> .. toctree::
> :maxdepth: 2
> :numbered:
> +
> + boot
Thanks,
Mauro
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