Date:   Thu, 10 Oct 2019 11:43:49 +0200
From:   Daniel Kiper <daniel.kiper@...cle.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, xen-devel@...ts.xenproject.org,
        ard.biesheuvel@...aro.org, boris.ostrovsky@...cle.com,
        bp@...en8.de, corbet@....net, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, eric.snowberg@...cle.com,
        hpa@...or.com, jgross@...e.com, konrad.wilk@...cle.com,
        mingo@...hat.com, ross.philipson@...cle.com, tglx@...utronix.de
Subject: Re: [PATCH v3 1/3] x86/boot: Introduce the kernel_info

On Wed, Oct 09, 2019 at 05:43:31PM -0700, Randy Dunlap wrote:
> Hi,
>
> Questions and comments below...
> Thanks.
>
> On 10/9/19 3:53 AM, Daniel Kiper wrote:
>
> > Suggested-by: H. Peter Anvin <hpa@...or.com>
> > Signed-off-by: Daniel Kiper <daniel.kiper@...cle.com>
> > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> > Reviewed-by: Ross Philipson <ross.philipson@...cle.com>
> > ---
>
> > ---
> >  Documentation/x86/boot.rst             | 121 +++++++++++++++++++++++++++++++++
> >  arch/x86/boot/Makefile                 |   2 +-
> >  arch/x86/boot/compressed/Makefile      |   4 +-
> >  arch/x86/boot/compressed/kernel_info.S |  17 +++++
> >  arch/x86/boot/header.S                 |   1 +
> >  arch/x86/boot/tools/build.c            |   5 ++
> >  arch/x86/include/uapi/asm/bootparam.h  |   1 +
> >  7 files changed, 148 insertions(+), 3 deletions(-)
> >  create mode 100644 arch/x86/boot/compressed/kernel_info.S
> >
> > diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
> > index 08a2f100c0e6..d5323a39f5e3 100644
> > --- a/Documentation/x86/boot.rst
> > +++ b/Documentation/x86/boot.rst
> > @@ -68,8 +68,25 @@ Protocol 2.12	(Kernel 3.8) Added the xloadflags field and extension fields
> >  Protocol 2.13	(Kernel 3.14) Support 32- and 64-bit flags being set in
> >  		xloadflags to support booting a 64-bit kernel from 32-bit
> >  		EFI
> > +
> > +Protocol 2.14:	BURNT BY INCORRECT COMMIT ae7e1238e68f2a472a125673ab506d49158c1889
> > +		(x86/boot: Add ACPI RSDP address to setup_header)
> > +		DO NOT USE!!! ASSUME SAME AS 2.13.
> > +
> > +Protocol 2.15:	(Kernel 5.5) Added the kernel_info.
> >  =============	============================================================
> >
> > +.. note::
> > +     The protocol version number should be changed only if the setup header
> > +     is changed. There is no need to update the version number if boot_params
> > +     or kernel_info are changed. Additionally, it is recommended to use
> > +     xloadflags (in this case the protocol version number should not be
> > +     updated either) or kernel_info to communicate supported Linux kernel
> > +     features to the boot loader. Due to very limited space available in
> > +     the original setup header every update to it should be considered
> > +     with great care. Starting from the protocol 2.15 the primary way to
> > +     communicate things to the boot loader is the kernel_info.
> > +
> >
> >  Memory Layout
> >  =============
> > @@ -207,6 +224,7 @@ Offset/Size	Proto		Name			Meaning
> >  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
> > +0268/4		2.15+		kernel_info_offset	Offset of the kernel_info
> >  ===========	========	=====================	============================================
> >
> >  .. note::
> > @@ -855,6 +873,109 @@ Offset/size:	0x264/4
> >
> >    See EFI HANDOVER PROTOCOL below for more details.
> >
> > +============	==================
> > +Field name:	kernel_info_offset
> > +Type:		read
> > +Offset/size:	0x268/4
> > +Protocol:	2.15+
> > +============	==================
> > +
> > +  This field is the offset from the beginning of the kernel image to the
> > +  kernel_info. It is embedded in the Linux image in the uncompressed
>                   ^^
>    What does      It   refer to, please?

s/It/The kernel_info structure/ Is it better?

> > +  protected mode region.
> > +
> > +
> > +The kernel_info
> > +===============
> > +
> > +The relationships between the headers are analogous to the various data
> > +sections:
> > +
> > +  setup_header = .data
> > +  boot_params/setup_data = .bss
> > +
> > +What is missing from the above list? That's right:
> > +
> > +  kernel_info = .rodata
> > +
> > +We have been (ab)using .data for things that could go into .rodata or .bss for
> > +a long time, for lack of alternatives and -- especially early on -- inertia.
> > +Also, the BIOS stub is responsible for creating boot_params, so it isn't
> > +available to a BIOS-based loader (setup_data is, though).
> > +
> > +setup_header is permanently limited to 144 bytes due to the reach of the
> > +2-byte jump field, which doubles as a length field for the structure, combined
> > +with the size of the "hole" in struct boot_params that a protected-mode loader
> > +or the BIOS stub has to copy it into. It is currently 119 bytes long, which
> > +leaves us with 25 very precious bytes. This isn't something that can be fixed
> > +without revising the boot protocol entirely, breaking backwards compatibility.
> > +
> > +boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
> > +by adding setup_data entries. It cannot be used to communicate properties of
> > +the kernel image, because it is .bss and has no image-provided content.
> > +
> > +kernel_info solves this by providing an extensible place for information about
> > +the kernel image. It is readonly, because the kernel cannot rely on a
> > +bootloader copying its contents anywhere, but that is OK; if it becomes
> > +necessary it can still contain data items that an enabled bootloader would be
> > +expected to copy into a setup_data chunk.
> > +
> > +All kernel_info data should be part of this structure. Fixed size data have to
> > +be put before kernel_info_var_len_data label. Variable size data have to be put
> > +behind kernel_info_var_len_data label. Each chunk of variable size data has to
>
>    s/behind/after/

OK.

> > +be prefixed with header/magic and its size, e.g.:
> > +
> > +  kernel_info:
> > +          .ascii  "LToP"          /* Header, Linux top (structure). */
> > +          .long   kernel_info_var_len_data - kernel_info
> > +          .long   kernel_info_end - kernel_info
> > +          .long   0x01234567      /* Some fixed size data for the bootloaders. */
> > +  kernel_info_var_len_data:
> > +  example_struct:                 /* Some variable size data for the bootloaders. */
> > +          .ascii  "EsTT"          /* Header/Magic. */
> > +          .long   example_struct_end - example_struct
> > +          .ascii  "Struct"
> > +          .long   0x89012345
> > +  example_struct_end:
> > +  example_strings:                /* Some variable size data for the bootloaders. */
> > +          .ascii  "EsTs"          /* Header/Magic. */
>
> Where do the Magic values "EsTT" and "EsTs" come from?
> where are they defined?

EsTT == Example STrucT
EsTs == Example STringS

Anyway, it can be anything which does not collide with existing variable
length data magics. There are none right now. So, it can be anything.
Maybe I should add something saying that.

Daniel

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