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Date:   Thu,  4 Jul 2019 18:36:10 +0200
From:   Daniel Kiper <daniel.kiper@...cle.com>
To:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     bp@...en8.de, corbet@....net, dpsmith@...rtussolutions.com,
        eric.snowberg@...cle.com, hpa@...or.com, kanth.ghatraju@...cle.com,
        konrad.wilk@...cle.com, mingo@...hat.com,
        ross.philipson@...cle.com, tglx@...utronix.de
Subject: [PATCH v2 1/3] x86/boot: Introduce 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.

This patch does not bump setup_header version in arch/x86/boot/header.S
because it will be followed by additional changes coming into the
Linux/x86 boot protocol.

Suggested-by: H. Peter Anvin <hpa@...or.com>
Signed-off-by: Daniel Kiper <daniel.kiper@...cle.com>
Reviewed-by: Eric Snowberg <eric.snowberg@...cle.com>
Reviewed-by: Ross Philipson <ross.philipson@...cle.com>
---
v2 - suggestions/fixes:
   - rename setup_header2 to kernel_info,
     (suggested by H. Peter Anvin),
   - change kernel_info.header value to "InfO" (0x4f666e49),
   - new kernel_info description in Documentation/x86/boot.rst,
     (suggested by H. Peter Anvin),
   - drop kernel_info_offset_update() as an overkill and
     update kernel_info offset directly from main(),
     (suggested by Eric Snowberg),
   - new commit message
     (suggested by H. Peter Anvin),
   - fix some commit message misspellings
     (suggested by Eric Snowberg).
---
 Documentation/x86/boot.rst             | 89 ++++++++++++++++++++++++++++++++++
 arch/x86/boot/Makefile                 |  2 +-
 arch/x86/boot/compressed/Makefile      |  4 +-
 arch/x86/boot/compressed/kernel_info.S | 12 +++++
 arch/x86/boot/header.S                 |  1 +
 arch/x86/boot/tools/build.c            |  5 ++
 arch/x86/include/uapi/asm/bootparam.h  |  1 +
 7 files changed, 111 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..a934a56f0516 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 5.3) 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 +873,77 @@ 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
+  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.
+
+It is recommended to not store large data chunks, e.g. strings, directly in the
+kernel_info struct. Such data should be placed outside of it and pointed from
+the kernel_info structure using offsets from the beginning of the structure,
+the kernel_info.header field.
+
+
+Details of the kernel_info Fields
+=================================
+
+============	========
+Field name:	header
+Offset/size:	0x0000/4
+============	========
+
+  Contains the magic number "InfO" (0x4f666e49).
+
+============	========
+Field name:	size
+Offset/size:	0x0004/4
+============	========
+
+  This field contains the size of the kernel_info including kernel_info.header.
+  It should be used by the boot loader to detect supported fields in the kernel_info.
+
 
 The Image Checksum
 ==================
diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index e2839b5c246c..c30a9b642a86 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -87,7 +87,7 @@ $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE
 
 SETUP_OBJS = $(addprefix $(obj)/,$(setup-y))
 
-sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
+sed-zoffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(startup_32\|startup_64\|efi32_stub_entry\|efi64_stub_entry\|efi_pe_entry\|input_data\|kernel_info\|_end\|_ehead\|_text\|z_.*\)$$/\#define ZO_\2 0x\1/p'
 
 quiet_cmd_zoffset = ZOFFSET $@
       cmd_zoffset = $(NM) $< | sed -n $(sed-zoffset) > $@
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
index 6b84afdd7538..fad3b18e2cc3 100644
--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -72,8 +72,8 @@ $(obj)/../voffset.h: vmlinux FORCE
 
 $(obj)/misc.o: $(obj)/../voffset.h
 
-vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/head_$(BITS).o $(obj)/misc.o \
-	$(obj)/string.o $(obj)/cmdline.o $(obj)/error.o \
+vmlinux-objs-y := $(obj)/vmlinux.lds $(obj)/kernel_info.o $(obj)/head_$(BITS).o \
+	$(obj)/misc.o $(obj)/string.o $(obj)/cmdline.o $(obj)/error.o \
 	$(obj)/piggy.o $(obj)/cpuflags.o
 
 vmlinux-objs-$(CONFIG_EARLY_PRINTK) += $(obj)/early_serial_console.o
diff --git a/arch/x86/boot/compressed/kernel_info.S b/arch/x86/boot/compressed/kernel_info.S
new file mode 100644
index 000000000000..3f1cb301b9ff
--- /dev/null
+++ b/arch/x86/boot/compressed/kernel_info.S
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+	.section ".rodata.kernel_info", "a"
+
+	.global kernel_info
+
+kernel_info:
+        /* Header. */
+	.ascii	"InfO"
+        /* Size. */
+	.long	kernel_info_end - kernel_info
+kernel_info_end:
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index 850b8762e889..ec6a25a43148 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -557,6 +557,7 @@ pref_address:		.quad LOAD_PHYSICAL_ADDR	# preferred load addr
 
 init_size:		.long INIT_SIZE		# kernel initialization size
 handover_offset:	.long 0			# Filled in by build.c
+kernel_info_offset:	.long 0			# Filled in by build.c
 
 # End of setup header #####################################################
 
diff --git a/arch/x86/boot/tools/build.c b/arch/x86/boot/tools/build.c
index a93d44e58f9c..55e669d29e54 100644
--- a/arch/x86/boot/tools/build.c
+++ b/arch/x86/boot/tools/build.c
@@ -56,6 +56,7 @@ u8 buf[SETUP_SECT_MAX*512];
 unsigned long efi32_stub_entry;
 unsigned long efi64_stub_entry;
 unsigned long efi_pe_entry;
+unsigned long kernel_info;
 unsigned long startup_64;
 
 /*----------------------------------------------------------------------*/
@@ -321,6 +322,7 @@ static void parse_zoffset(char *fname)
 		PARSE_ZOFS(p, efi32_stub_entry);
 		PARSE_ZOFS(p, efi64_stub_entry);
 		PARSE_ZOFS(p, efi_pe_entry);
+		PARSE_ZOFS(p, kernel_info);
 		PARSE_ZOFS(p, startup_64);
 
 		p = strchr(p, '\n');
@@ -410,6 +412,9 @@ int main(int argc, char ** argv)
 
 	efi_stub_entry_update();
 
+	/* Update kernel_info offset. */
+	put_unaligned_le32(kernel_info, &buf[0x268]);
+
 	crc = partial_crc32(buf, i, crc);
 	if (fwrite(buf, 1, i, dest) != i)
 		die("Writing setup failed");
diff --git a/arch/x86/include/uapi/asm/bootparam.h b/arch/x86/include/uapi/asm/bootparam.h
index 60733f137e9a..b05318112452 100644
--- a/arch/x86/include/uapi/asm/bootparam.h
+++ b/arch/x86/include/uapi/asm/bootparam.h
@@ -86,6 +86,7 @@ struct setup_header {
 	__u64	pref_address;
 	__u32	init_size;
 	__u32	handover_offset;
+	__u32	kernel_info_offset;
 } __attribute__((packed));
 
 struct sys_desc_table {
-- 
2.11.0

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