x86/boot: Introduce kernel_info

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

=============

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::

  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. The kernel_info structure 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.

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

after kernel_info_var_len_data label. Each chunk of variable size data has to

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  "0123"          /* 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  "ABCD"          /* Header/Magic. */

          .long   example_strings_end - example_strings

          .asciz  "String_0"

          .asciz  "String_1"

  example_strings_end:

  kernel_info_end:

This way the kernel_info is self-contained blob.

.. note::

     Each variable size data header/magic can be any 4-character string,

     without \0 at the end of the string, which does not collide with

     existing variable length data headers/magics.

Details of the kernel_info Fields

=================================

============	========

Field name:	header

Offset/size:	0x0000/4

============	========

  Contains the magic number "LToP" (0x506f544c).

============	========

Field name:	size

Offset/size:	0x0004/4

============	========

  This field contains the size of the kernel_info including kernel_info.header.

  It does not count kernel_info.kernel_info_var_len_data size. This field should be

  used by the bootloaders to detect supported fixed size fields in the kernel_info

  and beginning of kernel_info.kernel_info_var_len_data.

============	========

Field name:	size_total

Offset/size:	0x0008/4

============	========

  This field contains the size of the kernel_info including kernel_info.header

  and kernel_info.kernel_info_var_len_data.

The Image Checksum

==================

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) > $@

$(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

/* SPDX-License-Identifier: GPL-2.0 */

	.section ".rodata.kernel_info", "a"

	.global kernel_info

kernel_info:

	/* Header, Linux top (structure). */

	.ascii	"LToP"

	/* Size. */

	.long	kernel_info_var_len_data - kernel_info

	/* Size total. */

	.long	kernel_info_end - kernel_info

kernel_info_var_len_data:

	/* Empty for time being... */

kernel_info_end:

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 #####################################################