Re: [PATCH v4 38/63] Documentation: x86: convert boot.txt to reST

Mauro Carvalho Chehab
2019-04-24T17:36:44Z

Em Wed, 24 Apr 2019 00:29:07 +0800
Changbin Du <changbin.du@gmail.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@gmail.com>
> ---
> Documentation/x86/boot.rst | 1205 +++++++++++++++++++++++++++++++++++
> Documentation/x86/boot.txt | 1130 --------------------------------
> Documentation/x86/index.rst | 2 +
> 3 files changed, 1207 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..9f55e832bc47
> --- /dev/null
> +++ b/Documentation/x86/boot.rst
> @@ -0,0 +1,1205 @@
> +.. 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.

This is a side node, but you should really try to avoid replacing too
many lines, as it makes a lot harder for reviewers for no good reason.

For example, this is the way I would convert this changelog table:

@@ -10,6 +11,7 @@ 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.

@@ -64,33 +66,35 @@ 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
+=============== ===============================================================

This is simple enough, preserves the original author's intent and
makes a lot easier for reviewers to check what you changed.

> +
> +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 +------------------------+
> +
> +

I might be wrong, but it seems that you broke the above ascii
artwork.

> +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 +------------------------+

Same here: it sounds to me that you mistakenly replaced some tabs
by spaces.

> +
> +... where the address X is as low as the design of the boot loader
> +permits.

That seems to be the legend of the artwork. I would indent it, in
order to be shown inside the artwork.

> +
> +
> +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

This is a table. Please use table markups and fix some wrong indentation
there, as it makes a lot easier to read it on html, e-pub and pdf formats.

E. g. something 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.

Btw, (1), (2) and (3) here sounds to be footnotes. Perhaps you could use
ReST footnote markups, if ok for the 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

Marking this as a literal block sounds plain wrong to me. I suspect that
you could use this syntax instead:

:Field name: setup_sects
:Type: read
:Offset/size: 0x1f1/1
:Protocol: ALL

Or:

Field name: setup_sects
-----------------------

Type:
read
Offset/size:
0x1f1/1
Protocol:
ALL

Or (my favorite):

Field name: setup_sects
-----------------------

:Type: read
:Offset/size: 0x1f1/1
:Protocol: ALL

As it is more compact in text, and will provide a much better
html/pdf output. It will also make (IMHO) a lot easier for
people to read in text and seek for an specific field.

Of course, whatever we do here should be applied to all similar
structs inside this file.

> +
> +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

Clearly there's something wrong with the last 3 lines, as they aren't
following the expected indentation.

Anyway, IMO the best would be to use a table, instead:

== =======================================
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@zytor.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.

You need to either add blank lines or add a "- " before the
two if's above.

> +
> + 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.

Please indent it the same way as Bit 0.

> +
> + 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

This is already a table. Just add the markups for it, instead of using '::'

e. g.:

+ ========== ==============================
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

Again, tables. Just do:

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@zytor.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