[PATCH v2 02/27] Documentation: x86: convert boot.txt to reST

Changbin Du
2019-05-02T07:09:58Z

Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
---
Documentation/x86/{boot.txt => boot.rst} | 527 ++++++++++++++---------
Documentation/x86/index.rst | 2 +
2 files changed, 327 insertions(+), 202 deletions(-)
rename Documentation/x86/{boot.txt => boot.rst} (73%)

diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.rst
similarity index 73%
rename from Documentation/x86/boot.txt
rename to Documentation/x86/boot.rst
index f4c2a97bfdbd..d3730a11424b 100644
--- a/Documentation/x86/boot.txt
+++ b/Documentation/x86/boot.rst
@@ -1,5 +1,8 @@
- THE LINUX/x86 BOOT PROTOCOL
- ---------------------------
+.. 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
@@ -10,84 +13,86 @@ 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
+============= ============================================================
+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
+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 2.01 (Kernel 1.3.76) Added a heap overrun warning.

-Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
+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
+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 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.

-Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
+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
+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.
+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
+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
+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
+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 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
+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
+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 +------------------------+
-
+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,
@@ -112,36 +117,36 @@ zImage or old bzImage kernels, which need data written into the
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
+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
@@ -155,61 +160,63 @@ 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.
+=========== ======== ===================== ============================================
+Offset/Size Proto Name Meaning
+=========== ======== ===================== ============================================
+01F1/1 ALL(1) setup_sects The size of the setup in sectors
+01F2/2 ALL root_flags If set, the root is mounted readonly
+01F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
+01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
+01FA/2 ALL vid_mode Video mode control
+01FC/2 ALL root_dev Default root device number
+01FE/2 ALL boot_flag 0xAA55 magic number
+0200/2 2.00+ jump Jump instruction
+0202/4 2.00+ header Magic signature "HdrS"
+0206/2 2.00+ version Boot protocol version supported
+0208/4 2.00+ realmode_swtch Boot loader hook (see below)
+020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
+020E/2 2.00+ kernel_version Pointer to kernel version string
+0210/1 2.00+ type_of_loader Boot loader identifier
+0211/1 2.00+ loadflags Boot protocol option flags
+0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
+0214/4 2.00+ code32_start Boot loader hook (see below)
+0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
+021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
+0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
+0224/2 2.01+ heap_end_ptr Free memory after setup end
+0226/1 2.02+(3) ext_loader_ver Extended boot loader version
+0227/1 2.02+(3) ext_loader_type Extended boot loader ID
+0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
+022C/4 2.03+ initrd_addr_max Highest legal initrd address
+0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
+0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
+0235/1 2.10+ min_alignment Minimum alignment, as a power of two
+0236/2 2.12+ xloadflags Boot protocol option flags
+0238/4 2.06+ cmdline_size Maximum size of the kernel command line
+023C/4 2.07+ hardware_subarch Hardware subarchitecture
+0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
+0248/4 2.08+ payload_offset Offset of kernel payload
+024C/4 2.08+ payload_length Length of kernel payload
+0250/8 2.09+ setup_data 64-bit physical pointer to linked list
+ of struct setup_data
+0258/8 2.10+ pref_address Preferred loading address
+0260/4 2.10+ init_size Linear memory required during initialization
+0264/4 2.11+ handover_offset Offset of handover entry point
+=========== ======== ===================== ============================================
+
+.. note::
+ (1) For backwards compatibility, if the setup_sects field contains 0, the
+ real value is 4.
+
+ (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
+ field are unusable, which means the size of a bzImage kernel
+ cannot be determined.
+
+ (3) Ignored, but safe to set, for boot protocols 2.02-2.09.

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:
+following parameters should be assumed::

Image type = zImage
initrd not supported
@@ -221,7 +228,8 @@ setting fields in the header, you must make sure only to set fields
supported by the protocol version in use.

-**** DETAILS OF HEADER FIELDS
+Details of Harder Fileds
+========================

For each field, some are information from the kernel to the bootloader
("read"), some are expected to be filled out by the bootloader
@@ -235,106 +243,132 @@ 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
+============ =================
+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
@@ -344,17 +378,21 @@ Protocol: 2.00+
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:
+ 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.
+ 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
@@ -365,17 +403,20 @@ Protocol: 2.00+
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:
+ For example, for T = 0x15, V = 0x234, write::

- type_of_loader <- 0xE4
- ext_loader_type <- 0x05
- ext_loader_ver <- 0x23
+ 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)
+ == ==============================
+ 0 LILO
+ (0x00 reserved for pre-2.00 bootloader)
1 Loadlin
- 2 bootsect-loader (0x20, all other values reserved)
+ 2 bootsect-loader
+ (0x20, all other values reserved)
3 Syslinux
4 Etherboot/gPXE/iPXE
5 ELILO
@@ -386,55 +427,70 @@ Protocol: 2.00+
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
+ 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.
+ 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.
+
+ - 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
@@ -443,14 +499,16 @@ Protocol: 2.00-2.01
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
@@ -458,47 +516,57 @@ Protocol: 2.00+

This field can be modified for two purposes:

- 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
+ 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.
+ 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
@@ -510,10 +578,12 @@ Protocol: 2.02+
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
@@ -524,10 +594,12 @@ Protocol: 2.02+
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
@@ -540,10 +612,12 @@ Protocol: 2.02+
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
@@ -552,10 +626,12 @@ Protocol: 2.03+
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
@@ -567,25 +643,29 @@ Protocol: 2.05+ (read), 2.10+ (modify)
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 field with the alignment unit desired; typically::

kernel_alignment = 1 << min_alignment

@@ -593,44 +673,56 @@ Protocol: 2.10+
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+
+============ ==========
+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
@@ -639,25 +731,31 @@ Protocol: 2.07+
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.
@@ -670,29 +768,33 @@ Protocol: 2.08+
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:
+ as follow::

- struct setup_data {
- u64 next;
- u32 type;
- u32 len;
- u8 data[0];
- };
+ 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
@@ -704,10 +806,12 @@ Protocol: 2.09+
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
@@ -716,9 +820,11 @@ Protocol: 2.10+
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
@@ -727,16 +833,18 @@ Offset/size: 0x260/4
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:
+ The kernel runtime start address is determined by the following algorithm::

- if (relocatable_kernel)
+ if (relocatable_kernel)
runtime_start = align_up(load_address, kernel_alignment)
- else
+ 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
@@ -745,7 +853,8 @@ Offset/size: 0x264/4
See EFI HANDOVER PROTOCOL below for more details.

-**** THE IMAGE CHECKSUM
+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
@@ -754,7 +863,8 @@ 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
+=======================

The kernel command line has become an important way for the boot
loader to communicate with the kernel. Some of its options are also
@@ -774,19 +884,20 @@ 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 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).

- 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.
+ - 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
+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
@@ -802,10 +913,11 @@ 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.
+.. note::
+ 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.

@@ -818,24 +930,29 @@ The kernel command line should not be located below the real-mode
code, nor should it be located in high memory.

-**** SAMPLE BOOT CONFIGURATION
+Sample Boot Configuartion
+=========================

As a sample configuration, assume the following layout of the real
-mode segment:
+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:
+Such a boot loader should enter the following fields in the header::

unsigned long base_ptr; /* base address for real-mode segment */

@@ -894,7 +1011,8 @@ Such a boot loader should enter the following fields in the header:
}

-**** LOADING THE REST OF THE KERNEL
+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.)
@@ -902,7 +1020,7 @@ 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:
+bit (LOAD_HIGH) in the loadflags field is set::

is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
load_address = is_bzImage ? 0x100000 : 0x10000;
@@ -912,8 +1030,8 @@ 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
+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.
@@ -962,7 +1080,8 @@ or configuration-specified command line. Otherwise, "init=/bin/sh"
gets confused by the "auto" option.

-**** RUNNING THE KERNEL
+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
@@ -976,7 +1095,7 @@ 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:
+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 */
@@ -999,7 +1118,8 @@ switched off, especially if the loaded kernel has the floppy driver as
a demand-loaded module!

-**** ADVANCED BOOT LOADER HOOKS
+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
@@ -1028,7 +1148,8 @@ IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
(relocated, if appropriate.)

-**** 32-bit BOOT PROTOCOL
+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
@@ -1041,7 +1162,7 @@ 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:
+follow::

0x0202 + byte value at offset 0x0201

@@ -1065,7 +1186,8 @@ 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
+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.
@@ -1076,7 +1198,7 @@ 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:
+can be calculated as follows::

0x0202 + byte value at offset 0x0201

@@ -1103,7 +1225,8 @@ 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
+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)
@@ -1111,7 +1234,7 @@ 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,
+The function prototype for the handover entry point looks like this::

efi_main(void *handle, efi_system_table_t *table, struct boot_params *bp)

@@ -1120,11 +1243,11 @@ 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,
+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)
+ - 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/index.rst b/Documentation/x86/index.rst
index 9f34545a9c52..d7fc8efac192 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -7,3 +7,5 @@ x86-specific Documentation
.. toctree::
:maxdepth: 2
:numbered:
+
+ boot
--
2.20.1