Documentation: x86: convert boot.txt to reST
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>
Cc: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/x86/boot.rst b/Documentation/x86/boot.rst
new file mode 100644
index 0000000..08a2f10
--- /dev/null
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+.. 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.
+
+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
+============= ============================================================
+
+
+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/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::
+
+ 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 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
+("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.
+
+.. 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.
+
+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 Configuartion
+=========================
+
+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::
+
+ - hdr.code32_start
+ - hdr.cmd_line_ptr
+ - hdr.ramdisk_image (if applicable)
+ - hdr.ramdisk_size (if applicable)
+
+All other fields should be zero.