xref: /openbmc/linux/Documentation/arch/x86/boot.rst (revision d56b699d)
1.. SPDX-License-Identifier: GPL-2.0
2
3===========================
4The Linux/x86 Boot Protocol
5===========================
6
7On the x86 platform, the Linux kernel uses a rather complicated boot
8convention.  This has evolved partially due to historical aspects, as
9well as the desire in the early days to have the kernel itself be a
10bootable image, the complicated PC memory model and due to changed
11expectations in the PC industry caused by the effective demise of
12real-mode DOS as a mainstream operating system.
13
14Currently, the following versions of the Linux/x86 boot protocol exist.
15
16=============	============================================================
17Old kernels	zImage/Image support only.  Some very early kernels
18		may not even support a command line.
19
20Protocol 2.00	(Kernel 1.3.73) Added bzImage and initrd support, as
21		well as a formalized way to communicate between the
22		boot loader and the kernel.  setup.S made relocatable,
23		although the traditional setup area still assumed
24		writable.
25
26Protocol 2.01	(Kernel 1.3.76) Added a heap overrun warning.
27
28Protocol 2.02	(Kernel 2.4.0-test3-pre3) New command line protocol.
29		Lower the conventional memory ceiling.	No overwrite
30		of the traditional setup area, thus making booting
31		safe for systems which use the EBDA from SMM or 32-bit
32		BIOS entry points.  zImage deprecated but still
33		supported.
34
35Protocol 2.03	(Kernel 2.4.18-pre1) Explicitly makes the highest possible
36		initrd address available to the bootloader.
37
38Protocol 2.04	(Kernel 2.6.14) Extend the syssize field to four bytes.
39
40Protocol 2.05	(Kernel 2.6.20) Make protected mode kernel relocatable.
41		Introduce relocatable_kernel and kernel_alignment fields.
42
43Protocol 2.06	(Kernel 2.6.22) Added a field that contains the size of
44		the boot command line.
45
46Protocol 2.07	(Kernel 2.6.24) Added paravirtualised boot protocol.
47		Introduced hardware_subarch and hardware_subarch_data
48		and KEEP_SEGMENTS flag in load_flags.
49
50Protocol 2.08	(Kernel 2.6.26) Added crc32 checksum and ELF format
51		payload. Introduced payload_offset and payload_length
52		fields to aid in locating the payload.
53
54Protocol 2.09	(Kernel 2.6.26) Added a field of 64-bit physical
55		pointer to single linked list of struct	setup_data.
56
57Protocol 2.10	(Kernel 2.6.31) Added a protocol for relaxed alignment
58		beyond the kernel_alignment added, new init_size and
59		pref_address fields.  Added extended boot loader IDs.
60
61Protocol 2.11	(Kernel 3.6) Added a field for offset of EFI handover
62		protocol entry point.
63
64Protocol 2.12	(Kernel 3.8) Added the xloadflags field and extension fields
65		to struct boot_params for loading bzImage and ramdisk
66		above 4G in 64bit.
67
68Protocol 2.13	(Kernel 3.14) Support 32- and 64-bit flags being set in
69		xloadflags to support booting a 64-bit kernel from 32-bit
70		EFI
71
72Protocol 2.14	BURNT BY INCORRECT COMMIT
73                ae7e1238e68f2a472a125673ab506d49158c1889
74		(x86/boot: Add ACPI RSDP address to setup_header)
75		DO NOT USE!!! ASSUME SAME AS 2.13.
76
77Protocol 2.15	(Kernel 5.5) Added the kernel_info and kernel_info.setup_type_max.
78=============	============================================================
79
80.. note::
81     The protocol version number should be changed only if the setup header
82     is changed. There is no need to update the version number if boot_params
83     or kernel_info are changed. Additionally, it is recommended to use
84     xloadflags (in this case the protocol version number should not be
85     updated either) or kernel_info to communicate supported Linux kernel
86     features to the boot loader. Due to very limited space available in
87     the original setup header every update to it should be considered
88     with great care. Starting from the protocol 2.15 the primary way to
89     communicate things to the boot loader is the kernel_info.
90
91
92Memory Layout
93=============
94
95The traditional memory map for the kernel loader, used for Image or
96zImage kernels, typically looks like::
97
98		|			 |
99	0A0000	+------------------------+
100		|  Reserved for BIOS	 |	Do not use.  Reserved for BIOS EBDA.
101	09A000	+------------------------+
102		|  Command line		 |
103		|  Stack/heap		 |	For use by the kernel real-mode code.
104	098000	+------------------------+
105		|  Kernel setup		 |	The kernel real-mode code.
106	090200	+------------------------+
107		|  Kernel boot sector	 |	The kernel legacy boot sector.
108	090000	+------------------------+
109		|  Protected-mode kernel |	The bulk of the kernel image.
110	010000	+------------------------+
111		|  Boot loader		 |	<- Boot sector entry point 0000:7C00
112	001000	+------------------------+
113		|  Reserved for MBR/BIOS |
114	000800	+------------------------+
115		|  Typically used by MBR |
116	000600	+------------------------+
117		|  BIOS use only	 |
118	000000	+------------------------+
119
120When using bzImage, the protected-mode kernel was relocated to
1210x100000 ("high memory"), and the kernel real-mode block (boot sector,
122setup, and stack/heap) was made relocatable to any address between
1230x10000 and end of low memory. Unfortunately, in protocols 2.00 and
1242.01 the 0x90000+ memory range is still used internally by the kernel;
125the 2.02 protocol resolves that problem.
126
127It is desirable to keep the "memory ceiling" -- the highest point in
128low memory touched by the boot loader -- as low as possible, since
129some newer BIOSes have begun to allocate some rather large amounts of
130memory, called the Extended BIOS Data Area, near the top of low
131memory.	 The boot loader should use the "INT 12h" BIOS call to verify
132how much low memory is available.
133
134Unfortunately, if INT 12h reports that the amount of memory is too
135low, there is usually nothing the boot loader can do but to report an
136error to the user.  The boot loader should therefore be designed to
137take up as little space in low memory as it reasonably can.  For
138zImage or old bzImage kernels, which need data written into the
1390x90000 segment, the boot loader should make sure not to use memory
140above the 0x9A000 point; too many BIOSes will break above that point.
141
142For a modern bzImage kernel with boot protocol version >= 2.02, a
143memory layout like the following is suggested::
144
145		~                        ~
146		|  Protected-mode kernel |
147	100000  +------------------------+
148		|  I/O memory hole	 |
149	0A0000	+------------------------+
150		|  Reserved for BIOS	 |	Leave as much as possible unused
151		~                        ~
152		|  Command line		 |	(Can also be below the X+10000 mark)
153	X+10000	+------------------------+
154		|  Stack/heap		 |	For use by the kernel real-mode code.
155	X+08000	+------------------------+
156		|  Kernel setup		 |	The kernel real-mode code.
157		|  Kernel boot sector	 |	The kernel legacy boot sector.
158	X       +------------------------+
159		|  Boot loader		 |	<- Boot sector entry point 0000:7C00
160	001000	+------------------------+
161		|  Reserved for MBR/BIOS |
162	000800	+------------------------+
163		|  Typically used by MBR |
164	000600	+------------------------+
165		|  BIOS use only	 |
166	000000	+------------------------+
167
168  ... where the address X is as low as the design of the boot loader permits.
169
170
171The Real-Mode Kernel Header
172===========================
173
174In the following text, and anywhere in the kernel boot sequence, "a
175sector" refers to 512 bytes.  It is independent of the actual sector
176size of the underlying medium.
177
178The first step in loading a Linux kernel should be to load the
179real-mode code (boot sector and setup code) and then examine the
180following header at offset 0x01f1.  The real-mode code can total up to
18132K, although the boot loader may choose to load only the first two
182sectors (1K) and then examine the bootup sector size.
183
184The header looks like:
185
186===========	========	=====================	============================================
187Offset/Size	Proto		Name			Meaning
188===========	========	=====================	============================================
18901F1/1		ALL(1)		setup_sects		The size of the setup in sectors
19001F2/2		ALL		root_flags		If set, the root is mounted readonly
19101F4/4		2.04+(2)	syssize			The size of the 32-bit code in 16-byte paras
19201F8/2		ALL		ram_size		DO NOT USE - for bootsect.S use only
19301FA/2		ALL		vid_mode		Video mode control
19401FC/2		ALL		root_dev		Default root device number
19501FE/2		ALL		boot_flag		0xAA55 magic number
1960200/2		2.00+		jump			Jump instruction
1970202/4		2.00+		header			Magic signature "HdrS"
1980206/2		2.00+		version			Boot protocol version supported
1990208/4		2.00+		realmode_swtch		Boot loader hook (see below)
200020C/2		2.00+		start_sys_seg		The load-low segment (0x1000) (obsolete)
201020E/2		2.00+		kernel_version		Pointer to kernel version string
2020210/1		2.00+		type_of_loader		Boot loader identifier
2030211/1		2.00+		loadflags		Boot protocol option flags
2040212/2		2.00+		setup_move_size		Move to high memory size (used with hooks)
2050214/4		2.00+		code32_start		Boot loader hook (see below)
2060218/4		2.00+		ramdisk_image		initrd load address (set by boot loader)
207021C/4		2.00+		ramdisk_size		initrd size (set by boot loader)
2080220/4		2.00+		bootsect_kludge		DO NOT USE - for bootsect.S use only
2090224/2		2.01+		heap_end_ptr		Free memory after setup end
2100226/1		2.02+(3)	ext_loader_ver		Extended boot loader version
2110227/1		2.02+(3)	ext_loader_type		Extended boot loader ID
2120228/4		2.02+		cmd_line_ptr		32-bit pointer to the kernel command line
213022C/4		2.03+		initrd_addr_max		Highest legal initrd address
2140230/4		2.05+		kernel_alignment	Physical addr alignment required for kernel
2150234/1		2.05+		relocatable_kernel	Whether kernel is relocatable or not
2160235/1		2.10+		min_alignment		Minimum alignment, as a power of two
2170236/2		2.12+		xloadflags		Boot protocol option flags
2180238/4		2.06+		cmdline_size		Maximum size of the kernel command line
219023C/4		2.07+		hardware_subarch	Hardware subarchitecture
2200240/8		2.07+		hardware_subarch_data	Subarchitecture-specific data
2210248/4		2.08+		payload_offset		Offset of kernel payload
222024C/4		2.08+		payload_length		Length of kernel payload
2230250/8		2.09+		setup_data		64-bit physical pointer to linked list
224							of struct setup_data
2250258/8		2.10+		pref_address		Preferred loading address
2260260/4		2.10+		init_size		Linear memory required during initialization
2270264/4		2.11+		handover_offset		Offset of handover entry point
2280268/4		2.15+		kernel_info_offset	Offset of the kernel_info
229===========	========	=====================	============================================
230
231.. note::
232  (1) For backwards compatibility, if the setup_sects field contains 0, the
233      real value is 4.
234
235  (2) For boot protocol prior to 2.04, the upper two bytes of the syssize
236      field are unusable, which means the size of a bzImage kernel
237      cannot be determined.
238
239  (3) Ignored, but safe to set, for boot protocols 2.02-2.09.
240
241If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
242the boot protocol version is "old".  Loading an old kernel, the
243following parameters should be assumed::
244
245	Image type = zImage
246	initrd not supported
247	Real-mode kernel must be located at 0x90000.
248
249Otherwise, the "version" field contains the protocol version,
250e.g. protocol version 2.01 will contain 0x0201 in this field.  When
251setting fields in the header, you must make sure only to set fields
252supported by the protocol version in use.
253
254
255Details of Header Fields
256========================
257
258For each field, some are information from the kernel to the bootloader
259("read"), some are expected to be filled out by the bootloader
260("write"), and some are expected to be read and modified by the
261bootloader ("modify").
262
263All general purpose boot loaders should write the fields marked
264(obligatory).  Boot loaders who want to load the kernel at a
265nonstandard address should fill in the fields marked (reloc); other
266boot loaders can ignore those fields.
267
268The byte order of all fields is littleendian (this is x86, after all.)
269
270============	===========
271Field name:	setup_sects
272Type:		read
273Offset/size:	0x1f1/1
274Protocol:	ALL
275============	===========
276
277  The size of the setup code in 512-byte sectors.  If this field is
278  0, the real value is 4.  The real-mode code consists of the boot
279  sector (always one 512-byte sector) plus the setup code.
280
281============	=================
282Field name:	root_flags
283Type:		modify (optional)
284Offset/size:	0x1f2/2
285Protocol:	ALL
286============	=================
287
288  If this field is nonzero, the root defaults to readonly.  The use of
289  this field is deprecated; use the "ro" or "rw" options on the
290  command line instead.
291
292============	===============================================
293Field name:	syssize
294Type:		read
295Offset/size:	0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
296Protocol:	2.04+
297============	===============================================
298
299  The size of the protected-mode code in units of 16-byte paragraphs.
300  For protocol versions older than 2.04 this field is only two bytes
301  wide, and therefore cannot be trusted for the size of a kernel if
302  the LOAD_HIGH flag is set.
303
304============	===============
305Field name:	ram_size
306Type:		kernel internal
307Offset/size:	0x1f8/2
308Protocol:	ALL
309============	===============
310
311  This field is obsolete.
312
313============	===================
314Field name:	vid_mode
315Type:		modify (obligatory)
316Offset/size:	0x1fa/2
317============	===================
318
319  Please see the section on SPECIAL COMMAND LINE OPTIONS.
320
321============	=================
322Field name:	root_dev
323Type:		modify (optional)
324Offset/size:	0x1fc/2
325Protocol:	ALL
326============	=================
327
328  The default root device device number.  The use of this field is
329  deprecated, use the "root=" option on the command line instead.
330
331============	=========
332Field name:	boot_flag
333Type:		read
334Offset/size:	0x1fe/2
335Protocol:	ALL
336============	=========
337
338  Contains 0xAA55.  This is the closest thing old Linux kernels have
339  to a magic number.
340
341============	=======
342Field name:	jump
343Type:		read
344Offset/size:	0x200/2
345Protocol:	2.00+
346============	=======
347
348  Contains an x86 jump instruction, 0xEB followed by a signed offset
349  relative to byte 0x202.  This can be used to determine the size of
350  the header.
351
352============	=======
353Field name:	header
354Type:		read
355Offset/size:	0x202/4
356Protocol:	2.00+
357============	=======
358
359  Contains the magic number "HdrS" (0x53726448).
360
361============	=======
362Field name:	version
363Type:		read
364Offset/size:	0x206/2
365Protocol:	2.00+
366============	=======
367
368  Contains the boot protocol version, in (major << 8)+minor format,
369  e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
370  10.17.
371
372============	=================
373Field name:	realmode_swtch
374Type:		modify (optional)
375Offset/size:	0x208/4
376Protocol:	2.00+
377============	=================
378
379  Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
380
381============	=============
382Field name:	start_sys_seg
383Type:		read
384Offset/size:	0x20c/2
385Protocol:	2.00+
386============	=============
387
388  The load low segment (0x1000).  Obsolete.
389
390============	==============
391Field name:	kernel_version
392Type:		read
393Offset/size:	0x20e/2
394Protocol:	2.00+
395============	==============
396
397  If set to a nonzero value, contains a pointer to a NUL-terminated
398  human-readable kernel version number string, less 0x200.  This can
399  be used to display the kernel version to the user.  This value
400  should be less than (0x200*setup_sects).
401
402  For example, if this value is set to 0x1c00, the kernel version
403  number string can be found at offset 0x1e00 in the kernel file.
404  This is a valid value if and only if the "setup_sects" field
405  contains the value 15 or higher, as::
406
407	0x1c00  < 15*0x200 (= 0x1e00) but
408	0x1c00 >= 14*0x200 (= 0x1c00)
409
410	0x1c00 >> 9 = 14, So the minimum value for setup_secs is 15.
411
412============	==================
413Field name:	type_of_loader
414Type:		write (obligatory)
415Offset/size:	0x210/1
416Protocol:	2.00+
417============	==================
418
419  If your boot loader has an assigned id (see table below), enter
420  0xTV here, where T is an identifier for the boot loader and V is
421  a version number.  Otherwise, enter 0xFF here.
422
423  For boot loader IDs above T = 0xD, write T = 0xE to this field and
424  write the extended ID minus 0x10 to the ext_loader_type field.
425  Similarly, the ext_loader_ver field can be used to provide more than
426  four bits for the bootloader version.
427
428  For example, for T = 0x15, V = 0x234, write::
429
430	type_of_loader  <- 0xE4
431	ext_loader_type <- 0x05
432	ext_loader_ver  <- 0x23
433
434  Assigned boot loader ids (hexadecimal):
435
436	== =======================================
437	0  LILO
438	   (0x00 reserved for pre-2.00 bootloader)
439	1  Loadlin
440	2  bootsect-loader
441	   (0x20, all other values reserved)
442	3  Syslinux
443	4  Etherboot/gPXE/iPXE
444	5  ELILO
445	7  GRUB
446	8  U-Boot
447	9  Xen
448	A  Gujin
449	B  Qemu
450	C  Arcturus Networks uCbootloader
451	D  kexec-tools
452	E  Extended (see ext_loader_type)
453	F  Special (0xFF = undefined)
454	10 Reserved
455	11 Minimal Linux Bootloader
456	   <http://sebastian-plotz.blogspot.de>
457	12 OVMF UEFI virtualization stack
458	13 barebox
459	== =======================================
460
461  Please contact <hpa@zytor.com> if you need a bootloader ID value assigned.
462
463============	===================
464Field name:	loadflags
465Type:		modify (obligatory)
466Offset/size:	0x211/1
467Protocol:	2.00+
468============	===================
469
470  This field is a bitmask.
471
472  Bit 0 (read):	LOADED_HIGH
473
474	- If 0, the protected-mode code is loaded at 0x10000.
475	- If 1, the protected-mode code is loaded at 0x100000.
476
477  Bit 1 (kernel internal): KASLR_FLAG
478
479	- Used internally by the compressed kernel to communicate
480	  KASLR status to kernel proper.
481
482	    - If 1, KASLR enabled.
483	    - If 0, KASLR disabled.
484
485  Bit 5 (write): QUIET_FLAG
486
487	- If 0, print early messages.
488	- If 1, suppress early messages.
489
490		This requests to the kernel (decompressor and early
491		kernel) to not write early messages that require
492		accessing the display hardware directly.
493
494  Bit 6 (obsolete): KEEP_SEGMENTS
495
496	Protocol: 2.07+
497
498        - This flag is obsolete.
499
500  Bit 7 (write): CAN_USE_HEAP
501
502	Set this bit to 1 to indicate that the value entered in the
503	heap_end_ptr is valid.  If this field is clear, some setup code
504	functionality will be disabled.
505
506
507============	===================
508Field name:	setup_move_size
509Type:		modify (obligatory)
510Offset/size:	0x212/2
511Protocol:	2.00-2.01
512============	===================
513
514  When using protocol 2.00 or 2.01, if the real mode kernel is not
515  loaded at 0x90000, it gets moved there later in the loading
516  sequence.  Fill in this field if you want additional data (such as
517  the kernel command line) moved in addition to the real-mode kernel
518  itself.
519
520  The unit is bytes starting with the beginning of the boot sector.
521
522  This field is can be ignored when the protocol is 2.02 or higher, or
523  if the real-mode code is loaded at 0x90000.
524
525============	========================
526Field name:	code32_start
527Type:		modify (optional, reloc)
528Offset/size:	0x214/4
529Protocol:	2.00+
530============	========================
531
532  The address to jump to in protected mode.  This defaults to the load
533  address of the kernel, and can be used by the boot loader to
534  determine the proper load address.
535
536  This field can be modified for two purposes:
537
538    1. as a boot loader hook (see Advanced Boot Loader Hooks below.)
539
540    2. if a bootloader which does not install a hook loads a
541       relocatable kernel at a nonstandard address it will have to modify
542       this field to point to the load address.
543
544============	==================
545Field name:	ramdisk_image
546Type:		write (obligatory)
547Offset/size:	0x218/4
548Protocol:	2.00+
549============	==================
550
551  The 32-bit linear address of the initial ramdisk or ramfs.  Leave at
552  zero if there is no initial ramdisk/ramfs.
553
554============	==================
555Field name:	ramdisk_size
556Type:		write (obligatory)
557Offset/size:	0x21c/4
558Protocol:	2.00+
559============	==================
560
561  Size of the initial ramdisk or ramfs.  Leave at zero if there is no
562  initial ramdisk/ramfs.
563
564============	===============
565Field name:	bootsect_kludge
566Type:		kernel internal
567Offset/size:	0x220/4
568Protocol:	2.00+
569============	===============
570
571  This field is obsolete.
572
573============	==================
574Field name:	heap_end_ptr
575Type:		write (obligatory)
576Offset/size:	0x224/2
577Protocol:	2.01+
578============	==================
579
580  Set this field to the offset (from the beginning of the real-mode
581  code) of the end of the setup stack/heap, minus 0x0200.
582
583============	================
584Field name:	ext_loader_ver
585Type:		write (optional)
586Offset/size:	0x226/1
587Protocol:	2.02+
588============	================
589
590  This field is used as an extension of the version number in the
591  type_of_loader field.  The total version number is considered to be
592  (type_of_loader & 0x0f) + (ext_loader_ver << 4).
593
594  The use of this field is boot loader specific.  If not written, it
595  is zero.
596
597  Kernels prior to 2.6.31 did not recognize this field, but it is safe
598  to write for protocol version 2.02 or higher.
599
600============	=====================================================
601Field name:	ext_loader_type
602Type:		write (obligatory if (type_of_loader & 0xf0) == 0xe0)
603Offset/size:	0x227/1
604Protocol:	2.02+
605============	=====================================================
606
607  This field is used as an extension of the type number in
608  type_of_loader field.  If the type in type_of_loader is 0xE, then
609  the actual type is (ext_loader_type + 0x10).
610
611  This field is ignored if the type in type_of_loader is not 0xE.
612
613  Kernels prior to 2.6.31 did not recognize this field, but it is safe
614  to write for protocol version 2.02 or higher.
615
616============	==================
617Field name:	cmd_line_ptr
618Type:		write (obligatory)
619Offset/size:	0x228/4
620Protocol:	2.02+
621============	==================
622
623  Set this field to the linear address of the kernel command line.
624  The kernel command line can be located anywhere between the end of
625  the setup heap and 0xA0000; it does not have to be located in the
626  same 64K segment as the real-mode code itself.
627
628  Fill in this field even if your boot loader does not support a
629  command line, in which case you can point this to an empty string
630  (or better yet, to the string "auto".)  If this field is left at
631  zero, the kernel will assume that your boot loader does not support
632  the 2.02+ protocol.
633
634============	===============
635Field name:	initrd_addr_max
636Type:		read
637Offset/size:	0x22c/4
638Protocol:	2.03+
639============	===============
640
641  The maximum address that may be occupied by the initial
642  ramdisk/ramfs contents.  For boot protocols 2.02 or earlier, this
643  field is not present, and the maximum address is 0x37FFFFFF.  (This
644  address is defined as the address of the highest safe byte, so if
645  your ramdisk is exactly 131072 bytes long and this field is
646  0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
647
648============	============================
649Field name:	kernel_alignment
650Type:		read/modify (reloc)
651Offset/size:	0x230/4
652Protocol:	2.05+ (read), 2.10+ (modify)
653============	============================
654
655  Alignment unit required by the kernel (if relocatable_kernel is
656  true.)  A relocatable kernel that is loaded at an alignment
657  incompatible with the value in this field will be realigned during
658  kernel initialization.
659
660  Starting with protocol version 2.10, this reflects the kernel
661  alignment preferred for optimal performance; it is possible for the
662  loader to modify this field to permit a lesser alignment.  See the
663  min_alignment and pref_address field below.
664
665============	==================
666Field name:	relocatable_kernel
667Type:		read (reloc)
668Offset/size:	0x234/1
669Protocol:	2.05+
670============	==================
671
672  If this field is nonzero, the protected-mode part of the kernel can
673  be loaded at any address that satisfies the kernel_alignment field.
674  After loading, the boot loader must set the code32_start field to
675  point to the loaded code, or to a boot loader hook.
676
677============	=============
678Field name:	min_alignment
679Type:		read (reloc)
680Offset/size:	0x235/1
681Protocol:	2.10+
682============	=============
683
684  This field, if nonzero, indicates as a power of two the minimum
685  alignment required, as opposed to preferred, by the kernel to boot.
686  If a boot loader makes use of this field, it should update the
687  kernel_alignment field with the alignment unit desired; typically::
688
689	kernel_alignment = 1 << min_alignment
690
691  There may be a considerable performance cost with an excessively
692  misaligned kernel.  Therefore, a loader should typically try each
693  power-of-two alignment from kernel_alignment down to this alignment.
694
695============	==========
696Field name:	xloadflags
697Type:		read
698Offset/size:	0x236/2
699Protocol:	2.12+
700============	==========
701
702  This field is a bitmask.
703
704  Bit 0 (read):	XLF_KERNEL_64
705
706	- If 1, this kernel has the legacy 64-bit entry point at 0x200.
707
708  Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
709
710        - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
711
712  Bit 2 (read):	XLF_EFI_HANDOVER_32
713
714	- If 1, the kernel supports the 32-bit EFI handoff entry point
715          given at handover_offset.
716
717  Bit 3 (read): XLF_EFI_HANDOVER_64
718
719	- If 1, the kernel supports the 64-bit EFI handoff entry point
720          given at handover_offset + 0x200.
721
722  Bit 4 (read): XLF_EFI_KEXEC
723
724	- If 1, the kernel supports kexec EFI boot with EFI runtime support.
725
726
727============	============
728Field name:	cmdline_size
729Type:		read
730Offset/size:	0x238/4
731Protocol:	2.06+
732============	============
733
734  The maximum size of the command line without the terminating
735  zero. This means that the command line can contain at most
736  cmdline_size characters. With protocol version 2.05 and earlier, the
737  maximum size was 255.
738
739============	====================================
740Field name:	hardware_subarch
741Type:		write (optional, defaults to x86/PC)
742Offset/size:	0x23c/4
743Protocol:	2.07+
744============	====================================
745
746  In a paravirtualized environment the hardware low level architectural
747  pieces such as interrupt handling, page table handling, and
748  accessing process control registers needs to be done differently.
749
750  This field allows the bootloader to inform the kernel we are in one
751  one of those environments.
752
753  ==========	==============================
754  0x00000000	The default x86/PC environment
755  0x00000001	lguest
756  0x00000002	Xen
757  0x00000003	Moorestown MID
758  0x00000004	CE4100 TV Platform
759  ==========	==============================
760
761============	=========================
762Field name:	hardware_subarch_data
763Type:		write (subarch-dependent)
764Offset/size:	0x240/8
765Protocol:	2.07+
766============	=========================
767
768  A pointer to data that is specific to hardware subarch
769  This field is currently unused for the default x86/PC environment,
770  do not modify.
771
772============	==============
773Field name:	payload_offset
774Type:		read
775Offset/size:	0x248/4
776Protocol:	2.08+
777============	==============
778
779  If non-zero then this field contains the offset from the beginning
780  of the protected-mode code to the payload.
781
782  The payload may be compressed. The format of both the compressed and
783  uncompressed data should be determined using the standard magic
784  numbers.  The currently supported compression formats are gzip
785  (magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
786  (magic number 5D 00), XZ (magic number FD 37), LZ4 (magic number
787  02 21) and ZSTD (magic number 28 B5). The uncompressed payload is
788  currently always ELF (magic number 7F 45 4C 46).
789
790============	==============
791Field name:	payload_length
792Type:		read
793Offset/size:	0x24c/4
794Protocol:	2.08+
795============	==============
796
797  The length of the payload.
798
799============	===============
800Field name:	setup_data
801Type:		write (special)
802Offset/size:	0x250/8
803Protocol:	2.09+
804============	===============
805
806  The 64-bit physical pointer to NULL terminated single linked list of
807  struct setup_data. This is used to define a more extensible boot
808  parameters passing mechanism. The definition of struct setup_data is
809  as follow::
810
811	struct setup_data {
812		u64 next;
813		u32 type;
814		u32 len;
815		u8  data[0];
816	};
817
818  Where, the next is a 64-bit physical pointer to the next node of
819  linked list, the next field of the last node is 0; the type is used
820  to identify the contents of data; the len is the length of data
821  field; the data holds the real payload.
822
823  This list may be modified at a number of points during the bootup
824  process.  Therefore, when modifying this list one should always make
825  sure to consider the case where the linked list already contains
826  entries.
827
828  The setup_data is a bit awkward to use for extremely large data objects,
829  both because the setup_data header has to be adjacent to the data object
830  and because it has a 32-bit length field. However, it is important that
831  intermediate stages of the boot process have a way to identify which
832  chunks of memory are occupied by kernel data.
833
834  Thus setup_indirect struct and SETUP_INDIRECT type were introduced in
835  protocol 2.15::
836
837    struct setup_indirect {
838      __u32 type;
839      __u32 reserved;  /* Reserved, must be set to zero. */
840      __u64 len;
841      __u64 addr;
842    };
843
844  The type member is a SETUP_INDIRECT | SETUP_* type. However, it cannot be
845  SETUP_INDIRECT itself since making the setup_indirect a tree structure
846  could require a lot of stack space in something that needs to parse it
847  and stack space can be limited in boot contexts.
848
849  Let's give an example how to point to SETUP_E820_EXT data using setup_indirect.
850  In this case setup_data and setup_indirect will look like this::
851
852    struct setup_data {
853      __u64 next = 0 or <addr_of_next_setup_data_struct>;
854      __u32 type = SETUP_INDIRECT;
855      __u32 len = sizeof(setup_indirect);
856      __u8 data[sizeof(setup_indirect)] = struct setup_indirect {
857        __u32 type = SETUP_INDIRECT | SETUP_E820_EXT;
858        __u32 reserved = 0;
859        __u64 len = <len_of_SETUP_E820_EXT_data>;
860        __u64 addr = <addr_of_SETUP_E820_EXT_data>;
861      }
862    }
863
864.. note::
865     SETUP_INDIRECT | SETUP_NONE objects cannot be properly distinguished
866     from SETUP_INDIRECT itself. So, this kind of objects cannot be provided
867     by the bootloaders.
868
869============	============
870Field name:	pref_address
871Type:		read (reloc)
872Offset/size:	0x258/8
873Protocol:	2.10+
874============	============
875
876  This field, if nonzero, represents a preferred load address for the
877  kernel.  A relocating bootloader should attempt to load at this
878  address if possible.
879
880  A non-relocatable kernel will unconditionally move itself and to run
881  at this address.
882
883============	=======
884Field name:	init_size
885Type:		read
886Offset/size:	0x260/4
887============	=======
888
889  This field indicates the amount of linear contiguous memory starting
890  at the kernel runtime start address that the kernel needs before it
891  is capable of examining its memory map.  This is not the same thing
892  as the total amount of memory the kernel needs to boot, but it can
893  be used by a relocating boot loader to help select a safe load
894  address for the kernel.
895
896  The kernel runtime start address is determined by the following algorithm::
897
898	if (relocatable_kernel)
899	runtime_start = align_up(load_address, kernel_alignment)
900	else
901	runtime_start = pref_address
902
903============	===============
904Field name:	handover_offset
905Type:		read
906Offset/size:	0x264/4
907============	===============
908
909  This field is the offset from the beginning of the kernel image to
910  the EFI handover protocol entry point. Boot loaders using the EFI
911  handover protocol to boot the kernel should jump to this offset.
912
913  See EFI HANDOVER PROTOCOL below for more details.
914
915============	==================
916Field name:	kernel_info_offset
917Type:		read
918Offset/size:	0x268/4
919Protocol:	2.15+
920============	==================
921
922  This field is the offset from the beginning of the kernel image to the
923  kernel_info. The kernel_info structure is embedded in the Linux image
924  in the uncompressed protected mode region.
925
926
927The kernel_info
928===============
929
930The relationships between the headers are analogous to the various data
931sections:
932
933  setup_header = .data
934  boot_params/setup_data = .bss
935
936What is missing from the above list? That's right:
937
938  kernel_info = .rodata
939
940We have been (ab)using .data for things that could go into .rodata or .bss for
941a long time, for lack of alternatives and -- especially early on -- inertia.
942Also, the BIOS stub is responsible for creating boot_params, so it isn't
943available to a BIOS-based loader (setup_data is, though).
944
945setup_header is permanently limited to 144 bytes due to the reach of the
9462-byte jump field, which doubles as a length field for the structure, combined
947with the size of the "hole" in struct boot_params that a protected-mode loader
948or the BIOS stub has to copy it into. It is currently 119 bytes long, which
949leaves us with 25 very precious bytes. This isn't something that can be fixed
950without revising the boot protocol entirely, breaking backwards compatibility.
951
952boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
953by adding setup_data entries. It cannot be used to communicate properties of
954the kernel image, because it is .bss and has no image-provided content.
955
956kernel_info solves this by providing an extensible place for information about
957the kernel image. It is readonly, because the kernel cannot rely on a
958bootloader copying its contents anywhere, but that is OK; if it becomes
959necessary it can still contain data items that an enabled bootloader would be
960expected to copy into a setup_data chunk.
961
962All kernel_info data should be part of this structure. Fixed size data have to
963be put before kernel_info_var_len_data label. Variable size data have to be put
964after kernel_info_var_len_data label. Each chunk of variable size data has to
965be prefixed with header/magic and its size, e.g.::
966
967  kernel_info:
968          .ascii  "LToP"          /* Header, Linux top (structure). */
969          .long   kernel_info_var_len_data - kernel_info
970          .long   kernel_info_end - kernel_info
971          .long   0x01234567      /* Some fixed size data for the bootloaders. */
972  kernel_info_var_len_data:
973  example_struct:                 /* Some variable size data for the bootloaders. */
974          .ascii  "0123"          /* Header/Magic. */
975          .long   example_struct_end - example_struct
976          .ascii  "Struct"
977          .long   0x89012345
978  example_struct_end:
979  example_strings:                /* Some variable size data for the bootloaders. */
980          .ascii  "ABCD"          /* Header/Magic. */
981          .long   example_strings_end - example_strings
982          .asciz  "String_0"
983          .asciz  "String_1"
984  example_strings_end:
985  kernel_info_end:
986
987This way the kernel_info is self-contained blob.
988
989.. note::
990     Each variable size data header/magic can be any 4-character string,
991     without \0 at the end of the string, which does not collide with
992     existing variable length data headers/magics.
993
994
995Details of the kernel_info Fields
996=================================
997
998============	========
999Field name:	header
1000Offset/size:	0x0000/4
1001============	========
1002
1003  Contains the magic number "LToP" (0x506f544c).
1004
1005============	========
1006Field name:	size
1007Offset/size:	0x0004/4
1008============	========
1009
1010  This field contains the size of the kernel_info including kernel_info.header.
1011  It does not count kernel_info.kernel_info_var_len_data size. This field should be
1012  used by the bootloaders to detect supported fixed size fields in the kernel_info
1013  and beginning of kernel_info.kernel_info_var_len_data.
1014
1015============	========
1016Field name:	size_total
1017Offset/size:	0x0008/4
1018============	========
1019
1020  This field contains the size of the kernel_info including kernel_info.header
1021  and kernel_info.kernel_info_var_len_data.
1022
1023============	==============
1024Field name:	setup_type_max
1025Offset/size:	0x000c/4
1026============	==============
1027
1028  This field contains maximal allowed type for setup_data and setup_indirect structs.
1029
1030
1031The Image Checksum
1032==================
1033
1034From boot protocol version 2.08 onwards the CRC-32 is calculated over
1035the entire file using the characteristic polynomial 0x04C11DB7 and an
1036initial remainder of 0xffffffff.  The checksum is appended to the
1037file; therefore the CRC of the file up to the limit specified in the
1038syssize field of the header is always 0.
1039
1040
1041The Kernel Command Line
1042=======================
1043
1044The kernel command line has become an important way for the boot
1045loader to communicate with the kernel.  Some of its options are also
1046relevant to the boot loader itself, see "special command line options"
1047below.
1048
1049The kernel command line is a null-terminated string. The maximum
1050length can be retrieved from the field cmdline_size.  Before protocol
1051version 2.06, the maximum was 255 characters.  A string that is too
1052long will be automatically truncated by the kernel.
1053
1054If the boot protocol version is 2.02 or later, the address of the
1055kernel command line is given by the header field cmd_line_ptr (see
1056above.)  This address can be anywhere between the end of the setup
1057heap and 0xA0000.
1058
1059If the protocol version is *not* 2.02 or higher, the kernel
1060command line is entered using the following protocol:
1061
1062  - At offset 0x0020 (word), "cmd_line_magic", enter the magic
1063    number 0xA33F.
1064
1065  - At offset 0x0022 (word), "cmd_line_offset", enter the offset
1066    of the kernel command line (relative to the start of the
1067    real-mode kernel).
1068
1069  - The kernel command line *must* be within the memory region
1070    covered by setup_move_size, so you may need to adjust this
1071    field.
1072
1073
1074Memory Layout of The Real-Mode Code
1075===================================
1076
1077The real-mode code requires a stack/heap to be set up, as well as
1078memory allocated for the kernel command line.  This needs to be done
1079in the real-mode accessible memory in bottom megabyte.
1080
1081It should be noted that modern machines often have a sizable Extended
1082BIOS Data Area (EBDA).  As a result, it is advisable to use as little
1083of the low megabyte as possible.
1084
1085Unfortunately, under the following circumstances the 0x90000 memory
1086segment has to be used:
1087
1088	- When loading a zImage kernel ((loadflags & 0x01) == 0).
1089	- When loading a 2.01 or earlier boot protocol kernel.
1090
1091.. note::
1092     For the 2.00 and 2.01 boot protocols, the real-mode code
1093     can be loaded at another address, but it is internally
1094     relocated to 0x90000.  For the "old" protocol, the
1095     real-mode code must be loaded at 0x90000.
1096
1097When loading at 0x90000, avoid using memory above 0x9a000.
1098
1099For boot protocol 2.02 or higher, the command line does not have to be
1100located in the same 64K segment as the real-mode setup code; it is
1101thus permitted to give the stack/heap the full 64K segment and locate
1102the command line above it.
1103
1104The kernel command line should not be located below the real-mode
1105code, nor should it be located in high memory.
1106
1107
1108Sample Boot Configuration
1109=========================
1110
1111As a sample configuration, assume the following layout of the real
1112mode segment.
1113
1114    When loading below 0x90000, use the entire segment:
1115
1116        =============	===================
1117	0x0000-0x7fff	Real mode kernel
1118	0x8000-0xdfff	Stack and heap
1119	0xe000-0xffff	Kernel command line
1120	=============	===================
1121
1122    When loading at 0x90000 OR the protocol version is 2.01 or earlier:
1123
1124	=============	===================
1125	0x0000-0x7fff	Real mode kernel
1126	0x8000-0x97ff	Stack and heap
1127	0x9800-0x9fff	Kernel command line
1128	=============	===================
1129
1130Such a boot loader should enter the following fields in the header::
1131
1132	unsigned long base_ptr;	/* base address for real-mode segment */
1133
1134	if ( setup_sects == 0 ) {
1135		setup_sects = 4;
1136	}
1137
1138	if ( protocol >= 0x0200 ) {
1139		type_of_loader = <type code>;
1140		if ( loading_initrd ) {
1141			ramdisk_image = <initrd_address>;
1142			ramdisk_size = <initrd_size>;
1143		}
1144
1145		if ( protocol >= 0x0202 && loadflags & 0x01 )
1146			heap_end = 0xe000;
1147		else
1148			heap_end = 0x9800;
1149
1150		if ( protocol >= 0x0201 ) {
1151			heap_end_ptr = heap_end - 0x200;
1152			loadflags |= 0x80; /* CAN_USE_HEAP */
1153		}
1154
1155		if ( protocol >= 0x0202 ) {
1156			cmd_line_ptr = base_ptr + heap_end;
1157			strcpy(cmd_line_ptr, cmdline);
1158		} else {
1159			cmd_line_magic	= 0xA33F;
1160			cmd_line_offset = heap_end;
1161			setup_move_size = heap_end + strlen(cmdline)+1;
1162			strcpy(base_ptr+cmd_line_offset, cmdline);
1163		}
1164	} else {
1165		/* Very old kernel */
1166
1167		heap_end = 0x9800;
1168
1169		cmd_line_magic	= 0xA33F;
1170		cmd_line_offset = heap_end;
1171
1172		/* A very old kernel MUST have its real-mode code
1173		   loaded at 0x90000 */
1174
1175		if ( base_ptr != 0x90000 ) {
1176			/* Copy the real-mode kernel */
1177			memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1178			base_ptr = 0x90000;		 /* Relocated */
1179		}
1180
1181		strcpy(0x90000+cmd_line_offset, cmdline);
1182
1183		/* It is recommended to clear memory up to the 32K mark */
1184		memset(0x90000 + (setup_sects+1)*512, 0,
1185		       (64-(setup_sects+1))*512);
1186	}
1187
1188
1189Loading The Rest of The Kernel
1190==============================
1191
1192The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1193in the kernel file (again, if setup_sects == 0 the real value is 4.)
1194It should be loaded at address 0x10000 for Image/zImage kernels and
11950x100000 for bzImage kernels.
1196
1197The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1198bit (LOAD_HIGH) in the loadflags field is set::
1199
1200	is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1201	load_address = is_bzImage ? 0x100000 : 0x10000;
1202
1203Note that Image/zImage kernels can be up to 512K in size, and thus use
1204the entire 0x10000-0x90000 range of memory.  This means it is pretty
1205much a requirement for these kernels to load the real-mode part at
12060x90000.  bzImage kernels allow much more flexibility.
1207
1208Special Command Line Options
1209============================
1210
1211If the command line provided by the boot loader is entered by the
1212user, the user may expect the following command line options to work.
1213They should normally not be deleted from the kernel command line even
1214though not all of them are actually meaningful to the kernel.  Boot
1215loader authors who need additional command line options for the boot
1216loader itself should get them registered in
1217Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1218conflict with actual kernel options now or in the future.
1219
1220  vga=<mode>
1221	<mode> here is either an integer (in C notation, either
1222	decimal, octal, or hexadecimal) or one of the strings
1223	"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1224	(meaning 0xFFFD).  This value should be entered into the
1225	vid_mode field, as it is used by the kernel before the command
1226	line is parsed.
1227
1228  mem=<size>
1229	<size> is an integer in C notation optionally followed by
1230	(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1231	<< 30, << 40, << 50 or << 60).  This specifies the end of
1232	memory to the kernel. This affects the possible placement of
1233	an initrd, since an initrd should be placed near end of
1234	memory.  Note that this is an option to *both* the kernel and
1235	the bootloader!
1236
1237  initrd=<file>
1238	An initrd should be loaded.  The meaning of <file> is
1239	obviously bootloader-dependent, and some boot loaders
1240	(e.g. LILO) do not have such a command.
1241
1242In addition, some boot loaders add the following options to the
1243user-specified command line:
1244
1245  BOOT_IMAGE=<file>
1246	The boot image which was loaded.  Again, the meaning of <file>
1247	is obviously bootloader-dependent.
1248
1249  auto
1250	The kernel was booted without explicit user intervention.
1251
1252If these options are added by the boot loader, it is highly
1253recommended that they are located *first*, before the user-specified
1254or configuration-specified command line.  Otherwise, "init=/bin/sh"
1255gets confused by the "auto" option.
1256
1257
1258Running the Kernel
1259==================
1260
1261The kernel is started by jumping to the kernel entry point, which is
1262located at *segment* offset 0x20 from the start of the real mode
1263kernel.  This means that if you loaded your real-mode kernel code at
12640x90000, the kernel entry point is 9020:0000.
1265
1266At entry, ds = es = ss should point to the start of the real-mode
1267kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1268set up properly, normally pointing to the top of the heap, and
1269interrupts should be disabled.  Furthermore, to guard against bugs in
1270the kernel, it is recommended that the boot loader sets fs = gs = ds =
1271es = ss.
1272
1273In our example from above, we would do::
1274
1275	/* Note: in the case of the "old" kernel protocol, base_ptr must
1276	   be == 0x90000 at this point; see the previous sample code */
1277
1278	seg = base_ptr >> 4;
1279
1280	cli();	/* Enter with interrupts disabled! */
1281
1282	/* Set up the real-mode kernel stack */
1283	_SS = seg;
1284	_SP = heap_end;
1285
1286	_DS = _ES = _FS = _GS = seg;
1287	jmp_far(seg+0x20, 0);	/* Run the kernel */
1288
1289If your boot sector accesses a floppy drive, it is recommended to
1290switch off the floppy motor before running the kernel, since the
1291kernel boot leaves interrupts off and thus the motor will not be
1292switched off, especially if the loaded kernel has the floppy driver as
1293a demand-loaded module!
1294
1295
1296Advanced Boot Loader Hooks
1297==========================
1298
1299If the boot loader runs in a particularly hostile environment (such as
1300LOADLIN, which runs under DOS) it may be impossible to follow the
1301standard memory location requirements.  Such a boot loader may use the
1302following hooks that, if set, are invoked by the kernel at the
1303appropriate time.  The use of these hooks should probably be
1304considered an absolutely last resort!
1305
1306IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1307%edi across invocation.
1308
1309  realmode_swtch:
1310	A 16-bit real mode far subroutine invoked immediately before
1311	entering protected mode.  The default routine disables NMI, so
1312	your routine should probably do so, too.
1313
1314  code32_start:
1315	A 32-bit flat-mode routine *jumped* to immediately after the
1316	transition to protected mode, but before the kernel is
1317	uncompressed.  No segments, except CS, are guaranteed to be
1318	set up (current kernels do, but older ones do not); you should
1319	set them up to BOOT_DS (0x18) yourself.
1320
1321	After completing your hook, you should jump to the address
1322	that was in this field before your boot loader overwrote it
1323	(relocated, if appropriate.)
1324
1325
132632-bit Boot Protocol
1327====================
1328
1329For machine with some new BIOS other than legacy BIOS, such as EFI,
1330LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1331based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1332to be defined.
1333
1334In 32-bit boot protocol, the first step in loading a Linux kernel
1335should be to setup the boot parameters (struct boot_params,
1336traditionally known as "zero page"). The memory for struct boot_params
1337should be allocated and initialized to all zero. Then the setup header
1338from offset 0x01f1 of kernel image on should be loaded into struct
1339boot_params and examined. The end of setup header can be calculated as
1340follow::
1341
1342	0x0202 + byte value at offset 0x0201
1343
1344In addition to read/modify/write the setup header of the struct
1345boot_params as that of 16-bit boot protocol, the boot loader should
1346also fill the additional fields of the struct boot_params as
1347described in chapter Documentation/arch/x86/zero-page.rst.
1348
1349After setting up the struct boot_params, the boot loader can load the
135032/64-bit kernel in the same way as that of 16-bit boot protocol.
1351
1352In 32-bit boot protocol, the kernel is started by jumping to the
135332-bit kernel entry point, which is the start address of loaded
135432/64-bit kernel.
1355
1356At entry, the CPU must be in 32-bit protected mode with paging
1357disabled; a GDT must be loaded with the descriptors for selectors
1358__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1359segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1360must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1361must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1362address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1363
136464-bit Boot Protocol
1365====================
1366
1367For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1368and we need a 64-bit boot protocol.
1369
1370In 64-bit boot protocol, the first step in loading a Linux kernel
1371should be to setup the boot parameters (struct boot_params,
1372traditionally known as "zero page"). The memory for struct boot_params
1373could be allocated anywhere (even above 4G) and initialized to all zero.
1374Then, the setup header at offset 0x01f1 of kernel image on should be
1375loaded into struct boot_params and examined. The end of setup header
1376can be calculated as follows::
1377
1378	0x0202 + byte value at offset 0x0201
1379
1380In addition to read/modify/write the setup header of the struct
1381boot_params as that of 16-bit boot protocol, the boot loader should
1382also fill the additional fields of the struct boot_params as described
1383in chapter Documentation/arch/x86/zero-page.rst.
1384
1385After setting up the struct boot_params, the boot loader can load
138664-bit kernel in the same way as that of 16-bit boot protocol, but
1387kernel could be loaded above 4G.
1388
1389In 64-bit boot protocol, the kernel is started by jumping to the
139064-bit kernel entry point, which is the start address of loaded
139164-bit kernel plus 0x200.
1392
1393At entry, the CPU must be in 64-bit mode with paging enabled.
1394The range with setup_header.init_size from start address of loaded
1395kernel and zero page and command line buffer get ident mapping;
1396a GDT must be loaded with the descriptors for selectors
1397__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1398segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1399must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1400must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1401address of the struct boot_params.
1402
1403EFI Handover Protocol (deprecated)
1404==================================
1405
1406This protocol allows boot loaders to defer initialisation to the EFI
1407boot stub. The boot loader is required to load the kernel/initrd(s)
1408from the boot media and jump to the EFI handover protocol entry point
1409which is hdr->handover_offset bytes from the beginning of
1410startup_{32,64}.
1411
1412The boot loader MUST respect the kernel's PE/COFF metadata when it comes
1413to section alignment, the memory footprint of the executable image beyond
1414the size of the file itself, and any other aspect of the PE/COFF header
1415that may affect correct operation of the image as a PE/COFF binary in the
1416execution context provided by the EFI firmware.
1417
1418The function prototype for the handover entry point looks like this::
1419
1420    efi_stub_entry(void *handle, efi_system_table_t *table, struct boot_params *bp)
1421
1422'handle' is the EFI image handle passed to the boot loader by the EFI
1423firmware, 'table' is the EFI system table - these are the first two
1424arguments of the "handoff state" as described in section 2.3 of the
1425UEFI specification. 'bp' is the boot loader-allocated boot params.
1426
1427The boot loader *must* fill out the following fields in bp::
1428
1429  - hdr.cmd_line_ptr
1430  - hdr.ramdisk_image (if applicable)
1431  - hdr.ramdisk_size  (if applicable)
1432
1433All other fields should be zero.
1434
1435NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF
1436      entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd
1437      loading protocol (refer to [0] for an example of the bootloader side of
1438      this), which removes the need for any knowledge on the part of the EFI
1439      bootloader regarding the internal representation of boot_params or any
1440      requirements/limitations regarding the placement of the command line
1441      and ramdisk in memory, or the placement of the kernel image itself.
1442
1443[0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0
1444