1 .. SPDX-License-Identifier: GPL-2.0
2
3 ===========================
4 The Linux/x86 Boot Protocol
5 ===========================
6
7 On the x86 platform, the Linux kernel uses a rather complicated boot
8 convention. This has evolved partially due to historical aspects, as
9 well as the desire in the early days to have the kernel itself be a
10 bootable image, the complicated PC memory model and due to changed
11 expectations in the PC industry caused by the effective demise of
12 real-mode DOS as a mainstream operating system.
13
14 Currently, the following versions of the Linux/x86 boot protocol exist.
15
16 ============= ============================================================
17 Old kernels zImage/Image support only. Some very early kernels
18 may not even support a command line.
19
20 Protocol 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
26 Protocol 2.01 (Kernel 1.3.76) Added a heap overrun warning.
27
28 Protocol 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
35 Protocol 2.03 (Kernel 2.4.18-pre1) Explicitly makes the highest possible
36 initrd address available to the bootloader.
37
38 Protocol 2.04 (Kernel 2.6.14) Extend the syssize field to four bytes.
39
40 Protocol 2.05 (Kernel 2.6.20) Make protected mode kernel relocatable.
41 Introduce relocatable_kernel and kernel_alignment fields.
42
43 Protocol 2.06 (Kernel 2.6.22) Added a field that contains the size of
44 the boot command line.
45
46 Protocol 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
50 Protocol 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
54 Protocol 2.09 (Kernel 2.6.26) Added a field of 64-bit physical
55 pointer to single linked list of struct setup_data.
56
57 Protocol 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
61 Protocol 2.11 (Kernel 3.6) Added a field for offset of EFI handover
62 protocol entry point.
63
64 Protocol 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
68 Protocol 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
72 Protocol 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
77 Protocol 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
92 Memory Layout
93 =============
94
95 The traditional memory map for the kernel loader, used for Image or
96 zImage 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
120 When using bzImage, the protected-mode kernel was relocated to
121 0x100000 ("high memory"), and the kernel real-mode block (boot sector,
122 setup, and stack/heap) was made relocatable to any address between
123 0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
124 2.01 the 0x90000+ memory range is still used internally by the kernel;
125 the 2.02 protocol resolves that problem.
126
127 It is desirable to keep the "memory ceiling" -- the highest point in
128 low memory touched by the boot loader -- as low as possible, since
129 some newer BIOSes have begun to allocate some rather large amounts of
130 memory, called the Extended BIOS Data Area, near the top of low
131 memory. The boot loader should use the "INT 12h" BIOS call to verify
132 how much low memory is available.
133
134 Unfortunately, if INT 12h reports that the amount of memory is too
135 low, there is usually nothing the boot loader can do but to report an
136 error to the user. The boot loader should therefore be designed to
137 take up as little space in low memory as it reasonably can. For
138 zImage or old bzImage kernels, which need data written into the
139 0x90000 segment, the boot loader should make sure not to use memory
140 above the 0x9A000 point; too many BIOSes will break above that point.
141
142 For a modern bzImage kernel with boot protocol version >= 2.02, a
143 memory 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
171 The Real-Mode Kernel Header
172 ===========================
173
174 In the following text, and anywhere in the kernel boot sequence, "a
175 sector" refers to 512 bytes. It is independent of the actual sector
176 size of the underlying medium.
177
178 The first step in loading a Linux kernel should be to load the
179 real-mode code (boot sector and setup code) and then examine the
180 following header at offset 0x01f1. The real-mode code can total up to
181 32K, although the boot loader may choose to load only the first two
182 sectors (1K) and then examine the bootup sector size.
183
184 The header looks like:
185
186 =========== ======== ===================== ============================================
187 Offset/Size Proto Name Meaning
188 =========== ======== ===================== ============================================
189 01F1/1 ALL(1) setup_sects The size of the setup in sectors
190 01F2/2 ALL root_flags If set, the root is mounted readonly
191 01F4/4 2.04+(2) syssize The size of the 32-bit code in 16-byte paras
192 01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
193 01FA/2 ALL vid_mode Video mode control
194 01FC/2 ALL root_dev Default root device number
195 01FE/2 ALL boot_flag 0xAA55 magic number
196 0200/2 2.00+ jump Jump instruction
197 0202/4 2.00+ header Magic signature "HdrS"
198 0206/2 2.00+ version Boot protocol version supported
199 0208/4 2.00+ realmode_swtch Boot loader hook (see below)
200 020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
201 020E/2 2.00+ kernel_version Pointer to kernel version string
202 0210/1 2.00+ type_of_loader Boot loader identifier
203 0211/1 2.00+ loadflags Boot protocol option flags
204 0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
205 0214/4 2.00+ code32_start Boot loader hook (see below)
206 0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
207 021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
208 0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
209 0224/2 2.01+ heap_end_ptr Free memory after setup end
210 0226/1 2.02+(3) ext_loader_ver Extended boot loader version
211 0227/1 2.02+(3) ext_loader_type Extended boot loader ID
212 0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
213 022C/4 2.03+ initrd_addr_max Highest legal initrd address
214 0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
215 0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
216 0235/1 2.10+ min_alignment Minimum alignment, as a power of two
217 0236/2 2.12+ xloadflags Boot protocol option flags
218 0238/4 2.06+ cmdline_size Maximum size of the kernel command line
219 023C/4 2.07+ hardware_subarch Hardware subarchitecture
220 0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
221 0248/4 2.08+ payload_offset Offset of kernel payload
222 024C/4 2.08+ payload_length Length of kernel payload
223 0250/8 2.09+ setup_data 64-bit physical pointer to linked list
224 of struct setup_data
225 0258/8 2.10+ pref_address Preferred loading address
226 0260/4 2.10+ init_size Linear memory required during initialization
227 0264/4 2.11+ handover_offset Offset of handover entry point
228 0268/4 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
241 If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
242 the boot protocol version is "old". Loading an old kernel, the
243 following parameters should be assumed::
244
245 Image type = zImage
246 initrd not supported
247 Real-mode kernel must be located at 0x90000.
248
249 Otherwise, the "version" field contains the protocol version,
250 e.g. protocol version 2.01 will contain 0x0201 in this field. When
251 setting fields in the header, you must make sure only to set fields
252 supported by the protocol version in use.
253
254
255 Details of Header Fields
256 ========================
257
258 For 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
261 bootloader ("modify").
262
263 All general purpose boot loaders should write the fields marked
264 (obligatory). Boot loaders who want to load the kernel at a
265 nonstandard address should fill in the fields marked (reloc); other
266 boot loaders can ignore those fields.
267
268 The byte order of all fields is littleendian (this is x86, after all.)
269
270 ============ ===========
271 Field name: setup_sects
272 Type: read
273 Offset/size: 0x1f1/1
274 Protocol: 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 ============ =================
282 Field name: root_flags
283 Type: modify (optional)
284 Offset/size: 0x1f2/2
285 Protocol: 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 ============ ===============================================
293 Field name: syssize
294 Type: read
295 Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
296 Protocol: 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 ============ ===============
305 Field name: ram_size
306 Type: kernel internal
307 Offset/size: 0x1f8/2
308 Protocol: ALL
309 ============ ===============
310
311 This field is obsolete.
312
313 ============ ===================
314 Field name: vid_mode
315 Type: modify (obligatory)
316 Offset/size: 0x1fa/2
317 ============ ===================
318
319 Please see the section on SPECIAL COMMAND LINE OPTIONS.
320
321 ============ =================
322 Field name: root_dev
323 Type: modify (optional)
324 Offset/size: 0x1fc/2
325 Protocol: 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 ============ =========
332 Field name: boot_flag
333 Type: read
334 Offset/size: 0x1fe/2
335 Protocol: ALL
336 ============ =========
337
338 Contains 0xAA55. This is the closest thing old Linux kernels have
339 to a magic number.
340
341 ============ =======
342 Field name: jump
343 Type: read
344 Offset/size: 0x200/2
345 Protocol: 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 ============ =======
353 Field name: header
354 Type: read
355 Offset/size: 0x202/4
356 Protocol: 2.00+
357 ============ =======
358
359 Contains the magic number "HdrS" (0x53726448).
360
361 ============ =======
362 Field name: version
363 Type: read
364 Offset/size: 0x206/2
365 Protocol: 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 ============ =================
373 Field name: realmode_swtch
374 Type: modify (optional)
375 Offset/size: 0x208/4
376 Protocol: 2.00+
377 ============ =================
378
379 Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
380
381 ============ =============
382 Field name: start_sys_seg
383 Type: read
384 Offset/size: 0x20c/2
385 Protocol: 2.00+
386 ============ =============
387
388 The load low segment (0x1000). Obsolete.
389
390 ============ ==============
391 Field name: kernel_version
392 Type: read
393 Offset/size: 0x20e/2
394 Protocol: 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 ============ ==================
413 Field name: type_of_loader
414 Type: write (obligatory)
415 Offset/size: 0x210/1
416 Protocol: 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 ============ ===================
464 Field name: loadflags
465 Type: modify (obligatory)
466 Offset/size: 0x211/1
467 Protocol: 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 ============ ===================
508 Field name: setup_move_size
509 Type: modify (obligatory)
510 Offset/size: 0x212/2
511 Protocol: 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 ============ ========================
526 Field name: code32_start
527 Type: modify (optional, reloc)
528 Offset/size: 0x214/4
529 Protocol: 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 ============ ==================
545 Field name: ramdisk_image
546 Type: write (obligatory)
547 Offset/size: 0x218/4
548 Protocol: 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 ============ ==================
555 Field name: ramdisk_size
556 Type: write (obligatory)
557 Offset/size: 0x21c/4
558 Protocol: 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 ============ ===============
565 Field name: bootsect_kludge
566 Type: kernel internal
567 Offset/size: 0x220/4
568 Protocol: 2.00+
569 ============ ===============
570
571 This field is obsolete.
572
573 ============ ==================
574 Field name: heap_end_ptr
575 Type: write (obligatory)
576 Offset/size: 0x224/2
577 Protocol: 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 ============ ================
584 Field name: ext_loader_ver
585 Type: write (optional)
586 Offset/size: 0x226/1
587 Protocol: 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 ============ =====================================================
601 Field name: ext_loader_type
602 Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
603 Offset/size: 0x227/1
604 Protocol: 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 ============ ==================
617 Field name: cmd_line_ptr
618 Type: write (obligatory)
619 Offset/size: 0x228/4
620 Protocol: 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 ============ ===============
635 Field name: initrd_addr_max
636 Type: read
637 Offset/size: 0x22c/4
638 Protocol: 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 ============ ============================
649 Field name: kernel_alignment
650 Type: read/modify (reloc)
651 Offset/size: 0x230/4
652 Protocol: 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 ============ ==================
666 Field name: relocatable_kernel
667 Type: read (reloc)
668 Offset/size: 0x234/1
669 Protocol: 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 ============ =============
678 Field name: min_alignment
679 Type: read (reloc)
680 Offset/size: 0x235/1
681 Protocol: 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 ============ ==========
696 Field name: xloadflags
697 Type: read
698 Offset/size: 0x236/2
699 Protocol: 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 ============ ============
728 Field name: cmdline_size
729 Type: read
730 Offset/size: 0x238/4
731 Protocol: 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 ============ ====================================
740 Field name: hardware_subarch
741 Type: write (optional, defaults to x86/PC)
742 Offset/size: 0x23c/4
743 Protocol: 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 ============ =========================
762 Field name: hardware_subarch_data
763 Type: write (subarch-dependent)
764 Offset/size: 0x240/8
765 Protocol: 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 ============ ==============
773 Field name: payload_offset
774 Type: read
775 Offset/size: 0x248/4
776 Protocol: 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 ============ ==============
791 Field name: payload_length
792 Type: read
793 Offset/size: 0x24c/4
794 Protocol: 2.08+
795 ============ ==============
796
797 The length of the payload.
798
799 ============ ===============
800 Field name: setup_data
801 Type: write (special)
802 Offset/size: 0x250/8
803 Protocol: 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 ============ ============
870 Field name: pref_address
871 Type: read (reloc)
872 Offset/size: 0x258/8
873 Protocol: 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. A relocatable kernel will move itself to this address if it
882 loaded below this address.
883
884 ============ =======
885 Field name: init_size
886 Type: read
887 Offset/size: 0x260/4
888 ============ =======
889
890 This field indicates the amount of linear contiguous memory starting
891 at the kernel runtime start address that the kernel needs before it
892 is capable of examining its memory map. This is not the same thing
893 as the total amount of memory the kernel needs to boot, but it can
894 be used by a relocating boot loader to help select a safe load
895 address for the kernel.
896
897 The kernel runtime start address is determined by the following algorithm::
898
899 if (relocatable_kernel)
900 runtime_start = align_up(load_address, kernel_alignment)
901 else
902 runtime_start = pref_address
903
904 ============ ===============
905 Field name: handover_offset
906 Type: read
907 Offset/size: 0x264/4
908 ============ ===============
909
910 This field is the offset from the beginning of the kernel image to
911 the EFI handover protocol entry point. Boot loaders using the EFI
912 handover protocol to boot the kernel should jump to this offset.
913
914 See EFI HANDOVER PROTOCOL below for more details.
915
916 ============ ==================
917 Field name: kernel_info_offset
918 Type: read
919 Offset/size: 0x268/4
920 Protocol: 2.15+
921 ============ ==================
922
923 This field is the offset from the beginning of the kernel image to the
924 kernel_info. The kernel_info structure is embedded in the Linux image
925 in the uncompressed protected mode region.
926
927
928 The kernel_info
929 ===============
930
931 The relationships between the headers are analogous to the various data
932 sections:
933
934 setup_header = .data
935 boot_params/setup_data = .bss
936
937 What is missing from the above list? That's right:
938
939 kernel_info = .rodata
940
941 We have been (ab)using .data for things that could go into .rodata or .bss for
942 a long time, for lack of alternatives and -- especially early on -- inertia.
943 Also, the BIOS stub is responsible for creating boot_params, so it isn't
944 available to a BIOS-based loader (setup_data is, though).
945
946 setup_header is permanently limited to 144 bytes due to the reach of the
947 2-byte jump field, which doubles as a length field for the structure, combined
948 with the size of the "hole" in struct boot_params that a protected-mode loader
949 or the BIOS stub has to copy it into. It is currently 119 bytes long, which
950 leaves us with 25 very precious bytes. This isn't something that can be fixed
951 without revising the boot protocol entirely, breaking backwards compatibility.
952
953 boot_params proper is limited to 4096 bytes, but can be arbitrarily extended
954 by adding setup_data entries. It cannot be used to communicate properties of
955 the kernel image, because it is .bss and has no image-provided content.
956
957 kernel_info solves this by providing an extensible place for information about
958 the kernel image. It is readonly, because the kernel cannot rely on a
959 bootloader copying its contents anywhere, but that is OK; if it becomes
960 necessary it can still contain data items that an enabled bootloader would be
961 expected to copy into a setup_data chunk.
962
963 All kernel_info data should be part of this structure. Fixed size data have to
964 be put before kernel_info_var_len_data label. Variable size data have to be put
965 after kernel_info_var_len_data label. Each chunk of variable size data has to
966 be prefixed with header/magic and its size, e.g.::
967
968 kernel_info:
969 .ascii "LToP" /* Header, Linux top (structure). */
970 .long kernel_info_var_len_data - kernel_info
971 .long kernel_info_end - kernel_info
972 .long 0x01234567 /* Some fixed size data for the bootloaders. */
973 kernel_info_var_len_data:
974 example_struct: /* Some variable size data for the bootloaders. */
975 .ascii "0123" /* Header/Magic. */
976 .long example_struct_end - example_struct
977 .ascii "Struct"
978 .long 0x89012345
979 example_struct_end:
980 example_strings: /* Some variable size data for the bootloaders. */
981 .ascii "ABCD" /* Header/Magic. */
982 .long example_strings_end - example_strings
983 .asciz "String_0"
984 .asciz "String_1"
985 example_strings_end:
986 kernel_info_end:
987
988 This way the kernel_info is self-contained blob.
989
990 .. note::
991 Each variable size data header/magic can be any 4-character string,
992 without \0 at the end of the string, which does not collide with
993 existing variable length data headers/magics.
994
995
996 Details of the kernel_info Fields
997 =================================
998
999 ============ ========
1000 Field name: header
1001 Offset/size: 0x0000/4
1002 ============ ========
1003
1004 Contains the magic number "LToP" (0x506f544c).
1005
1006 ============ ========
1007 Field name: size
1008 Offset/size: 0x0004/4
1009 ============ ========
1010
1011 This field contains the size of the kernel_info including kernel_info.header.
1012 It does not count kernel_info.kernel_info_var_len_data size. This field should be
1013 used by the bootloaders to detect supported fixed size fields in the kernel_info
1014 and beginning of kernel_info.kernel_info_var_len_data.
1015
1016 ============ ========
1017 Field name: size_total
1018 Offset/size: 0x0008/4
1019 ============ ========
1020
1021 This field contains the size of the kernel_info including kernel_info.header
1022 and kernel_info.kernel_info_var_len_data.
1023
1024 ============ ==============
1025 Field name: setup_type_max
1026 Offset/size: 0x000c/4
1027 ============ ==============
1028
1029 This field contains maximal allowed type for setup_data and setup_indirect structs.
1030
1031
1032 The Image Checksum
1033 ==================
1034
1035 From boot protocol version 2.08 onwards the CRC-32 is calculated over
1036 the entire file using the characteristic polynomial 0x04C11DB7 and an
1037 initial remainder of 0xffffffff. The checksum is appended to the
1038 file; therefore the CRC of the file up to the limit specified in the
1039 syssize field of the header is always 0.
1040
1041
1042 The Kernel Command Line
1043 =======================
1044
1045 The kernel command line has become an important way for the boot
1046 loader to communicate with the kernel. Some of its options are also
1047 relevant to the boot loader itself, see "special command line options"
1048 below.
1049
1050 The kernel command line is a null-terminated string. The maximum
1051 length can be retrieved from the field cmdline_size. Before protocol
1052 version 2.06, the maximum was 255 characters. A string that is too
1053 long will be automatically truncated by the kernel.
1054
1055 If the boot protocol version is 2.02 or later, the address of the
1056 kernel command line is given by the header field cmd_line_ptr (see
1057 above.) This address can be anywhere between the end of the setup
1058 heap and 0xA0000.
1059
1060 If the protocol version is *not* 2.02 or higher, the kernel
1061 command line is entered using the following protocol:
1062
1063 - At offset 0x0020 (word), "cmd_line_magic", enter the magic
1064 number 0xA33F.
1065
1066 - At offset 0x0022 (word), "cmd_line_offset", enter the offset
1067 of the kernel command line (relative to the start of the
1068 real-mode kernel).
1069
1070 - The kernel command line *must* be within the memory region
1071 covered by setup_move_size, so you may need to adjust this
1072 field.
1073
1074
1075 Memory Layout of The Real-Mode Code
1076 ===================================
1077
1078 The real-mode code requires a stack/heap to be set up, as well as
1079 memory allocated for the kernel command line. This needs to be done
1080 in the real-mode accessible memory in bottom megabyte.
1081
1082 It should be noted that modern machines often have a sizable Extended
1083 BIOS Data Area (EBDA). As a result, it is advisable to use as little
1084 of the low megabyte as possible.
1085
1086 Unfortunately, under the following circumstances the 0x90000 memory
1087 segment has to be used:
1088
1089 - When loading a zImage kernel ((loadflags & 0x01) == 0).
1090 - When loading a 2.01 or earlier boot protocol kernel.
1091
1092 .. note::
1093 For the 2.00 and 2.01 boot protocols, the real-mode code
1094 can be loaded at another address, but it is internally
1095 relocated to 0x90000. For the "old" protocol, the
1096 real-mode code must be loaded at 0x90000.
1097
1098 When loading at 0x90000, avoid using memory above 0x9a000.
1099
1100 For boot protocol 2.02 or higher, the command line does not have to be
1101 located in the same 64K segment as the real-mode setup code; it is
1102 thus permitted to give the stack/heap the full 64K segment and locate
1103 the command line above it.
1104
1105 The kernel command line should not be located below the real-mode
1106 code, nor should it be located in high memory.
1107
1108
1109 Sample Boot Configuration
1110 =========================
1111
1112 As a sample configuration, assume the following layout of the real
1113 mode segment.
1114
1115 When loading below 0x90000, use the entire segment:
1116
1117 ============= ===================
1118 0x0000-0x7fff Real mode kernel
1119 0x8000-0xdfff Stack and heap
1120 0xe000-0xffff Kernel command line
1121 ============= ===================
1122
1123 When loading at 0x90000 OR the protocol version is 2.01 or earlier:
1124
1125 ============= ===================
1126 0x0000-0x7fff Real mode kernel
1127 0x8000-0x97ff Stack and heap
1128 0x9800-0x9fff Kernel command line
1129 ============= ===================
1130
1131 Such a boot loader should enter the following fields in the header::
1132
1133 unsigned long base_ptr; /* base address for real-mode segment */
1134
1135 if ( setup_sects == 0 ) {
1136 setup_sects = 4;
1137 }
1138
1139 if ( protocol >= 0x0200 ) {
1140 type_of_loader = <type code>;
1141 if ( loading_initrd ) {
1142 ramdisk_image = <initrd_address>;
1143 ramdisk_size = <initrd_size>;
1144 }
1145
1146 if ( protocol >= 0x0202 && loadflags & 0x01 )
1147 heap_end = 0xe000;
1148 else
1149 heap_end = 0x9800;
1150
1151 if ( protocol >= 0x0201 ) {
1152 heap_end_ptr = heap_end - 0x200;
1153 loadflags |= 0x80; /* CAN_USE_HEAP */
1154 }
1155
1156 if ( protocol >= 0x0202 ) {
1157 cmd_line_ptr = base_ptr + heap_end;
1158 strcpy(cmd_line_ptr, cmdline);
1159 } else {
1160 cmd_line_magic = 0xA33F;
1161 cmd_line_offset = heap_end;
1162 setup_move_size = heap_end + strlen(cmdline)+1;
1163 strcpy(base_ptr+cmd_line_offset, cmdline);
1164 }
1165 } else {
1166 /* Very old kernel */
1167
1168 heap_end = 0x9800;
1169
1170 cmd_line_magic = 0xA33F;
1171 cmd_line_offset = heap_end;
1172
1173 /* A very old kernel MUST have its real-mode code
1174 loaded at 0x90000 */
1175
1176 if ( base_ptr != 0x90000 ) {
1177 /* Copy the real-mode kernel */
1178 memcpy(0x90000, base_ptr, (setup_sects+1)*512);
1179 base_ptr = 0x90000; /* Relocated */
1180 }
1181
1182 strcpy(0x90000+cmd_line_offset, cmdline);
1183
1184 /* It is recommended to clear memory up to the 32K mark */
1185 memset(0x90000 + (setup_sects+1)*512, 0,
1186 (64-(setup_sects+1))*512);
1187 }
1188
1189
1190 Loading The Rest of The Kernel
1191 ==============================
1192
1193 The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
1194 in the kernel file (again, if setup_sects == 0 the real value is 4.)
1195 It should be loaded at address 0x10000 for Image/zImage kernels and
1196 0x100000 for bzImage kernels.
1197
1198 The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
1199 bit (LOAD_HIGH) in the loadflags field is set::
1200
1201 is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
1202 load_address = is_bzImage ? 0x100000 : 0x10000;
1203
1204 Note that Image/zImage kernels can be up to 512K in size, and thus use
1205 the entire 0x10000-0x90000 range of memory. This means it is pretty
1206 much a requirement for these kernels to load the real-mode part at
1207 0x90000. bzImage kernels allow much more flexibility.
1208
1209 Special Command Line Options
1210 ============================
1211
1212 If the command line provided by the boot loader is entered by the
1213 user, the user may expect the following command line options to work.
1214 They should normally not be deleted from the kernel command line even
1215 though not all of them are actually meaningful to the kernel. Boot
1216 loader authors who need additional command line options for the boot
1217 loader itself should get them registered in
1218 Documentation/admin-guide/kernel-parameters.rst to make sure they will not
1219 conflict with actual kernel options now or in the future.
1220
1221 vga=<mode>
1222 <mode> here is either an integer (in C notation, either
1223 decimal, octal, or hexadecimal) or one of the strings
1224 "normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
1225 (meaning 0xFFFD). This value should be entered into the
1226 vid_mode field, as it is used by the kernel before the command
1227 line is parsed.
1228
1229 mem=<size>
1230 <size> is an integer in C notation optionally followed by
1231 (case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
1232 << 30, << 40, << 50 or << 60). This specifies the end of
1233 memory to the kernel. This affects the possible placement of
1234 an initrd, since an initrd should be placed near end of
1235 memory. Note that this is an option to *both* the kernel and
1236 the bootloader!
1237
1238 initrd=<file>
1239 An initrd should be loaded. The meaning of <file> is
1240 obviously bootloader-dependent, and some boot loaders
1241 (e.g. LILO) do not have such a command.
1242
1243 In addition, some boot loaders add the following options to the
1244 user-specified command line:
1245
1246 BOOT_IMAGE=<file>
1247 The boot image which was loaded. Again, the meaning of <file>
1248 is obviously bootloader-dependent.
1249
1250 auto
1251 The kernel was booted without explicit user intervention.
1252
1253 If these options are added by the boot loader, it is highly
1254 recommended that they are located *first*, before the user-specified
1255 or configuration-specified command line. Otherwise, "init=/bin/sh"
1256 gets confused by the "auto" option.
1257
1258
1259 Running the Kernel
1260 ==================
1261
1262 The kernel is started by jumping to the kernel entry point, which is
1263 located at *segment* offset 0x20 from the start of the real mode
1264 kernel. This means that if you loaded your real-mode kernel code at
1265 0x90000, the kernel entry point is 9020:0000.
1266
1267 At entry, ds = es = ss should point to the start of the real-mode
1268 kernel code (0x9000 if the code is loaded at 0x90000), sp should be
1269 set up properly, normally pointing to the top of the heap, and
1270 interrupts should be disabled. Furthermore, to guard against bugs in
1271 the kernel, it is recommended that the boot loader sets fs = gs = ds =
1272 es = ss.
1273
1274 In our example from above, we would do::
1275
1276 /* Note: in the case of the "old" kernel protocol, base_ptr must
1277 be == 0x90000 at this point; see the previous sample code */
1278
1279 seg = base_ptr >> 4;
1280
1281 cli(); /* Enter with interrupts disabled! */
1282
1283 /* Set up the real-mode kernel stack */
1284 _SS = seg;
1285 _SP = heap_end;
1286
1287 _DS = _ES = _FS = _GS = seg;
1288 jmp_far(seg+0x20, 0); /* Run the kernel */
1289
1290 If your boot sector accesses a floppy drive, it is recommended to
1291 switch off the floppy motor before running the kernel, since the
1292 kernel boot leaves interrupts off and thus the motor will not be
1293 switched off, especially if the loaded kernel has the floppy driver as
1294 a demand-loaded module!
1295
1296
1297 Advanced Boot Loader Hooks
1298 ==========================
1299
1300 If the boot loader runs in a particularly hostile environment (such as
1301 LOADLIN, which runs under DOS) it may be impossible to follow the
1302 standard memory location requirements. Such a boot loader may use the
1303 following hooks that, if set, are invoked by the kernel at the
1304 appropriate time. The use of these hooks should probably be
1305 considered an absolutely last resort!
1306
1307 IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
1308 %edi across invocation.
1309
1310 realmode_swtch:
1311 A 16-bit real mode far subroutine invoked immediately before
1312 entering protected mode. The default routine disables NMI, so
1313 your routine should probably do so, too.
1314
1315 code32_start:
1316 A 32-bit flat-mode routine *jumped* to immediately after the
1317 transition to protected mode, but before the kernel is
1318 uncompressed. No segments, except CS, are guaranteed to be
1319 set up (current kernels do, but older ones do not); you should
1320 set them up to BOOT_DS (0x18) yourself.
1321
1322 After completing your hook, you should jump to the address
1323 that was in this field before your boot loader overwrote it
1324 (relocated, if appropriate.)
1325
1326
1327 32-bit Boot Protocol
1328 ====================
1329
1330 For machine with some new BIOS other than legacy BIOS, such as EFI,
1331 LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
1332 based on legacy BIOS can not be used, so a 32-bit boot protocol needs
1333 to be defined.
1334
1335 In 32-bit boot protocol, the first step in loading a Linux kernel
1336 should be to setup the boot parameters (struct boot_params,
1337 traditionally known as "zero page"). The memory for struct boot_params
1338 should be allocated and initialized to all zero. Then the setup header
1339 from offset 0x01f1 of kernel image on should be loaded into struct
1340 boot_params and examined. The end of setup header can be calculated as
1341 follow::
1342
1343 0x0202 + byte value at offset 0x0201
1344
1345 In addition to read/modify/write the setup header of the struct
1346 boot_params as that of 16-bit boot protocol, the boot loader should
1347 also fill the additional fields of the struct boot_params as
1348 described in chapter Documentation/arch/x86/zero-page.rst.
1349
1350 After setting up the struct boot_params, the boot loader can load the
1351 32/64-bit kernel in the same way as that of 16-bit boot protocol.
1352
1353 In 32-bit boot protocol, the kernel is started by jumping to the
1354 32-bit kernel entry point, which is the start address of loaded
1355 32/64-bit kernel.
1356
1357 At entry, the CPU must be in 32-bit protected mode with paging
1358 disabled; a GDT must be loaded with the descriptors for selectors
1359 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1360 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1361 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1362 must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
1363 address of the struct boot_params; %ebp, %edi and %ebx must be zero.
1364
1365 64-bit Boot Protocol
1366 ====================
1367
1368 For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
1369 and we need a 64-bit boot protocol.
1370
1371 In 64-bit boot protocol, the first step in loading a Linux kernel
1372 should be to setup the boot parameters (struct boot_params,
1373 traditionally known as "zero page"). The memory for struct boot_params
1374 could be allocated anywhere (even above 4G) and initialized to all zero.
1375 Then, the setup header at offset 0x01f1 of kernel image on should be
1376 loaded into struct boot_params and examined. The end of setup header
1377 can be calculated as follows::
1378
1379 0x0202 + byte value at offset 0x0201
1380
1381 In addition to read/modify/write the setup header of the struct
1382 boot_params as that of 16-bit boot protocol, the boot loader should
1383 also fill the additional fields of the struct boot_params as described
1384 in chapter Documentation/arch/x86/zero-page.rst.
1385
1386 After setting up the struct boot_params, the boot loader can load
1387 64-bit kernel in the same way as that of 16-bit boot protocol, but
1388 kernel could be loaded above 4G.
1389
1390 In 64-bit boot protocol, the kernel is started by jumping to the
1391 64-bit kernel entry point, which is the start address of loaded
1392 64-bit kernel plus 0x200.
1393
1394 At entry, the CPU must be in 64-bit mode with paging enabled.
1395 The range with setup_header.init_size from start address of loaded
1396 kernel and zero page and command line buffer get ident mapping;
1397 a GDT must be loaded with the descriptors for selectors
1398 __BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
1399 segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
1400 must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
1401 must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
1402 address of the struct boot_params.
1403
1404 EFI Handover Protocol (deprecated)
1405 ==================================
1406
1407 This protocol allows boot loaders to defer initialisation to the EFI
1408 boot stub. The boot loader is required to load the kernel/initrd(s)
1409 from the boot media and jump to the EFI handover protocol entry point
1410 which is hdr->handover_offset bytes from the beginning of
1411 startup_{32,64}.
1412
1413 The boot loader MUST respect the kernel's PE/COFF metadata when it comes
1414 to section alignment, the memory footprint of the executable image beyond
1415 the size of the file itself, and any other aspect of the PE/COFF header
1416 that may affect correct operation of the image as a PE/COFF binary in the
1417 execution context provided by the EFI firmware.
1418
1419 The function prototype for the handover entry point looks like this::
1420
1421 efi_stub_entry(void *handle, efi_system_table_t *table, struct boot_params *bp)
1422
1423 'handle' is the EFI image handle passed to the boot loader by the EFI
1424 firmware, 'table' is the EFI system table - these are the first two
1425 arguments of the "handoff state" as described in section 2.3 of the
1426 UEFI specification. 'bp' is the boot loader-allocated boot params.
1427
1428 The boot loader *must* fill out the following fields in bp::
1429
1430 - hdr.cmd_line_ptr
1431 - hdr.ramdisk_image (if applicable)
1432 - hdr.ramdisk_size (if applicable)
1433
1434 All other fields should be zero.
1435
1436 NOTE: The EFI Handover Protocol is deprecated in favour of the ordinary PE/COFF
1437 entry point, combined with the LINUX_EFI_INITRD_MEDIA_GUID based initrd
1438 loading protocol (refer to [0] for an example of the bootloader side of
1439 this), which removes the need for any knowledge on the part of the EFI
1440 bootloader regarding the internal representation of boot_params or any
1441 requirements/limitations regarding the placement of the command line
1442 and ramdisk in memory, or the placement of the kernel image itself.
1443
1444 [0] https://github.com/u-boot/u-boot/commit/ec80b4735a593961fe701cc3a5d717d4739b0fd0
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