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/* SPDX-License-Identifier: GPL-2.0 */
* linux/boot/head.S
* Copyright (C) 1991, 1992, 1993 Linus Torvalds
* head.S contains the 32-bit startup code.
* NOTE!!! Startup happens at absolute address 0x00001000, which is also where
* the page directory will exist. The startup code will be overwritten by
* the page directory. [According to comments etc elsewhere on a compressed
* kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC]
* Page 0 is deliberately kept safe, since System Management Mode code in
* laptops may need to access the BIOS data stored there. This is also
* useful for future device drivers that either access the BIOS via VM86
* mode.
* High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
#include <linux/init.h>
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/page_types.h>
#include <asm/boot.h>
#include <asm/asm-offsets.h>
#include <asm/bootparam.h>
* The 32-bit x86 assembler in binutils 2.26 will generate R_386_GOT32X
* relocation to get the symbol address in PIC. When the compressed x86
* kernel isn't built as PIC, the linker optimizes R_386_GOT32X
* relocations to their fixed symbol addresses. However, when the
* compressed x86 kernel is loaded at a different address, it leads
* to the following load failure:
* Failed to allocate space for phdrs
* during the decompression stage.
* If the compressed x86 kernel is relocatable at run-time, it should be
* compiled with -fPIE, instead of -fPIC, if possible and should be built as
* Position Independent Executable (PIE) so that linker won't optimize
* R_386_GOT32X relocation to its fixed symbol address. Older
* linkers generate R_386_32 relocations against locally defined symbols,
* _bss, _ebss, _got and _egot, in PIE. It isn't wrong, just less
* optimal than R_386_RELATIVE. But the x86 kernel fails to properly handle
* R_386_32 relocations when relocating the kernel. To generate
* R_386_RELATIVE relocations, we mark _bss, _ebss, _got and _egot as
* hidden:
.hidden _bss
.hidden _ebss
.hidden _got
.hidden _egot
* Calculate the delta between where we were compiled to run
* at and where we were actually loaded at. This can only be done
* with a short local call on x86. Nothing else will tell us what
* address we are running at. The reserved chunk of the real-mode
* data at 0x1e4 (defined as a scratch field) are used as the stack
* for this calculation. Only 4 bytes are needed.
leal (BP_scratch+4)(%esi), %esp
call 1f
1: popl %edx
subl $1b, %edx
/* Load new GDT */
leal gdt(%edx), %eax
movl %eax, 2(%eax)
lgdt (%eax)
/* Load segment registers with our descriptors */
movl $__BOOT_DS, %eax
movl %eax, %ds
movl %eax, %es
movl %eax, %fs
movl %eax, %gs
movl %eax, %ss
* %edx contains the address we are loaded at by the boot loader and %ebx
* contains the address where we should move the kernel image temporarily
* for safe in-place decompression. %ebp contains the address that the kernel
* will be decompressed to.
movl %edx, %ebx
* If we were loaded via the EFI LoadImage service, startup_32() will be at an
* offset to the start of the space allocated for the image. efi_pe_entry() will
* set up image_offset to tell us where the image actually starts, so that we
* can use the full available buffer.
* image_offset = startup_32 - image_base
* Otherwise image_offset will be zero and has no effect on the calculations.
subl image_offset(%edx), %ebx
movl BP_kernel_alignment(%esi), %eax
decl %eax
addl %eax, %ebx
notl %eax
andl %eax, %ebx
jae 1f
movl %ebx, %ebp // Save the output address for later
/* Target address to relocate to for decompression */
addl BP_init_size(%esi), %ebx
subl $_end, %ebx
/* Set up the stack */
leal boot_stack_end(%ebx), %esp
/* Zero EFLAGS */
pushl $0
* Copy the compressed kernel to the end of our buffer
* where decompression in place becomes safe.
pushl %esi
leal (_bss-4)(%edx), %esi
leal (_bss-4)(%ebx), %edi
movl $(_bss - startup_32), %ecx
shrl $2, %ecx
rep movsl
popl %esi
* The GDT may get overwritten either during the copy we just did or
* during extract_kernel below. To avoid any issues, repoint the GDTR
* to the new copy of the GDT.
leal gdt(%ebx), %eax
movl %eax, 2(%eax)
lgdt (%eax)
* Jump to the relocated address.
leal .Lrelocated(%ebx), %eax
jmp *%eax
add $0x4, %esp
movl 8(%esp), %esi /* save boot_params pointer */
call efi_main
leal startup_32(%eax), %eax
jmp *%eax
* Clear BSS (stack is currently empty)
xorl %eax, %eax
leal _bss(%ebx), %edi
leal _ebss(%ebx), %ecx
subl %edi, %ecx
shrl $2, %ecx
rep stosl
* Adjust our own GOT
leal _got(%ebx), %edx
leal _egot(%ebx), %ecx
cmpl %ecx, %edx
jae 2f
addl %ebx, (%edx)
addl $4, %edx
jmp 1b
* Do the extraction, and jump to the new kernel..
/* push arguments for extract_kernel: */
pushl $z_output_len /* decompressed length, end of relocs */
pushl %ebp /* output address */
pushl $z_input_len /* input_len */
leal input_data(%ebx), %eax
pushl %eax /* input_data */
leal boot_heap(%ebx), %eax
pushl %eax /* heap area */
pushl %esi /* real mode pointer */
call extract_kernel /* returns kernel location in %eax */
addl $24, %esp
* Jump to the extracted kernel.
xorl %ebx, %ebx
jmp *%eax
.balign 8
.word gdt_end - gdt - 1
.long 0
.word 0
.quad 0x0000000000000000 /* Reserved */
.quad 0x00cf9a000000ffff /* __KERNEL_CS */
.quad 0x00cf92000000ffff /* __KERNEL_DS */
SYM_DATA(image_offset, .long 0)
* Stack and heap for uncompression
.balign 4
.fill BOOT_HEAP_SIZE, 1, 0
.fill BOOT_STACK_SIZE, 1, 0