1 /* SPDX-License-Identifier: GPL-2.0 */
5 * Copyright (C) 1991, 1992, 1993 Linus Torvalds
9 * head.S contains the 32-bit startup code.
11 * NOTE!!! Startup happens at absolute address 0x00001000, which is also where
12 * the page directory will exist. The startup code will be overwritten by
13 * the page directory. [According to comments etc elsewhere on a compressed
14 * kernel it will end up at 0x1000 + 1Mb I hope so as I assume this. - AC]
16 * Page 0 is deliberately kept safe, since System Management Mode code in
17 * laptops may need to access the BIOS data stored there. This is also
18 * useful for future device drivers that either access the BIOS via VM86
23 * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
27 #include <linux/init.h>
28 #include <linux/linkage.h>
29 #include <asm/segment.h>
30 #include <asm/page_types.h>
32 #include <asm/asm-offsets.h>
33 #include <asm/bootparam.h>
36 * The 32-bit x86 assembler in binutils 2.26 will generate R_386_GOT32X
37 * relocation to get the symbol address in PIC. When the compressed x86
38 * kernel isn't built as PIC, the linker optimizes R_386_GOT32X
39 * relocations to their fixed symbol addresses. However, when the
40 * compressed x86 kernel is loaded at a different address, it leads
41 * to the following load failure:
43 * Failed to allocate space for phdrs
45 * during the decompression stage.
47 * If the compressed x86 kernel is relocatable at run-time, it should be
48 * compiled with -fPIE, instead of -fPIC, if possible and should be built as
49 * Position Independent Executable (PIE) so that linker won't optimize
50 * R_386_GOT32X relocation to its fixed symbol address. Older
51 * linkers generate R_386_32 relocations against locally defined symbols,
52 * _bss, _ebss, _got and _egot, in PIE. It isn't wrong, just less
53 * optimal than R_386_RELATIVE. But the x86 kernel fails to properly handle
54 * R_386_32 relocations when relocating the kernel. To generate
55 * R_386_RELATIVE relocations, we mark _bss, _ebss, _got and _egot as
64 SYM_FUNC_START(startup_32)
69 * Calculate the delta between where we were compiled to run
70 * at and where we were actually loaded at. This can only be done
71 * with a short local call on x86. Nothing else will tell us what
72 * address we are running at. The reserved chunk of the real-mode
73 * data at 0x1e4 (defined as a scratch field) are used as the stack
74 * for this calculation. Only 4 bytes are needed.
76 leal (BP_scratch+4)(%esi), %esp
86 /* Load segment registers with our descriptors */
95 * %ebp contains the address we are loaded at by the boot loader and %ebx
96 * contains the address where we should move the kernel image temporarily
97 * for safe in-place decompression.
100 #ifdef CONFIG_RELOCATABLE
102 movl BP_kernel_alignment(%esi), %eax
107 cmpl $LOAD_PHYSICAL_ADDR, %ebx
110 movl $LOAD_PHYSICAL_ADDR, %ebx
113 /* Target address to relocate to for decompression */
114 movl BP_init_size(%esi), %eax
118 /* Set up the stack */
119 leal boot_stack_end(%ebx), %esp
126 * Copy the compressed kernel to the end of our buffer
127 * where decompression in place becomes safe.
130 leal (_bss-4)(%ebp), %esi
131 leal (_bss-4)(%ebx), %edi
132 movl $(_bss - startup_32), %ecx
140 * The GDT may get overwritten either during the copy we just did or
141 * during extract_kernel below. To avoid any issues, repoint the GDTR
142 * to the new copy of the GDT. EAX still contains the previously
143 * calculated relocation offset of init_size - _end.
150 * Jump to the relocated address.
152 leal .Lrelocated(%ebx), %eax
154 SYM_FUNC_END(startup_32)
156 #ifdef CONFIG_EFI_STUB
157 SYM_FUNC_START(efi32_stub_entry)
158 SYM_FUNC_START_ALIAS(efi_stub_entry)
162 movl BP_code32_start(%esi), %eax
163 leal startup_32(%eax), %eax
165 SYM_FUNC_END(efi32_stub_entry)
166 SYM_FUNC_END_ALIAS(efi_stub_entry)
170 SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
173 * Clear BSS (stack is currently empty)
176 leal _bss(%ebx), %edi
177 leal _ebss(%ebx), %ecx
185 leal _got(%ebx), %edx
186 leal _egot(%ebx), %ecx
196 * Do the extraction, and jump to the new kernel..
198 /* push arguments for extract_kernel: */
199 pushl $z_output_len /* decompressed length, end of relocs */
201 leal _end(%ebx), %eax
202 subl BP_init_size(%esi), %eax
203 pushl %eax /* output address */
205 pushl $z_input_len /* input_len */
206 leal input_data(%ebx), %eax
207 pushl %eax /* input_data */
208 leal boot_heap(%ebx), %eax
209 pushl %eax /* heap area */
210 pushl %esi /* real mode pointer */
211 call extract_kernel /* returns kernel location in %eax */
215 * Jump to the extracted kernel.
219 SYM_FUNC_END(.Lrelocated)
223 SYM_DATA_START_LOCAL(gdt)
224 .word gdt_end - gdt - 1
227 .quad 0x0000000000000000 /* Reserved */
228 .quad 0x00cf9a000000ffff /* __KERNEL_CS */
229 .quad 0x00cf92000000ffff /* __KERNEL_DS */
230 SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
233 * Stack and heap for uncompression
238 .fill BOOT_HEAP_SIZE, 1, 0
240 .fill BOOT_STACK_SIZE, 1, 0