1/* SPDX-License-Identifier: GPL-2.0 */
2/*
3 * linux/boot/head.S
4 *
5 * Copyright (C) 1991, 1992, 1993 Linus Torvalds
6 */
7
8/*
9 * head.S contains the 32-bit startup code.
10 *
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]
15 *
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
19 * mode.
20 */
21
22/*
23 * High loaded stuff by Hans Lermen & Werner Almesberger, Feb. 1996
24 */
25 .text
26
27#include <linux/init.h>
28#include <linux/linkage.h>
29#include <asm/segment.h>
30#include <asm/page_types.h>
31#include <asm/boot.h>
32#include <asm/asm-offsets.h>
33#include <asm/bootparam.h>
34
35/*
36 * These symbols needed to be marked as .hidden to prevent the BFD linker from
37 * generating R_386_32 (rather than R_386_RELATIVE) relocations for them when
38 * the 32-bit compressed kernel is linked as PIE. This is no longer necessary,
39 * but it doesn't hurt to keep them .hidden.
40 */
41 .hidden _bss
42 .hidden _ebss
43 .hidden _end
44
45 __HEAD
46SYM_FUNC_START(startup_32)
47 cld
48 cli
49
50/*
51 * Calculate the delta between where we were compiled to run
52 * at and where we were actually loaded at. This can only be done
53 * with a short local call on x86. Nothing else will tell us what
54 * address we are running at. The reserved chunk of the real-mode
55 * data at 0x1e4 (defined as a scratch field) are used as the stack
56 * for this calculation. Only 4 bytes are needed.
57 */
58 leal (BP_scratch+4)(%esi), %esp
59 call 1f
601: popl %edx
61 addl $_GLOBAL_OFFSET_TABLE_+(.-1b), %edx
62
63 /* Load new GDT */
64 leal gdt@GOTOFF(%edx), %eax
65 movl %eax, 2(%eax)
66 lgdt (%eax)
67
68 /* Load segment registers with our descriptors */
69 movl $__BOOT_DS, %eax
70 movl %eax, %ds
71 movl %eax, %es
72 movl %eax, %fs
73 movl %eax, %gs
74 movl %eax, %ss
75
76/*
77 * %edx contains the address we are loaded at by the boot loader (plus the
78 * offset to the GOT). The below code calculates %ebx to be the address where
79 * we should move the kernel image temporarily for safe in-place decompression
80 * (again, plus the offset to the GOT).
81 *
82 * %ebp is calculated to be the address that the kernel will be decompressed to.
83 */
84
85#ifdef CONFIG_RELOCATABLE
86 leal startup_32@GOTOFF(%edx), %ebx
87 movl BP_kernel_alignment(%esi), %eax
88 decl %eax
89 addl %eax, %ebx
90 notl %eax
91 andl %eax, %ebx
92 cmpl $LOAD_PHYSICAL_ADDR, %ebx
93 jae 1f
94#endif
95 movl $LOAD_PHYSICAL_ADDR, %ebx
961:
97
98 movl %ebx, %ebp // Save the output address for later
99 /* Target address to relocate to for decompression */
100 addl BP_init_size(%esi), %ebx
101 subl $_end@GOTOFF, %ebx
102
103 /* Set up the stack */
104 leal boot_stack_end@GOTOFF(%ebx), %esp
105
106 /* Zero EFLAGS */
107 pushl $0
108 popfl
109
110/*
111 * Copy the compressed kernel to the end of our buffer
112 * where decompression in place becomes safe.
113 */
114 pushl %esi
115 leal (_bss@GOTOFF-4)(%edx), %esi
116 leal (_bss@GOTOFF-4)(%ebx), %edi
117 movl $(_bss - startup_32), %ecx
118 shrl $2, %ecx
119 std
120 rep movsl
121 cld
122 popl %esi
123
124 /*
125 * The GDT may get overwritten either during the copy we just did or
126 * during extract_kernel below. To avoid any issues, repoint the GDTR
127 * to the new copy of the GDT.
128 */
129 leal gdt@GOTOFF(%ebx), %eax
130 movl %eax, 2(%eax)
131 lgdt (%eax)
132
133/*
134 * Jump to the relocated address.
135 */
136 leal .Lrelocated@GOTOFF(%ebx), %eax
137 jmp *%eax
138SYM_FUNC_END(startup_32)
139
140 .text
141SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
142
143/*
144 * Clear BSS (stack is currently empty)
145 */
146 xorl %eax, %eax
147 leal _bss@GOTOFF(%ebx), %edi
148 leal _ebss@GOTOFF(%ebx), %ecx
149 subl %edi, %ecx
150 shrl $2, %ecx
151 rep stosl
152
153/*
154 * Do the extraction, and jump to the new kernel..
155 */
156 /* push arguments for extract_kernel: */
157
158 pushl %ebp /* output address */
159 pushl %esi /* real mode pointer */
160 call extract_kernel /* returns kernel entry point in %eax */
161 addl $24, %esp
162
163/*
164 * Jump to the extracted kernel.
165 */
166 xorl %ebx, %ebx
167 jmp *%eax
168SYM_FUNC_END(.Lrelocated)
169
170 .data
171 .balign 8
172SYM_DATA_START_LOCAL(gdt)
173 .word gdt_end - gdt - 1
174 .long 0
175 .word 0
176 .quad 0x0000000000000000 /* Reserved */
177 .quad 0x00cf9a000000ffff /* __KERNEL_CS */
178 .quad 0x00cf92000000ffff /* __KERNEL_DS */
179SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
180
181/*
182 * Stack and heap for uncompression
183 */
184 .bss
185 .balign 4
186boot_stack:
187 .fill BOOT_STACK_SIZE, 1, 0
188boot_stack_end:
189

source code of linux/arch/x86/boot/compressed/head_32.S