tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1/* SPDX-License-Identifier: GPL-2.0 */
2#include <linux/linkage.h>
3#include <asm/segment.h>
4#include <asm/page_types.h>
5#include <asm/processor-flags.h>
6#include <asm/msr-index.h>
7#include "realmode.h"
8
9/*
10 * The following code and data reboots the machine by switching to real
11 * mode and jumping to the BIOS reset entry point, as if the CPU has
12 * really been reset. The previous version asked the keyboard
13 * controller to pulse the CPU reset line, which is more thorough, but
14 * doesn't work with at least one type of 486 motherboard. It is easy
15 * to stop this code working; hence the copious comments.
16 *
17 * This code is called with the restart type (0 = BIOS, 1 = APM) in
18 * the primary argument register (%eax for 32 bit, %edi for 64 bit).
19 */
20 .section ".text32", "ax"
21 .code32
22SYM_CODE_START(machine_real_restart_asm)
23
24#ifdef CONFIG_X86_64
25 /* Switch to trampoline GDT as it is guaranteed < 4 GiB */
26 movl $__KERNEL_DS, %eax
27 movl %eax, %ds
28 lgdtl pa_tr_gdt
29
30 /* Disable paging to drop us out of long mode */
31 movl %cr0, %eax
32 andl $~X86_CR0_PG, %eax
33 movl %eax, %cr0
34 ljmpl $__KERNEL32_CS, $pa_machine_real_restart_paging_off
35
36SYM_INNER_LABEL(machine_real_restart_paging_off, SYM_L_GLOBAL)
37 xorl %eax, %eax
38 xorl %edx, %edx
39 movl $MSR_EFER, %ecx
40 wrmsr
41
42 movl %edi, %eax
43
44#endif /* CONFIG_X86_64 */
45
46 /* Set up the IDT for real mode. */
47 lidtl pa_machine_real_restart_idt
48
49 /*
50 * Set up a GDT from which we can load segment descriptors for real
51 * mode. The GDT is not used in real mode; it is just needed here to
52 * prepare the descriptors.
53 */
54 lgdtl pa_machine_real_restart_gdt
55
56 /*
57 * Load the data segment registers with 16-bit compatible values
58 */
59 movl $16, %ecx
60 movl %ecx, %ds
61 movl %ecx, %es
62 movl %ecx, %fs
63 movl %ecx, %gs
64 movl %ecx, %ss
65 ljmpw $8, $1f
66SYM_CODE_END(machine_real_restart_asm)
67
68/*
69 * This is 16-bit protected mode code to disable paging and the cache,
70 * switch to real mode and jump to the BIOS reset code.
71 *
72 * The instruction that switches to real mode by writing to CR0 must be
73 * followed immediately by a far jump instruction, which set CS to a
74 * valid value for real mode, and flushes the prefetch queue to avoid
75 * running instructions that have already been decoded in protected
76 * mode.
77 *
78 * Clears all the flags except ET, especially PG (paging), PE
79 * (protected-mode enable) and TS (task switch for coprocessor state
80 * save). Flushes the TLB after paging has been disabled. Sets CD and
81 * NW, to disable the cache on a 486, and invalidates the cache. This
82 * is more like the state of a 486 after reset. I don't know if
83 * something else should be done for other chips.
84 *
85 * More could be done here to set up the registers as if a CPU reset had
86 * occurred; hopefully real BIOSs don't assume much. This is not the
87 * actual BIOS entry point, anyway (that is at 0xfffffff0).
88 *
89 * Most of this work is probably excessive, but it is what is tested.
90 */
91 .text
92 .code16
93
94 .balign 16
95machine_real_restart_asm16:
961:
97 xorl %ecx, %ecx
98 movl %cr0, %edx
99 andl $0x00000011, %edx
100 orl $0x60000000, %edx
101 movl %edx, %cr0
102 movl %ecx, %cr3
103 movl %cr0, %edx
104 testl $0x60000000, %edx /* If no cache bits -> no wbinvd */
105 jz 2f
106 wbinvd
1072:
108 andb $0x10, %dl
109 movl %edx, %cr0
110 LJMPW_RM(3f)
1113:
112 andw %ax, %ax
113 jz bios
114
115apm:
116 movw $0x1000, %ax
117 movw %ax, %ss
118 movw $0xf000, %sp
119 movw $0x5307, %ax
120 movw $0x0001, %bx
121 movw $0x0003, %cx
122 int $0x15
123 /* This should never return... */
124
125bios:
126 ljmpw $0xf000, $0xfff0
127
128 .section ".rodata", "a"
129
130 .balign 16
131SYM_DATA_START(machine_real_restart_idt)
132 .word 0xffff /* Length - real mode default value */
133 .long 0 /* Base - real mode default value */
134SYM_DATA_END(machine_real_restart_idt)
135
136 .balign 16
137SYM_DATA_START(machine_real_restart_gdt)
138 /* Self-pointer */
139 .word 0xffff /* Length - real mode default value */
140 .long pa_machine_real_restart_gdt
141 .word 0
142
143 /*
144 * 16-bit code segment pointing to real_mode_seg
145 * Selector value 8
146 */
147 .word 0xffff /* Limit */
148 .long 0x9b000000 + pa_real_mode_base
149 .word 0
150
151 /*
152 * 16-bit data segment with the selector value 16 = 0x10 and
153 * base value 0x100; since this is consistent with real mode
154 * semantics we don't have to reload the segments once CR0.PE = 0.
155 */
156 .quad GDT_ENTRY(0x0093, 0x100, 0xffff)
157SYM_DATA_END(machine_real_restart_gdt)