1/* SPDX-License-Identifier: GPL-2.0 */
2#include <linux/linkage.h>
3#include <asm/desc_defs.h>
4#include <asm/segment.h>
5#include <asm/page_types.h>
6#include <asm/processor-flags.h>
7#include <asm/msr-index.h>
8#include "realmode.h"
9
10/*
11 * The following code and data reboots the machine by switching to real
12 * mode and jumping to the BIOS reset entry point, as if the CPU has
13 * really been reset.  The previous version asked the keyboard
14 * controller to pulse the CPU reset line, which is more thorough, but
15 * doesn't work with at least one type of 486 motherboard.  It is easy
16 * to stop this code working; hence the copious comments.
17 *
18 * This code is called with the restart type (0 = BIOS, 1 = APM) in
19 * the primary argument register (%eax for 32 bit, %edi for 64 bit).
20 */
21	.section ".text32", "ax"
22	.code32
23SYM_CODE_START(machine_real_restart_asm)
24
25#ifdef CONFIG_X86_64
26	/* Switch to trampoline GDT as it is guaranteed < 4 GiB */
27	movl	$__KERNEL_DS, %eax
28	movl	%eax, %ds
29	lgdtl	pa_tr_gdt
30
31	/* Disable paging to drop us out of long mode */
32	movl	%cr0, %eax
33	andl	$~X86_CR0_PG, %eax
34	movl	%eax, %cr0
35	ljmpl	$__KERNEL32_CS, $pa_machine_real_restart_paging_off
36
37SYM_INNER_LABEL(machine_real_restart_paging_off, SYM_L_GLOBAL)
38	xorl	%eax, %eax
39	xorl	%edx, %edx
40	movl	$MSR_EFER, %ecx
41	wrmsr
42
43	movl	%edi, %eax
44
45#endif /* CONFIG_X86_64 */
46
47	/* Set up the IDT for real mode. */
48	lidtl	pa_machine_real_restart_idt
49
50	/*
51	 * Set up a GDT from which we can load segment descriptors for real
52	 * mode.  The GDT is not used in real mode; it is just needed here to
53	 * prepare the descriptors.
54	 */
55	lgdtl	pa_machine_real_restart_gdt
56
57	/*
58	 * Load the data segment registers with 16-bit compatible values
59	 */
60	movl	$16, %ecx
61	movl	%ecx, %ds
62	movl	%ecx, %es
63	movl	%ecx, %fs
64	movl	%ecx, %gs
65	movl	%ecx, %ss
66	ljmpw	$8, $1f
67SYM_CODE_END(machine_real_restart_asm)
68
69/*
70 * This is 16-bit protected mode code to disable paging and the cache,
71 * switch to real mode and jump to the BIOS reset code.
72 *
73 * The instruction that switches to real mode by writing to CR0 must be
74 * followed immediately by a far jump instruction, which set CS to a
75 * valid value for real mode, and flushes the prefetch queue to avoid
76 * running instructions that have already been decoded in protected
77 * mode.
78 *
79 * Clears all the flags except ET, especially PG (paging), PE
80 * (protected-mode enable) and TS (task switch for coprocessor state
81 * save).  Flushes the TLB after paging has been disabled.  Sets CD and
82 * NW, to disable the cache on a 486, and invalidates the cache.  This
83 * is more like the state of a 486 after reset.  I don't know if
84 * something else should be done for other chips.
85 *
86 * More could be done here to set up the registers as if a CPU reset had
87 * occurred; hopefully real BIOSs don't assume much.  This is not the
88 * actual BIOS entry point, anyway (that is at 0xfffffff0).
89 *
90 * Most of this work is probably excessive, but it is what is tested.
91 */
92	.text
93	.code16
94
95	.balign	16
96machine_real_restart_asm16:
971:
98	xorl	%ecx, %ecx
99	movl	%cr0, %edx
100	andl	$0x00000011, %edx
101	orl	$0x60000000, %edx
102	movl	%edx, %cr0
103	movl	%ecx, %cr3
104	movl	%cr0, %edx
105	testl	$0x60000000, %edx	/* If no cache bits -> no wbinvd */
106	jz	2f
107	wbinvd
1082:
109	andb	$0x10, %dl
110	movl	%edx, %cr0
111	LJMPW_RM(3f)
1123:
113	andw	%ax, %ax
114	jz	bios
115
116apm:
117	movw	$0x1000, %ax
118	movw	%ax, %ss
119	movw	$0xf000, %sp
120	movw	$0x5307, %ax
121	movw	$0x0001, %bx
122	movw	$0x0003, %cx
123	int	$0x15
124	/* This should never return... */
125
126bios:
127	ljmpw	$0xf000, $0xfff0
128
129	.section ".rodata", "a"
130
131	.balign	16
132SYM_DATA_START(machine_real_restart_idt)
133	.word	0xffff		/* Length - real mode default value */
134	.long	0		/* Base - real mode default value */
135SYM_DATA_END(machine_real_restart_idt)
136
137	.balign	16
138SYM_DATA_START(machine_real_restart_gdt)
139	/* Self-pointer */
140	.word	0xffff		/* Length - real mode default value */
141	.long	pa_machine_real_restart_gdt
142	.word	0
143
144	/*
145	 * 16-bit code segment pointing to real_mode_seg
146	 * Selector value 8
147	 */
148	.word	0xffff		/* Limit */
149	.long	0x9b000000 + pa_real_mode_base
150	.word	0
151
152	/*
153	 * 16-bit data segment with the selector value 16 = 0x10 and
154	 * base value 0x100; since this is consistent with real mode
155	 * semantics we don't have to reload the segments once CR0.PE = 0.
156	 */
157	.quad	GDT_ENTRY(DESC_DATA16, 0x100, 0xffff)
158SYM_DATA_END(machine_real_restart_gdt)
159