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)
158