reboot.S

  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)