/* SPDX-License-Identifier: GPL-2.0 */
#include <linux/linkage.h>
#include <asm/segment.h>
#include <asm/page_types.h>
#include <asm/processor-flags.h>
#include <asm/msr-index.h>
#include "realmode.h"
/*
* The following code and data reboots the machine by switching to real
* mode and jumping to the BIOS reset entry point, as if the CPU has
* really been reset. The previous version asked the keyboard
* controller to pulse the CPU reset line, which is more thorough, but
* doesn't work with at least one type of 486 motherboard. It is easy
* to stop this code working; hence the copious comments.
*
* This code is called with the restart type (0 = BIOS, 1 = APM) in
* the primary argument register (%eax for 32 bit, %edi for 64 bit).
*/
.section ".text32", "ax"
.code32
ENTRY(machine_real_restart_asm)
#ifdef [31mCONFIG_X86_64[0m
/* Switch to trampoline GDT as it is guaranteed < 4 GiB */
movl $__KERNEL_DS, %eax
movl %eax, %ds
lgdtl pa_tr_gdt
/* Disable paging to drop us out of long mode */
movl %cr0, %eax
andl $~X86_CR0_PG, %eax
movl %eax, %cr0
ljmpl $__KERNEL32_CS, $pa_machine_real_restart_paging_off
GLOBAL(machine_real_restart_paging_off)
xorl %eax, %eax
xorl %edx, %edx
movl $MSR_EFER, %ecx
wrmsr
movl %edi, %eax
#endif /* CONFIG_X86_64 */
/* Set up the IDT for real mode. */
lidtl pa_machine_real_restart_idt
/*
* Set up a GDT from which we can load segment descriptors for real
* mode. The GDT is not used in real mode; it is just needed here to
* prepare the descriptors.
*/
lgdtl pa_machine_real_restart_gdt
/*
* Load the data segment registers with 16-bit compatible values
*/
movl $16, %ecx
movl %ecx, %ds
movl %ecx, %es
movl %ecx, %fs
movl %ecx, %gs
movl %ecx, %ss
ljmpw $8, $1f
/*
* This is 16-bit protected mode code to disable paging and the cache,
* switch to real mode and jump to the BIOS reset code.
*
* The instruction that switches to real mode by writing to CR0 must be
* followed immediately by a far jump instruction, which set CS to a
* valid value for real mode, and flushes the prefetch queue to avoid
* running instructions that have already been decoded in protected
* mode.
*
* Clears all the flags except ET, especially PG (paging), PE
* (protected-mode enable) and TS (task switch for coprocessor state
* save). Flushes the TLB after paging has been disabled. Sets CD and
* NW, to disable the cache on a 486, and invalidates the cache. This
* is more like the state of a 486 after reset. I don't know if
* something else should be done for other chips.
*
* More could be done here to set up the registers as if a CPU reset had
* occurred; hopefully real BIOSs don't assume much. This is not the
* actual BIOS entry point, anyway (that is at 0xfffffff0).
*
* Most of this work is probably excessive, but it is what is tested.
*/
.text
.code16
.balign 16
machine_real_restart_asm16:
1:
xorl %ecx, %ecx
movl %cr0, %edx
andl $0x00000011, %edx
orl $0x60000000, %edx
movl %edx, %cr0
movl %ecx, %cr3
movl %cr0, %edx
testl $0x60000000, %edx /* If no cache bits -> no wbinvd */
jz 2f
wbinvd
2:
andb $0x10, %dl
movl %edx, %cr0
LJMPW_RM(3f)
3:
andw %ax, %ax
jz bios
apm:
movw $0x1000, %ax
movw %ax, %ss
movw $0xf000, %sp
movw $0x5307, %ax
movw $0x0001, %bx
movw $0x0003, %cx
int $0x15
/* This should never return... */
bios:
ljmpw $0xf000, $0xfff0
.section ".rodata", "a"
.balign 16
GLOBAL(machine_real_restart_idt)
.word 0xffff /* Length - real mode default value */
.long 0 /* Base - real mode default value */
END(machine_real_restart_idt)
.balign 16
GLOBAL(machine_real_restart_gdt)
/* Self-pointer */
.word 0xffff /* Length - real mode default value */
.long pa_machine_real_restart_gdt
.word 0
/*
* 16-bit code segment pointing to real_mode_seg
* Selector value 8
*/
.word 0xffff /* Limit */
.long 0x9b000000 + pa_real_mode_base
.word 0
/*
* 16-bit data segment with the selector value 16 = 0x10 and
* base value 0x100; since this is consistent with real mode
* semantics we don't have to reload the segments once CR0.PE = 0.
*/
.quad GDT_ENTRY(0x0093, 0x100, 0xffff)
END(machine_real_restart_gdt)