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