diff options
Diffstat (limited to 'arch/x86/realmode/rm/reboot.S')
| -rw-r--r-- | arch/x86/realmode/rm/reboot.S | 157 |
1 files changed, 157 insertions, 0 deletions
diff --git a/arch/x86/realmode/rm/reboot.S b/arch/x86/realmode/rm/reboot.S new file mode 100644 index 000000000..f10515b10 --- /dev/null +++ b/arch/x86/realmode/rm/reboot.S @@ -0,0 +1,157 @@ +/* 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 +SYM_CODE_START(machine_real_restart_asm) + +#ifdef CONFIG_X86_64 + /* 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 + +SYM_INNER_LABEL(machine_real_restart_paging_off, SYM_L_GLOBAL) + 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 +SYM_CODE_END(machine_real_restart_asm) + +/* + * 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 +SYM_DATA_START(machine_real_restart_idt) + .word 0xffff /* Length - real mode default value */ + .long 0 /* Base - real mode default value */ +SYM_DATA_END(machine_real_restart_idt) + + .balign 16 +SYM_DATA_START(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) +SYM_DATA_END(machine_real_restart_gdt) |