1#include <linux/linkage.h> 2#include <asm/segment.h> 3#include <asm/page_types.h> 4#include <asm/processor-flags.h> 5#include <asm/msr-index.h> 6#include "realmode.h" 7 8/* 9 * The following code and data reboots the machine by switching to real 10 * mode and jumping to the BIOS reset entry point, as if the CPU has 11 * really been reset. The previous version asked the keyboard 12 * controller to pulse the CPU reset line, which is more thorough, but 13 * doesn't work with at least one type of 486 motherboard. It is easy 14 * to stop this code working; hence the copious comments. 15 * 16 * This code is called with the restart type (0 = BIOS, 1 = APM) in 17 * the primary argument register (%eax for 32 bit, %edi for 64 bit). 18 */ 19 .section ".text32", "ax" 20 .code32 21ENTRY(machine_real_restart_asm) 22 23#ifdef CONFIG_X86_64 24 /* Switch to trampoline GDT as it is guaranteed < 4 GiB */ 25 movl $__KERNEL_DS, %eax 26 movl %eax, %ds 27 lgdtl pa_tr_gdt 28 29 /* Disable paging to drop us out of long mode */ 30 movl %cr0, %eax 31 andl $~X86_CR0_PG, %eax 32 movl %eax, %cr0 33 ljmpl $__KERNEL32_CS, $pa_machine_real_restart_paging_off 34 35GLOBAL(machine_real_restart_paging_off) 36 xorl %eax, %eax 37 xorl %edx, %edx 38 movl $MSR_EFER, %ecx 39 wrmsr 40 41 movl %edi, %eax 42 43#endif /* CONFIG_X86_64 */ 44 45 /* Set up the IDT for real mode. */ 46 lidtl pa_machine_real_restart_idt 47 48 /* 49 * Set up a GDT from which we can load segment descriptors for real 50 * mode. The GDT is not used in real mode; it is just needed here to 51 * prepare the descriptors. 52 */ 53 lgdtl pa_machine_real_restart_gdt 54 55 /* 56 * Load the data segment registers with 16-bit compatible values 57 */ 58 movl $16, %ecx 59 movl %ecx, %ds 60 movl %ecx, %es 61 movl %ecx, %fs 62 movl %ecx, %gs 63 movl %ecx, %ss 64 ljmpw $8, $1f 65 66/* 67 * This is 16-bit protected mode code to disable paging and the cache, 68 * switch to real mode and jump to the BIOS reset code. 69 * 70 * The instruction that switches to real mode by writing to CR0 must be 71 * followed immediately by a far jump instruction, which set CS to a 72 * valid value for real mode, and flushes the prefetch queue to avoid 73 * running instructions that have already been decoded in protected 74 * mode. 75 * 76 * Clears all the flags except ET, especially PG (paging), PE 77 * (protected-mode enable) and TS (task switch for coprocessor state 78 * save). Flushes the TLB after paging has been disabled. Sets CD and 79 * NW, to disable the cache on a 486, and invalidates the cache. This 80 * is more like the state of a 486 after reset. I don't know if 81 * something else should be done for other chips. 82 * 83 * More could be done here to set up the registers as if a CPU reset had 84 * occurred; hopefully real BIOSs don't assume much. This is not the 85 * actual BIOS entry point, anyway (that is at 0xfffffff0). 86 * 87 * Most of this work is probably excessive, but it is what is tested. 88 */ 89 .text 90 .code16 91 92 .balign 16 93machine_real_restart_asm16: 941: 95 xorl %ecx, %ecx 96 movl %cr0, %edx 97 andl $0x00000011, %edx 98 orl $0x60000000, %edx 99 movl %edx, %cr0 100 movl %ecx, %cr3 101 movl %cr0, %edx 102 testl $0x60000000, %edx /* If no cache bits -> no wbinvd */ 103 jz 2f 104 wbinvd 1052: 106 andb $0x10, %dl 107 movl %edx, %cr0 108 LJMPW_RM(3f) 1093: 110 andw %ax, %ax 111 jz bios 112 113apm: 114 movw $0x1000, %ax 115 movw %ax, %ss 116 movw $0xf000, %sp 117 movw $0x5307, %ax 118 movw $0x0001, %bx 119 movw $0x0003, %cx 120 int $0x15 121 /* This should never return... */ 122 123bios: 124 ljmpw $0xf000, $0xfff0 125 126 .section ".rodata", "a" 127 128 .balign 16 129GLOBAL(machine_real_restart_idt) 130 .word 0xffff /* Length - real mode default value */ 131 .long 0 /* Base - real mode default value */ 132END(machine_real_restart_idt) 133 134 .balign 16 135GLOBAL(machine_real_restart_gdt) 136 /* Self-pointer */ 137 .word 0xffff /* Length - real mode default value */ 138 .long pa_machine_real_restart_gdt 139 .word 0 140 141 /* 142 * 16-bit code segment pointing to real_mode_seg 143 * Selector value 8 144 */ 145 .word 0xffff /* Limit */ 146 .long 0x9b000000 + pa_real_mode_base 147 .word 0 148 149 /* 150 * 16-bit data segment with the selector value 16 = 0x10 and 151 * base value 0x100; since this is consistent with real mode 152 * semantics we don't have to reload the segments once CR0.PE = 0. 153 */ 154 .quad GDT_ENTRY(0x0093, 0x100, 0xffff) 155END(machine_real_restart_gdt) 156