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-rw-r--r--arch/x86/kernel/reboot_32.S135
1 files changed, 135 insertions, 0 deletions
diff --git a/arch/x86/kernel/reboot_32.S b/arch/x86/kernel/reboot_32.S
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+++ b/arch/x86/kernel/reboot_32.S
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+#include <linux/linkage.h>
+#include <linux/init.h>
+#include <asm/segment.h>
+#include <asm/page_types.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 %eax.
+ */
+ .section ".x86_trampoline","a"
+ .balign 16
+ .code32
+ENTRY(machine_real_restart_asm)
+r_base = .
+ /* Get our own relocated address */
+ call 1f
+1: popl %ebx
+ subl $1b, %ebx
+
+ /* Compute the equivalent real-mode segment */
+ movl %ebx, %ecx
+ shrl $4, %ecx
+
+ /* Patch post-real-mode segment jump */
+ movw dispatch_table(%ebx,%eax,2),%ax
+ movw %ax, 101f(%ebx)
+ movw %cx, 102f(%ebx)
+
+ /* Set up the IDT for real mode. */
+ lidtl machine_real_restart_idt(%ebx)
+
+ /*
+ * 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 machine_real_restart_gdt(%ebx)
+
+ /*
+ * 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
+ ljmpl $8, $1f - r_base
+
+/*
+ * 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.
+ */
+ .code16
+1:
+ xorl %ecx, %ecx
+ movl %cr0, %eax
+ andl $0x00000011, %eax
+ orl $0x60000000, %eax
+ movl %eax, %cr0
+ movl %ecx, %cr3
+ movl %cr0, %edx
+ andl $0x60000000, %edx /* If no cache bits -> no wbinvd */
+ jz 2f
+ wbinvd
+2:
+ andb $0x10, %al
+ movl %eax, %cr0
+ .byte 0xea /* ljmpw */
+101: .word 0 /* Offset */
+102: .word 0 /* Segment */
+
+bios:
+ ljmpw $0xf000, $0xfff0
+
+apm:
+ movw $0x1000, %ax
+ movw %ax, %ss
+ movw $0xf000, %sp
+ movw $0x5307, %ax
+ movw $0x0001, %bx
+ movw $0x0003, %cx
+ int $0x15
+
+END(machine_real_restart_asm)
+
+ .balign 16
+ /* These must match <asm/reboot.h */
+dispatch_table:
+ .word bios - r_base
+ .word apm - r_base
+END(dispatch_table)
+
+ .balign 16
+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
+ENTRY(machine_real_restart_gdt)
+ .quad 0 /* Self-pointer, filled in by PM code */
+ .quad 0 /* 16-bit code segment, filled in by PM code */
+ /*
+ * 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)
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