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/*
 * Switch back to real mode and call the BIOS reboot vector.
 * This is a trampoline copied around in process.c 
 * Written 2003 by Andi Kleen, SuSE Labs.               
 */                     
        
#include <asm/msr.h>

#define R(x) x-warm_reboot(%ebx)
#define R64(x) x-warm_reboot(%rbx)
                
        /* running in identity mapping and in the first 64k of memory
           and in compatibility mode. This must be position independent */

        /* Follows 14.7 "Leaving Long Mode" in the AMD x86-64 manual, volume 2
           and 8.9.2 "Switching Back to Real-Address Mode" in the Intel IA32
           manual, volume 2 */

        /* ebx: self pointer to warm_reboot */

        .globl warm_reboot
warm_reboot:
        addl  %ebx,  R64(real_mode_desc)        /* relocate tables */
        addl  %ebx,2+R64(warm_gdt_desc)         

        movq  %cr0,%rax
        btr  $31,%rax           
        movq %rax,%cr0          /* disable paging */
        jmp  1f                 /* flush prefetch queue */
        
        .code32 
1:      movl $MSR_EFER,%ecx
        rdmsr
        andl $~((1<<_EFER_LME)|(1<<_EFER_SCE)|(1<<_EFER_NX)),%eax
        wrmsr                   /* disable long mode in EFER */
                
        xorl %eax,%eax
        movl %eax,%cr3          /* flush tlb */

        /* Running protected mode without paging now */
        
        wbinvd                  /* flush caches. Needed? */

        lidt R(warm_idt_desc)
        lgdt R(warm_gdt_desc)
        
        movl $0x10,%ecx         /* load segment registers with real mode settings */
        movl %ecx,%ds
        movl %ecx,%es
        movl %ecx,%fs
        movl %ecx,%gs
        movl %ecx,%ss
        
        lea  R(real_mode_desc),%eax             
        ljmp *(%eax)

        .code16:
real_mode:
        xorl %eax,%eax
        movl %eax,%cr0
        
        /* some people claim $0xf000,0xfff0 is better. Use what 32bit linux uses. */
        /* code as bytes because gas has problems with it */
        .byte   0xea,0xf0,0xff,0x00,0xf0        /* ljmp  0xf000:0xfff0 */

real_mode_desc:
        .long  real_mode - warm_reboot
        .short 8        
warm_gdt_desc:
        .short 8*3
        .long warm_gdt - warm_reboot
warm_gdt:       
        .quad   0
        .quad   0x00009a000000ffff      /* 16-bit real-mode 64k code at 0x00000000 */
        .quad   0x000092000100ffff      /* 16-bit real-mode 64k data at 0x00000100 */
        
warm_idt_desc:
        .short 0x3ff
        .long  0
                                
        .globl warm_reboot_end
warm_reboot_end: