1 /*
   2  * Parts copyright Michael Brown <mbrown@fensystems.co.uk>
   3  *
   4  * Copyright 2020 Joyent, Inc.
   5  */
   6 
   7 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
   8 
   9 /* CR0: protection enabled */
  10 #define CR0_PE ( 1 << 0 )
  11 
  12 /* CR0: paging */
  13 #define CR0_PG ( 1 << 31 )
  14 
  15 /* CR4: physical address extensions */
  16 #define CR4_PSE ( 1 << 4 )
  17 #define CR4_PAE ( 1 << 5 )
  18 #define CR4_PGE ( 1 << 7 )
  19 
  20 /* Extended feature enable MSR (EFER) */
  21 #define MSR_EFER 0xc0000080
  22 
  23 /* EFER: long mode enable */
  24 #define EFER_LME ( 1 << 8 )
  25 
  26 #define GDTSEL_CODE 0x8
  27 #define GDTSEL_DATA 0x10
  28 
  29 #if defined(PXE_EFI) && defined(__x86_64__)
  30 
  31         .section ".text", "ax", @progbits
  32 
  33         .data
  34 
  35 entry_gdt:
  36         /* null entry */
  37         .word   0x0, 0x0
  38         .byte   0x0, 0x0, 0x0, 0x0
  39 
  40         /* 32 bit protected mode code segment */
  41         .word   0xffff, 0x0
  42         .byte   0x0, 0x9f, 0xcf, 0x0
  43 
  44         /* 32 bit protected mode data segment */
  45         .word   0xffff, 0x0
  46         .byte   0x0, 0x93, 0xcf, 0x0
  47 
  48 entry_gdt_end:
  49         .equ    entry_gdt_length, entry_gdt_end - entry_gdt
  50 
  51 entry_gdtr:
  52         .word entry_gdt_length - 1
  53 entry_gdt_base:
  54         .quad 0
  55 
  56         /*
  57          * %rdi -> struct mb2 *
  58          * %rsi -> stack pointer to switch to
  59          * %rdx -> &multiboot2_enter_kernel
  60          */
  61         .code64
  62         .globl multiboot2_bounce
  63 
  64 multiboot2_bounce:
  65         movq    %rsi, %rsp
  66         jmp     *%rdx
  67 
  68         /*
  69          * %rdi -> multiboot2 magic
  70          * %rsi -> multiboot info pointer
  71          * %rdx -> entry address (32 bits)
  72          *
  73          *
  74          * We need to transition from our 64-bit environment into the one
  75          * defined by the multiboot2 spec, section 3.3. Namely, drop down to
  76          * 32-bit protected mode with a basic GDT, paging disabled, interrupts
  77          * off, and
  78          *
  79          * %eax -> multiboot2 magic
  80          * %ebx -> multiboot info pointer (physical)
  81          */
  82         .globl multiboot2_entry
  83 
  84 multiboot2_entry:
  85         cli
  86 
  87         movq    %rsi, %rbx /* mb2 infop */
  88         movq    %rdx, %rsi /* entry address */
  89 
  90         /* Load the mb2-mandated code and data segments.  */
  91         leaq    entry_gdt_base(%rip), %rcx
  92         leaq    entry_gdt(%rip), %rax
  93         movq    %rax, (%rcx)
  94 
  95         leaq    entry_gdtr(%rip), %rax
  96         lgdt    (%rax)
  97 
  98         /* Load our new %cs. */
  99         movq    %rsp, %rax
 100         pushq   $GDTSEL_DATA
 101         pushq   %rax
 102         pushf
 103         pushq   $GDTSEL_CODE
 104         lea     newcs(%rip), %rax
 105         pushq   %rax
 106         iretq
 107 
 108 
 109         .code32
 110 newcs:
 111 
 112         movw    $GDTSEL_DATA, %ax
 113         movw    %ax, %ds
 114         movw    %ax, %es
 115         movw    %ax, %fs
 116         movw    %ax, %gs
 117         movw    %ax, %ss
 118 
 119         /* Disable paging */
 120         movl    %cr0, %eax
 121         andl    $~CR0_PG, %eax
 122         movl    %eax, %cr0
 123 
 124         movl    %cr4, %eax
 125         andb    $~(CR4_PAE | CR4_PGE | CR4_PSE), %al
 126         movl    %eax, %cr4
 127 
 128         /* Disable long mode (clobbers %eax, %edx) */
 129         movl    $MSR_EFER, %ecx
 130         rdmsr
 131         andw    $~EFER_LME, %ax
 132         wrmsr
 133 
 134         /* %ebx still has our infop */
 135         movl    %edi, %eax
 136         jmp     *%esi
 137 
 138 #endif /* PXE_EFI && __x86_64__ */