github.com/u-root/u-root@v7.0.1-0.20200915234505-ad7babab0a8e+incompatible/pkg/boot/multiboot/internal/trampoline/trampoline_linux_amd64.s (about)

     1  // Copyright 2018 the u-root Authors. All rights reserved
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // We want all the trampoline's assembly code to be located
     6  // in a contiguous byte range in a compiled binary.
     7  // Go compiler does not guarantee that. Still, current version
     8  // of compiler puts all pieces together.
     9  
    10  #include "textflag.h"
    11  
    12  #define MSR_EFER	0xC0000080
    13  #define EFER_LME	0xFFFFFEFF
    14  #define CR0_PG		0x0FFFFFFF
    15  
    16  #define DATA_SEGMENT	0x00CF92000000FFFF
    17  #define CODE_SEGMENT	0x00CF9A000000FFFF
    18  
    19  TEXT ·start(SB),NOSPLIT,$0
    20  	// Create GDT pointer on stack.
    21  	LEAQ	gdt(SB), CX
    22  	SHLQ	$16, CX
    23  	ORQ	$(4*8 - 1), CX
    24  	PUSHQ	CX
    25  
    26  	LGDT	(SP)
    27  
    28  	// Store value of multiboot info addr in BX.
    29  	// Don't modify BX.
    30  	MOVL	·info(SB), BX
    31  
    32  	// Store value of mu(l)tiboot magic in SI.
    33  	// Don't modify SI.
    34  	MOVL	·magic(SB), SI
    35  
    36  	// Far return doesn't work on QEMU in 64-bit mode,
    37  	// let's do far jump.
    38  	//
    39  	// In a regular plan9 assembly we can do something like:
    40  	//	BYTE	$0xFF; BYTE $0x2D
    41  	//	LONG	$bootaddr(SB)
    42  	// TEXT bootaddr(SB),NOSPLIT,$0
    43  	//	LONG	$boot(SB)
    44  	//	LONG	$0x8
    45  	//
    46  	// Go compiler doesn't let us do it.
    47  	//
    48  	// Setup offset to make a far jump from boot(SB)
    49  	// to a final kernel in a 32-bit mode.
    50  	MOVL	·entry(SB), AX
    51  	MOVL	AX, farjump32+1(SB)
    52  
    53  	// Setup offset to make a far jump to boot(SB)
    54  	// to switch from 64-bit mode to 32-bit mode.
    55  	LEAQ	boot(SB), CX
    56  	MOVL	CX, farjump64+6(SB)
    57  	JMP	farjump64(SB)
    58  
    59  
    60  TEXT boot(SB),NOSPLIT,$0
    61  	// We are in 32-bit mode now.
    62  	//
    63  	// Be careful editing this code!!! Go compiler
    64  	// interprets all commands as 64-bit commands.
    65  
    66  	// Disable paging.
    67  	MOVL	CR0, AX
    68  	ANDL	$CR0_PG, AX
    69  	MOVL	AX, CR0
    70  
    71  	// Disable long mode.
    72  	MOVL	$MSR_EFER, CX
    73  	RDMSR
    74  	ANDL	$EFER_LME, AX
    75  	WRMSR
    76  
    77  	// Disable PAE.
    78  	XORL	AX, AX
    79  	MOVL	AX, CR4
    80  
    81  	// Load data segments.
    82  	MOVL	$0x10, AX // GDT 0x10 data segment
    83  	BYTE	$0x8e; BYTE $0xd8 // MOVL AX, DS
    84  	BYTE	$0x8e; BYTE $0xc0 // MOVL AX, ES
    85  	BYTE	$0x8e; BYTE $0xd0 // MOVL AX, SS
    86  	BYTE	$0x8e; BYTE $0xe0 // MOVL AX, FS
    87  	BYTE	$0x8e; BYTE $0xe8 // MOVL AX, GS
    88  
    89  	// We stored the magic in SI before the far jump.
    90  	MOVL	SI, AX
    91  	JMP	farjump32(SB)
    92  
    93  TEXT farjump64(SB),NOSPLIT,$0
    94  	BYTE	$0xFF; BYTE $0x2D; LONG $0x0 // ljmp *(ip)
    95  
    96  	LONG	$0x0 // farjump64+6(SB)
    97  	LONG	$0x8 // code segment
    98  
    99  TEXT farjump32(SB),NOSPLIT,$0
   100  	// ljmp $0x18, offset
   101  	BYTE	$0xEA
   102  	LONG	$0x0 // farjump32+1(SB)
   103  	WORD	$0x18 // code segment
   104  
   105  TEXT gdt(SB),NOSPLIT,$0
   106  	QUAD	$0x0		// 0x0 null entry
   107  	QUAD	$CODE_SEGMENT	// 0x8
   108  	QUAD	$DATA_SEGMENT	// 0x10
   109  	QUAD	$CODE_SEGMENT	// 0x18
   110  
   111  TEXT ·info(SB),NOSPLIT,$0
   112  	LONG	$0x0
   113  
   114  TEXT ·entry(SB),NOSPLIT,$0
   115  	LONG	$0x0
   116  
   117  TEXT ·magic(SB),NOSPLIT,$0
   118  	LONG	$0x0
   119  
   120  TEXT ·end(SB),NOSPLIT,$0