#include "BaseLibInternals.h"

;------------------------------------------------------------------------------
;
; Copyright (c) 2006 - 2013, Intel Corporation. All rights reserved.<BR>
; This program and the accompanying materials
; are licensed and made available under the terms and conditions of the BSD License
; which accompanies this distribution.  The full text of the license may be found at
; http://opensource.org/licenses/bsd-license.php.
;
; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
;
; Module Name:
;
;   Thunk.asm
;
; Abstract:
;
;   Real mode thunk
;
;------------------------------------------------------------------------------

global ASM_PFX(m16Size)
global ASM_PFX(mThunk16Attr)
global ASM_PFX(m16Gdt)
global ASM_PFX(m16GdtrBase)
global ASM_PFX(mTransition)
global ASM_PFX(m16Start)

struc IA32_REGS

  ._EDI:       resd      1
  ._ESI:       resd      1
  ._EBP:       resd      1
  ._ESP:       resd      1
  ._EBX:       resd      1
  ._EDX:       resd      1
  ._ECX:       resd      1
  ._EAX:       resd      1
  ._DS:        resw      1
  ._ES:        resw      1
  ._FS:        resw      1
  ._GS:        resw      1
  ._EFLAGS:    resq      1
  ._EIP:       resd      1
  ._CS:        resw      1
  ._SS:        resw      1
  .size:

endstruc

SECTION .data

;
; These are global constant to convey information to C code.
;
ASM_PFX(m16Size)         DW      InternalAsmThunk16 - ASM_PFX(m16Start)
ASM_PFX(mThunk16Attr)    DW      _BackFromUserCode.ThunkAttr - ASM_PFX(m16Start)
ASM_PFX(m16Gdt)          DW      _NullSeg - ASM_PFX(m16Start)
ASM_PFX(m16GdtrBase)     DW      _16GdtrBase - ASM_PFX(m16Start)
ASM_PFX(mTransition)     DW      _EntryPoint - ASM_PFX(m16Start)

SECTION .text

ASM_PFX(m16Start):

SavedGdt:
            dw  0
            dq  0

;------------------------------------------------------------------------------
; _BackFromUserCode() takes control in real mode after 'retf' has been executed
; by user code. It will be shadowed to somewhere in memory below 1MB.
;------------------------------------------------------------------------------
_BackFromUserCode:
    ;
    ; The order of saved registers on the stack matches the order they appears
    ; in IA32_REGS structure. This facilitates wrapper function to extract them
    ; into that structure.
    ;
    ; Some instructions for manipulation of segment registers have to be written
    ; in opcode since 64-bit MASM prevents accesses to those registers.
    ;
    DB      16h                         ; push ss
    DB      0eh                         ; push cs
    DB      66h
    call    .Base                       ; push eip
.Base:
    DB      66h
    push    0                           ; reserved high order 32 bits of EFlags
    pushfw                              ; pushfd actually
    cli                                 ; disable interrupts
    push    gs
    push    fs
    DB      6                           ; push es
    DB      1eh                         ; push ds
    DB      66h, 60h                    ; pushad
    DB      66h, 0bah                   ; mov edx, imm32
.ThunkAttr:     dd   0
    test    dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15
    jz      .1
    mov     eax, 15cd2401h              ; mov ax, 2401h & int 15h
    cli                                 ; disable interrupts
    jnc     .2
.1:
    test    dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_KBD_CTRL
    jz      .2
    in      al, 92h
    or      al, 2
    out     92h, al                     ; deactivate A20M#
.2:
    xor     ax, ax                      ; xor eax, eax
    mov     eax, ss                     ; mov ax, ss
    lea     bp, [esp + IA32_REGS.size]
    ;
    ; rsi in the following 2 instructions is indeed bp in 16-bit code
    ;
    mov     [rsi - IA32_REGS.size + IA32_REGS._ESP], bp
    DB      66h
    mov     ebx, [rsi - IA32_REGS.size + IA32_REGS._EIP]
    shl     ax, 4                       ; shl eax, 4
    add     bp, ax                      ; add ebp, eax
    mov     ax, cs
    shl     ax, 4
    lea     ax, [eax + ebx + (.64BitCode - .Base)]
    DB      66h, 2eh, 89h, 87h          ; mov cs:[bx + (.64Eip - .Base)], eax
    DW      .64Eip - .Base
    DB      66h, 0b8h                   ; mov eax, imm32
.SavedCr4:  DD      0
    mov     cr4, rax
    ;
    ; rdi in the instruction below is indeed bx in 16-bit code
    ;
    DB      66h, 2eh                    ; 2eh is "cs:" segment override
    lgdt    [rdi + (SavedGdt - .Base)]
    DB      66h
    mov     ecx, 0c0000080h
    rdmsr
    or      ah, 1
    wrmsr
    DB      66h, 0b8h                   ; mov eax, imm32
.SavedCr0:  DD      0
    mov     cr0, rax
    DB      66h, 0eah                   ; jmp far cs:.64Bit
.64Eip:     DD      0
.SavedCs:   DW      0
.64BitCode:
    db      090h 
    db      048h, 0bch                 ; mov rsp, imm64
.SavedSp:   DQ   0                     ; restore stack
    nop
    ret

_EntryPoint:
        DD      _ToUserCode - ASM_PFX(m16Start)
        DW      CODE16
_16Gdtr:
        DW      GDT_SIZE - 1
_16GdtrBase:
        DQ      _NullSeg
_16Idtr:
        DW      (1 << 10) - 1
        DD      0

;------------------------------------------------------------------------------
; _ToUserCode() takes control in real mode before passing control to user code.
; It will be shadowed to somewhere in memory below 1MB.
;------------------------------------------------------------------------------
_ToUserCode:
    mov     ss, edx                     ; set new segment selectors
    mov     ds, edx
    mov     es, edx
    mov     fs, edx
    mov     gs, edx
    DB      66h
    mov     ecx, 0c0000080h
    mov     cr0, rax                    ; real mode starts at next instruction
    rdmsr
    and     ah, ~1
    wrmsr
    mov     cr4, rbp
    mov     ss, esi                     ; set up 16-bit stack segment
    mov     sp, bx                      ; set up 16-bit stack pointer
    DB      66h                         ; make the following call 32-bit
    call    .Base                       ; push eip
.Base:
    pop     bp                          ; ebp <- address of .Base
    push    qword [esp + IA32_REGS.size + 2]
    lea     eax, [rsi + (.RealMode - .Base)]    ; rsi is "bp" in 16-bit code
    push    rax
    retf                                ; execution begins at next instruction
.RealMode:
    DB      66h, 2eh                    ; CS and operand size override
    lidt    [rsi + (_16Idtr - .Base)]
    DB      66h, 61h                    ; popad
    DB      1fh                         ; pop ds
    DB      07h                         ; pop es
    pop     fs
    pop     gs
    popfw                               ; popfd
    lea     sp, [esp + 4]               ; skip high order 32 bits of EFlags
    DB      66h                         ; make the following retf 32-bit
    retf                                ; transfer control to user code

ALIGN   8

CODE16  equ _16Code - $
DATA16  equ _16Data - $
DATA32  equ _32Data - $

_NullSeg    DQ      0
_16Code:
            DW      -1
            DW      0
            DB      0
            DB      9bh
            DB      8fh                 ; 16-bit segment, 4GB limit
            DB      0
_16Data:
            DW      -1
            DW      0
            DB      0
            DB      93h
            DB      8fh                 ; 16-bit segment, 4GB limit
            DB      0
_32Data:
            DW      -1
            DW      0
            DB      0
            DB      93h
            DB      0cfh                ; 16-bit segment, 4GB limit
            DB      0

GDT_SIZE equ $ - _NullSeg

;------------------------------------------------------------------------------
; IA32_REGISTER_SET *
; EFIAPI
; InternalAsmThunk16 (
;   IN      IA32_REGISTER_SET         *RegisterSet,
;   IN OUT  VOID                      *Transition
;   );
;------------------------------------------------------------------------------
global ASM_PFX(InternalAsmThunk16)
ASM_PFX(InternalAsmThunk16):
    push    rbp
    push    rbx
    push    rsi
    push    rdi
    
    mov     ebx, ds
    push    rbx          ; Save ds segment register on the stack
    mov     ebx, es
    push    rbx          ; Save es segment register on the stack
    mov     ebx, ss
    push    rbx          ; Save ss segment register on the stack
    
    push    fs
    push    gs
    mov     rsi, rcx
    movzx   r8d, word [rsi + IA32_REGS._SS]
    mov     edi, [rsi + IA32_REGS._ESP]
    lea     rdi, [edi - (IA32_REGS.size + 4)]
    imul    eax, r8d, 16                ; eax <- r8d(stack segment) * 16
    mov     ebx, edi                    ; ebx <- stack for 16-bit code
    push    IA32_REGS.size / 4
    add     edi, eax                    ; edi <- linear address of 16-bit stack
    pop     rcx
    rep     movsd                       ; copy RegSet
    lea     ecx, [rdx + (_BackFromUserCode.SavedCr4 - ASM_PFX(m16Start))]
    mov     eax, edx                    ; eax <- transition code address
    and     edx, 0fh
    shl     eax, 12                     ; segment address in high order 16 bits
    lea     ax, [rdx + (_BackFromUserCode - ASM_PFX(m16Start))]  ; offset address
    stosd                               ; [edi] <- return address of user code
  
    sgdt    [rsp + 60h]       ; save GDT stack in argument space
    movzx   r10, word [rsp + 60h]   ; r10 <- GDT limit 
    lea     r11, [rcx + (InternalAsmThunk16 - _BackFromUserCode.SavedCr4) + 0xf]
    and     r11, ~0xf            ; r11 <- 16-byte aligned shadowed GDT table in real mode buffer
    
    mov     [rcx + (SavedGdt - _BackFromUserCode.SavedCr4)], r10w      ; save the limit of shadowed GDT table
    mov     [rcx + (SavedGdt - _BackFromUserCode.SavedCr4) + 2], r11  ; save the base address of shadowed GDT table
    
    mov     rsi, [rsp + 62h]  ; rsi <- the original GDT base address
    xchg    rcx, r10                    ; save rcx to r10 and initialize rcx to be the limit of GDT table
    inc     rcx                         ; rcx <- the size of memory to copy
    xchg    rdi, r11                    ; save rdi to r11 and initialize rdi to the base address of shadowed GDT table
    rep     movsb                       ; perform memory copy to shadow GDT table
    mov     rcx, r10                    ; restore the orignal rcx before memory copy
    mov     rdi, r11                    ; restore the original rdi before memory copy
    
    sidt    [rsp + 50h]       ; save IDT stack in argument space
    mov     rax, cr0
    mov     [rcx + (_BackFromUserCode.SavedCr0 - _BackFromUserCode.SavedCr4)], eax
    and     eax, 7ffffffeh              ; clear PE, PG bits
    mov     rbp, cr4
    mov     [rcx], ebp                  ; save CR4 in _BackFromUserCode.SavedCr4
    and     ebp, ~30h                ; clear PAE, PSE bits
    mov     esi, r8d                    ; esi <- 16-bit stack segment
    DB      6ah, DATA32                 ; push DATA32
    pop     rdx                         ; rdx <- 32-bit data segment selector
    lgdt    [rcx + (_16Gdtr - _BackFromUserCode.SavedCr4)]
    mov     ss, edx
    pushfq
    lea     edx, [rdx + DATA16 - DATA32]
    lea     r8, [REL .RetFromRealMode]
    push    r8
    mov     r8d, cs
    mov     [rcx + (_BackFromUserCode.SavedCs - _BackFromUserCode.SavedCr4)], r8w
    mov     [rcx + (_BackFromUserCode.SavedSp - _BackFromUserCode.SavedCr4)], rsp
    jmp     dword far [rcx + (_EntryPoint - _BackFromUserCode.SavedCr4)]
.RetFromRealMode:
    popfq
    lgdt    [rsp + 60h]       ; restore protected mode GDTR
    lidt    [rsp + 50h]       ; restore protected mode IDTR
    lea     eax, [rbp - IA32_REGS.size]
    pop     gs
    pop     fs
    pop     rbx
    mov     ss, ebx
    pop     rbx
    mov     es, ebx
    pop     rbx
    mov     ds, ebx

    pop     rdi
    pop     rsi
    pop     rbx
    pop     rbp

    ret