1. Added comments to ASM files

2. Fixed a bug in 64-bit AsmDisablePaging64(), which may cause a #GP exception.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@2206 6f19259b-4bc3-4df7-8a09-765794883524

1 files changed, 20 insertions, 14 deletions

diff --git a/MdePkg/Library/BaseLib/X64/Thunk16.asm b/MdePkg/Library/BaseLib/X64/Thunk16.asm
index 642310869b..556a8b47e3 100644
--- a/MdePkg/Library/BaseLib/X64/Thunk16.asm
+++ b/MdePkg/Library/BaseLib/X64/Thunk16.asm
@@ -66,6 +66,14 @@ SavedGdt    LABEL   FWORD
 ; by user code. It will be shadowed to somewhere in memory below 1MB.

 ;------------------------------------------------------------------------------

 _BackFromUserCode   PROC

+    ;

+    ; 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

@@ -115,7 +123,7 @@ SavedCr4    DD      ?
     ;

     ; rdi in the instruction below is indeed bx in 16-bit code

     ;

-    DB      66h, 2eh

+    DB      66h, 2eh                    ; 2eh is "cs:" segment override

     lgdt    fword ptr [rdi + (SavedGdt - @Base)]

     DB      66h

     mov     ecx, 0c0000080h

@@ -129,9 +137,8 @@ SavedCr0    DD      ?
 @64Eip      DD      ?

 SavedCs     DW      ?

 @64BitCode:

-    DB      48h, 0b8h                   ; mov rax, imm64

-SavedRip    DQ      ?

-    jmp     rax                         ; return to caller

+    mov     rsp, r8                     ; restore stack

+    ret

 _BackFromUserCode   ENDP

 

 _EntryPoint DD      _ToUserCode - m16Start

@@ -160,14 +167,14 @@ _ToUserCode PROC
     mov     cr4, rbp

     mov     ss, esi                     ; set up 16-bit stack segment

     mov     sp, bx                      ; set up 16-bit stack pointer

-    DB      66h

+    DB      66h                         ; make the following call 32-bit

     call    @Base                       ; push eip

 @Base:

     pop     bp                          ; ebp <- address of @Base

     push    [esp + sizeof (IA32_REGS) + 2]

-    lea     eax, [rsi + (@RealMode - @Base)]

+    lea     eax, [rsi + (@RealMode - @Base)]    ; rsi is "bp" in 16-bit code

     push    rax

-    retf

+    retf                                ; execution begins at next instruction

 @RealMode:

     DB      66h, 2eh                    ; CS and operand size override

     lidt    fword ptr [rsi + (_16Idtr - @Base)]

@@ -178,7 +185,7 @@ _ToUserCode PROC
     pop     gs

     popf                                ; popfd

     lea     sp, [esp + 4]               ; skip high order 32 bits of EFlags

-    DB      66h

+    DB      66h                         ; make the following retf 32-bit

     retf                                ; transfer control to user code

 _ToUserCode ENDP

 

@@ -220,8 +227,8 @@ GDT_SIZE = $ - _NullSeg
 ;   );

 ;------------------------------------------------------------------------------

 InternalAsmThunk16  PROC    USES    rbp rbx rsi rdi

-    mov     r10d, ds

-    mov     r11d, es

+    mov     r10d, ds                    ; r9 ~ r11 are not accessible in 16-bit

+    mov     r11d, es                    ; so use them for saving seg registers

     mov     r9d, ss

     push    fs

     push    gs

@@ -238,8 +245,8 @@ InternalAsmThunk16  PROC    USES    rbp rbx rsi rdi
     lea     ecx, [rdx + (SavedCr4 - m16Start)]

     mov     eax, edx                    ; eax <- transition code address

     and     edx, 0fh

-    shl     eax, 12

-    lea     ax, [rdx + (_BackFromUserCode - m16Start)]

+    shl     eax, 12                     ; segment address in high order 16 bits

+    lea     ax, [rdx + (_BackFromUserCode - m16Start)]  ; offset address

     stosd                               ; [edi] <- return address of user code

     sgdt    fword ptr [rcx + (SavedGdt - SavedCr4)]

     sidt    fword ptr [rsp + 38h]       ; save IDT stack in argument space

@@ -257,13 +264,12 @@ InternalAsmThunk16  PROC    USES    rbp rbx rsi rdi
     pushfq

     lea     edx, [rdx + DATA16 - DATA32]

     lea     r8, @RetFromRealMode

-    mov     [rcx + (SavedRip - SavedCr4)], r8

+    push    r8

     mov     r8d, cs

     mov     [rcx + (SavedCs - SavedCr4)], r8w

     mov     r8, rsp

     jmp     fword ptr [rcx + (_EntryPoint - SavedCr4)]

 @RetFromRealMode:

-    mov     rsp, r8

     popfq

     lidt    fword ptr [rsp + 38h]       ; restore protected mode IDTR

     lea     eax, [rbp - sizeof (IA32_REGS)]