[PATCH 2/4] UefiCpuPkg/CpuExceptionHandlerLib: Setup single step in #PF handler

[Jian J Wang <jian.j.wang@intel.com>]

Once the #PF handler has set the page to be 'present', there should
be a way to reset it to 'not-present'. 'TF' bit in EFLAGS can be used
for this purpose. 'TF' bit will be set in interrupted function context
so that it can be triggered once the cpu control returns back to the
instruction causing #PF and re-execute it.

This is an necessary step to implement non-stop mode for Heap Guard
and NULL Pointer Detection feature.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 .../Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm   | 7 +++++++
 .../Library/CpuExceptionHandlerLib/Ia32/ExceptionTssEntryAsm.nasm  | 4 +---
 .../Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm    | 4 ++++
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
index 45d6474091..6fcf5fb23f 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
@@ -383,6 +383,13 @@ ErrorCodeAndVectorOnStack:
     pop     dword [ebp - 4]
     mov     esp, ebp
     pop     ebp
+
+; Enable TF bit after page fault handler runs
+    cmp     dword [esp], 14       ; #PF?
+    jne     .5
+    bts     dword [esp + 16], 8   ; EFLAGS
+
+.5:
     add     esp, 8
     cmp     dword [esp - 16], 0   ; check EXCEPTION_HANDLER_CONTEXT.OldIdtHandler
     jz      DoReturn
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionTssEntryAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionTssEntryAsm.nasm
index 62bcedea1a..7aac29c7e7 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionTssEntryAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionTssEntryAsm.nasm
@@ -355,10 +355,8 @@ o16 mov     [ecx + IA32_TSS._SS], ax
     movzx  ebx, word [ecx + IA32_TSS._CS]
     mov    [eax - 0x8], ebx                      ; create CS in old stack
     mov    ebx, dword [ecx + IA32_TSS.EFLAGS]
-    bts    ebx, 8
+    bts    ebx, 8                                ; Set TF
     mov    [eax - 0x4], ebx                      ; create eflags in old stack
-    mov    dword [ecx + IA32_TSS.EFLAGS], ebx    ; update eflags in old TSS
-    mov    eax, dword [ecx + IA32_TSS._ESP]      ; Get old stack pointer
     sub    eax, 0xc                              ; minus 12 byte
     mov    dword [ecx + IA32_TSS._ESP], eax      ; Set new stack pointer
 
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
index 7b97810d10..f842af2336 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -336,6 +336,10 @@ HasErrorCode:
     pop     r15
 
     mov     rsp, rbp
+    cmp     qword [rbp + 8], 14 ; #PF?
+    jne     .1
+    bts     qword [rsp + 40], 8 ; RFLAGS.TF
+.1:
     pop     rbp
     add     rsp, 16
     cmp     qword [rsp - 32], 0  ; check EXCEPTION_HANDLER_CONTEXT.OldIdtHandler
-- 
2.16.2.windows.1