[Patch V3] UefiCpuPkg/CpuExceptionHandlerLib: Fix split lock


John E Lofgren
 

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2150
V3 changes:
change to mov instruction (non locking instuction) instead
of xchg to simplify design.

V2 changes:
Add xchg 16 bit instructions to handle sgdt and sidt base
63:48 bits and 47:32 bits.
Add comment to explain why xchg 64bit isnt being used

Split lock happens when a locking instruction is used on mis-aligned data
that crosses two cachelines. If close source platform enables Alignment Check
Exception(#AC), They can hit a double fault due to split lock being in
CpuExceptionHandlerLib.

sigt and sgdt saves 10 bytes to memory, 8 bytes is base and 2 bytes is limit.
The data is mis-aligned, can cross two cacheline, and a xchg
instruction(locking instuction) is being utilize.

Signed-off-by: John E Lofgren <john.e.lofgren@intel.com>
---
UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
index 4db1a09f28..19198f2731 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -184,17 +184,19 @@ HasErrorCode:
push rax
push rax
sidt [rsp]
- xchg rax, [rsp + 2]
- xchg rax, [rsp]
- xchg rax, [rsp + 8]
+ mov bx, word [rsp]
+ mov rax, qword [rsp + 2]
+ mov qword [rsp], rax
+ mov word [rsp + 8], bx

xor rax, rax
push rax
push rax
sgdt [rsp]
- xchg rax, [rsp + 2]
- xchg rax, [rsp]
- xchg rax, [rsp + 8]
+ mov bx, word [rsp]
+ mov rax, qword [rsp + 2]
+ mov qword [rsp], rax
+ mov word [rsp + 8], bx

;; UINT64 Ldtr, Tr;
xor rax, rax
--
2.16.2.windows.1

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