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From: "John E Lofgren" <john.e.lofgren@intel.com>
To: devel@edk2.groups.io
Subject: [Patch V3] UefiCpuPkg/CpuExceptionHandlerLib: Fix split lock
Date: Tue, 17 Sep 2019 15:49:10 -0700	[thread overview]
Message-ID: <20190917224910.28040-1-john.e.lofgren@intel.com> (raw)

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 | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
index 4db1a09f28..7b7642b290 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -180,21 +180,29 @@ HasErrorCode:
     push    qword [rbp + 24]
 
 ;; UINT64  Gdtr[2], Idtr[2];
+    ; sidt and sgdt saves 10 bytes to memory, 8 bytes = base and 2 bytes = limit.
+    ; To avoid #AC split lock when separating base and limit into their
+    ; own separate 64 bit memory, we can’t use 64 bit xchg since base [63:48] bits
+    ; may cross the cache line.
     xor     rax, rax
     push    rax
     push    rax
     sidt    [rsp]
-    xchg    rax, [rsp + 2]
-    xchg    rax, [rsp]
-    xchg    rax, [rsp + 8]
+    xchg    eax, [rsp + 2]
+    xchg    eax, [rsp]
+    xchg    eax, [rsp + 8]
+    xchg     ax, [rsp + 6]
+    xchg     ax, [rsp + 4]
 
     xor     rax, rax
     push    rax
     push    rax
     sgdt    [rsp]
-    xchg    rax, [rsp + 2]
-    xchg    rax, [rsp]
-    xchg    rax, [rsp + 8]
+    xchg    eax, [rsp + 2]
+    xchg    eax, [rsp]
+    xchg    eax, [rsp + 8]
+    xchg     ax, [rsp + 6]
+    xchg     ax, [rsp + 4]
 
 ;; UINT64  Ldtr, Tr;
     xor     rax, rax
-- 
2.16.2.windows.1


             reply	other threads:[~2019-09-17 22:49 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-17 22:49 John E Lofgren [this message]
2019-09-18  8:52 ` [edk2-devel] [Patch V3] UefiCpuPkg/CpuExceptionHandlerLib: Fix split lock Laszlo Ersek
2019-09-18 15:23   ` John E Lofgren
  -- strict thread matches above, loose matches on Subject: below --
2019-09-18 15:43 John E Lofgren

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