From f56379f3610e51a01a18c00e4a0d5fded3af0657 Mon Sep 17 00:00:00 2001
From: Jeff Fan <jeff.fan@intel.com>
Date: Fri, 25 Nov 2016 12:58:36 +0800
Subject: [PATCH] UefiCpuPkg/DxeMpLib: Fix bug when getting target C-State from
 eax

AP will get target C-State from eax[7:4]. We do shift in ebx firstly before set
to eax. It will lead ebx is incorrect in the next time.

The fix is to set ebx to eax firstly and does shift in eax. Thus, ebx could keep
original value.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
---
 UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm b/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
index f8f471233e..34c07a6d51 100644
--- a/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
+++ b/UefiCpuPkg/Library/MpInitLib/X64/MpFuncs.nasm
@@ -266,8 +266,8 @@ MwaitLoop:
     xor        ecx, ecx           ; ecx = 0
     xor        edx, edx           ; edx = 0
     monitor
-    shl        ebx, 4
     mov        eax, ebx           ; Mwait Cx, Target C-State per eax[7:4]
+    shl        eax, 4
     mwait
     jmp        MwaitLoop
 HltLoop:
-- 
2.39.2