[PATCH v2 1/2] UefiCpuPkg/MpInitLib: Restore IDT context for APs.

["Dong, Eric" <eric.dong@intel.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2683

This patch fixes an assertion because AP can't find the CpuMpData.
When AP is waken up through Init-Sipi-Sipi, AP's IDT should
be restored to pre-allocated buffer so AP can get the CpuMpData
through the IDT base address.
Current code already has logic to handle this when CpuMpData->
InitFlag is ApInitConfig but misses the logic
when CpuMpData->InitFlag is ApInitReconfig.
This patch fixes this gap.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Chandana Kumar <chandana.c.kumar@intel.com>
Signed-off-by: Eric Dong <eric.dong@intel.com>
---
V2:
1. Enhance code comments.

 UefiCpuPkg/Library/MpInitLib/MpLib.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index 64a4c3546e..2e87aa1f06 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
@@ -692,6 +692,16 @@ ApWakeupFunction (
         //
         RestoreVolatileRegisters (&CpuMpData->CpuData[ProcessorNumber].VolatileRegisters, TRUE);
       } else {
+        if (CpuMpData->InitFlag == ApInitReconfig) {
+          //
+          // Initialize AP volatile registers in ApInitReconfig path.
+          // ApInitReconfig happens when:
+          // 1. AP is re-enabled after it's disabled, in either PEI or DXE phase.
+          // 2. AP is initialized in DXE phase.
+          //
+          RestoreVolatileRegisters (&CpuMpData->CpuData[0].VolatileRegisters, FALSE);
+        }
+
         //
         // The CPU driver might not flush TLB for APs on spot after updating
         // page attributes. AP in mwait loop mode needs to take care of it when
-- 
2.23.0.windows.1