[PATCH 1/2] UefiCpuPkg/MpInitLib: force flushing TLB for AP in mwait loop mode

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

The reason doing this is that we found that calling StartupAllAps() to
flush TLB for all APs in CpuDxe driver after changing page attributes
will spend a lot of time to complete. If there are many page attributes
update requests, the whole system performance will be slowed down
explicitly, including any shell command and UI operation.

The solution is removing the flush operation for AP in CpuDxe driver.
Since TLB is always flushed in HLT loop mode, we just need to enforce
a TLB flush for mwait loop mode.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 UefiCpuPkg/Library/MpInitLib/MpLib.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/UefiCpuPkg/Library/MpInitLib/MpLib.c b/UefiCpuPkg/Library/MpInitLib/MpLib.c
index 6231968c74..175a4b49e5 100644
--- a/UefiCpuPkg/Library/MpInitLib/MpLib.c
+++ b/UefiCpuPkg/Library/MpInitLib/MpLib.c
@@ -630,6 +630,12 @@ ApWakeupFunction (
         // Restore AP's volatile registers saved
         //
         RestoreVolatileRegisters (&CpuMpData->CpuData[ProcessorNumber].VolatileRegisters, TRUE);
+      } else {
+        //
+        // Due to performance concern, the CPU driver might not flush TLB for
+        // APs on spot. AP itself needs to take care of it when woken up.
+        //
+        CpuFlushTlb ();
       }
 
       if (GetApState (&CpuMpData->CpuData[ProcessorNumber]) == CpuStateReady) {
-- 
2.15.1.windows.2