UefiCpuPkg/PiSmmCpuDxeSmm: Clarification for BSP & APs Sync Flow

This patch does not impact functionality. It aims to clarify the
synchronization flow between the BSP and APs to enhance code
readability and understanding:

Steps #6 and #11 are the basic synchronization requirements for all
cases.

Steps #1 is additional requirements if the MmCpuSyncModeTradition
mode is selected.

Steps #1, #2, #3, #4, #5, #7, #8, #9, and #10 are additional
requirements if the system needs to configure the MTRR.

Steps #9 and #10 are additional requirements if the system needs to
support the mSmmDebugAgentSupport.

Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>

UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c

@@ -522,13 +522,13 @@ BSPHandler (
     //
     // Wait for all APs to get ready for programming MTRRs
     //
-    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #1: Wait APs
 
     if (SmmCpuFeaturesNeedConfigureMtrrs ()) {
       //
       // Signal all APs it's time for backup MTRRs
       //
-      ReleaseAllAPs ();
+      ReleaseAllAPs (); /// #2: Signal APs
 
       //
       // SmmCpuSyncWaitForAPs() may wait for ever if an AP happens to enter SMM at
@@ -543,12 +543,12 @@ BSPHandler (
       //
       // Wait for all APs to complete their MTRR saving
       //
-      SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+      SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #3: Wait APs
 
       //
       // Let all processors program SMM MTRRs together
       //
-      ReleaseAllAPs ();
+      ReleaseAllAPs (); /// #4: Signal APs
 
       //
       // SmmCpuSyncWaitForAPs() may wait for ever if an AP happens to enter SMM at
@@ -560,7 +560,7 @@ BSPHandler (
       //
       // Wait for all APs to complete their MTRR programming
       //
-      SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+      SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #5: Wait APs
     }
   }
 
@@ -625,18 +625,18 @@ BSPHandler (
   // Notify all APs to exit
   //
   *mSmmMpSyncData->InsideSmm = FALSE;
-  ReleaseAllAPs ();
+  ReleaseAllAPs (); /// #6: Signal APs
 
   if (SmmCpuFeaturesNeedConfigureMtrrs ()) {
     //
     // Wait for all APs the readiness to program MTRRs
     //
-    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #7: Wait APs
 
     //
     // Signal APs to restore MTRRs
     //
-    ReleaseAllAPs ();
+    ReleaseAllAPs (); /// #8: Signal APs
 
     //
     // Restore OS MTRRs
@@ -649,12 +649,12 @@ BSPHandler (
     //
     // Wait for all APs to complete their pending tasks including MTRR programming if needed.
     //
-    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+    SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #9: Wait APs
 
     //
     // Signal APs to Reset states/semaphore for this processor
     //
-    ReleaseAllAPs ();
+    ReleaseAllAPs (); /// #10: Signal APs
   }
 
   if (mSmmDebugAgentSupport) {
@@ -679,7 +679,7 @@ BSPHandler (
   // Gather APs to exit SMM synchronously. Note the Present flag is cleared by now but
   // WaitForAllAps does not depend on the Present flag.
   //
-  SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex);
+  SmmCpuSyncWaitForAPs (mSmmMpSyncData->SyncContext, ApCount, CpuIndex); /// #11: Wait APs
 
   //
   // At this point, all APs should have exited from APHandler().
@@ -805,14 +805,14 @@ APHandler (
     //
     // Notify BSP of arrival at this point
     //
-    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #1: Signal BSP
   }
 
   if (SmmCpuFeaturesNeedConfigureMtrrs ()) {
     //
     // Wait for the signal from BSP to backup MTRRs
     //
-    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #2: Wait BSP
 
     //
     // Backup OS MTRRs
@@ -822,12 +822,12 @@ APHandler (
     //
     // Signal BSP the completion of this AP
     //
-    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #3: Signal BSP
 
     //
     // Wait for BSP's signal to program MTRRs
     //
-    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #4: Wait BSP
 
     //
     // Replace OS MTRRs with SMI MTRRs
@@ -837,14 +837,14 @@ APHandler (
     //
     // Signal BSP the completion of this AP
     //
-    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #5: Signal BSP
   }
 
   while (TRUE) {
     //
     // Wait for something to happen
     //
-    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #6: Wait BSP
 
     //
     // Check if BSP wants to exit SMM
@@ -884,12 +884,12 @@ APHandler (
     //
     // Notify BSP the readiness of this AP to program MTRRs
     //
-    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #7: Signal BSP
 
     //
     // Wait for the signal from BSP to program MTRRs
     //
-    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #8: Wait BSP
 
     //
     // Restore OS MTRRs
@@ -902,12 +902,12 @@ APHandler (
     //
     // Notify BSP the readiness of this AP to Reset states/semaphore for this processor
     //
-    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #9: Signal BSP
 
     //
     // Wait for the signal from BSP to Reset states/semaphore for this processor
     //
-    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+    SmmCpuSyncWaitForBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #10: Wait BSP
   }
 
   //
@@ -918,7 +918,7 @@ APHandler (
   //
   // Notify BSP the readiness of this AP to exit SMM
   //
-  SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex);
+  SmmCpuSyncReleaseBsp (mSmmMpSyncData->SyncContext, CpuIndex, BspIndex); /// #11: Signal BSP
 }
 
 /**