Merged PR 5474: Add code to block on AP execution if a task is in pro…

…gress.

## Description

This adds a blocking loop to AP status check to ensure all APs are completed with any non-blocking tasks before allowing dispatch of new tasks.

- Breaking change?
  No.

## How This Was Tested

Tested by dispatching a long-running non-blocking task, and then attempting to dispatch a second task. Prior to this change, second task would proceed with dispatch, interfering with the first task. With this change, the second task waits in CheckAndUpdateApsStatus() until the first task completes.

## Integration Instructions

N/A

UefiCpuPkg/CpuMpPei/CpuMpPei.c

@@ -290,10 +290,14 @@ PeiStartupThisAP (
   The execution is in non-blocking mode. The BSP continues executing immediately
   after starting the AP.
 
-  Caller is responsible for ensuring that the scheduled AP task is complete (via
-  caller-specific logic) before dispatching further tasks on the AP using this
-  or other routines in the API.
-
+  If an attempt is made to dispatch a blocking or non-blcoking task on the AP
+  while it is running a non-blocking task, that dispatch will block until the
+  AP completes the current task.
+
+  No timeout is specified - failure of the AP to complete the task is fatal. If
+  the AP crashes or fails to return from Procedure, then the next attempt to
+  dispatch blocking or non-blocking tasks on the AP will hang waiting on the AP.
+  No attempt is made to reset or recover the AP in this state.
 
   @param[in] PeiServices          An indirect pointer to the PEI Services Table
                                   published by the PEI Foundation.

UefiCpuPkg/Library/MpInitLib/PeiMpLib.c

@@ -383,11 +383,18 @@ CheckAndUpdateApsStatus (
       continue;
     }
 
-    Status = CheckThisAP (ProcessorNumber);
+    //
+    // Block until all CPUs are ready. This ensures that we don't attempt to dispatch
+    // tasks on CPUs that are executing a non-blocking task. NOTE: this implies
+    // that only one non-blocking AP dispatch may be outstanding at a time.
+    //
+    do {
+      Status = CheckThisAP (ProcessorNumber);
 
-    if (Status != EFI_NOT_READY) {
-      CpuMpData->CpuData[ProcessorNumber].WaitEvent = NULL;
-    }
+      if (Status != EFI_NOT_READY) {
+        CpuMpData->CpuData[ProcessorNumber].WaitEvent = NULL;
+      }
+    } while (Status == EFI_NOT_READY);
   }
 
   // MU_CHANGE - End Add basic support for non-blocking AP dispatch in PEI.