UefiCpuPkg/MpInitLib: Implementation of MpInitLibSwitchBSP()

v4:
  1. Simply the internal function SwitchBSPWorker()'s comment header
     due to it is duplicated with MpInitLibSwitchBSP().

v3:
  1. Rename MpInitLibSwitchBsp to MpInitLibSwitchBSP.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Feng Tian <feng.tian@intel.com>
Cc: Giri P Mudusuru <giri.p.mudusuru@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Michael Kinney <michael.d.kinney@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Michael Kinney <michael.d.kinney@intel.com>

UefiCpuPkg/Library/MpInitLib/DxeMpLib.c

@@ -389,7 +389,31 @@ MpInitLibSwitchBSP (
   IN BOOLEAN                   EnableOldBSP
   )
 {
-  return EFI_UNSUPPORTED;
+  EFI_STATUS            Status;
+  BOOLEAN               OldInterruptState;
+
+  //
+  // Before send both BSP and AP to a procedure to exchange their roles,
+  // interrupt must be disabled. This is because during the exchange role
+  // process, 2 CPU may use 1 stack. If interrupt happens, the stack will
+  // be corrupted, since interrupt return address will be pushed to stack
+  // by hardware.
+  //
+  OldInterruptState = SaveAndDisableInterrupts ();
+
+  //
+  // Mask LINT0 & LINT1 for the old BSP
+  //
+  DisableLvtInterrupts ();
+
+  Status = SwitchBSPWorker (ProcessorNumber, EnableOldBSP);
+
+  //
+  // Restore interrupt state.
+  //
+  SetInterruptState (OldInterruptState);
+
+  return Status;
 }
 
 /**