Add a PCD to allow the AP wakeup buffer to be reserved (#209)

## Description

Creates a new PCD which when set will cause the AP wakeup buffer to be
allocated as a reserved type.

- [x] Impacts functionality?
- [ ] Impacts security?
- [ ] Includes tests?
- [ ] Includes documentation?
- [ ] Breaking change?

## How This Was Tested

Tested on custom Q35 image

## Integration Instructions

N/A

UefiCpuPkg/Library/MpInitLib/DxeMpInitLib.inf

@@ -81,6 +81,7 @@
   gUefiCpuPkgTokenSpaceGuid.PcdCpuApStatusCheckIntervalInMicroSeconds  ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdGhcbHypervisorFeatures                  ## CONSUMES
   gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase                       ## SOMETIMES_CONSUMES
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuApWakeupBufferReserved               ## CONSUMES MU_CHANGE
   # MU_CHANGE START Remove memory protection PCD references
   # gEfiMdeModulePkgTokenSpaceGuid.PcdCpuStackGuard                      ## CONSUMES
   # MU_CHANGE END

UefiCpuPkg/Library/MpInitLib/DxeMpLib.c

@@ -291,8 +291,12 @@ GetWakeupBuffer (
   EFI_PHYSICAL_ADDRESS  StartAddress;
   EFI_MEMORY_TYPE       MemoryType;
 
-  if (ConfidentialComputingGuestHas (CCAttrAmdSevEs) &&
-      !ConfidentialComputingGuestHas (CCAttrAmdSevSnp))
+  // MU_CHANGE START Add PCD to make wakeup buffer reserved
+  if (PcdGetBool (PcdCpuApWakeupBufferReserved)) {
+    MemoryType = EfiReservedMemoryType;
+    // MU_CHANGE END
+  } else if (ConfidentialComputingGuestHas (CCAttrAmdSevEs) &&
+             !ConfidentialComputingGuestHas (CCAttrAmdSevSnp))
   {
     //
     // An SEV-ES-only guest requires the memory to be reserved. SEV-SNP, which

UefiCpuPkg/UefiCpuPkg.dec

@@ -179,9 +179,9 @@
   # @Prompt Support SmmFeatureControl.
   gUefiCpuPkgTokenSpaceGuid.PcdSmmFeatureControlEnable|TRUE|BOOLEAN|0x32132110
   ## MS_CHANGE
-  ## If TRUE, Install the protocol to communicate to the Smm Exception 
-  # handler so automated testing can be performed. 
-  # 
+  ## If TRUE, Install the protocol to communicate to the Smm Exception
+  # handler so automated testing can be performed.
+  #
   gUefiCpuPkgTokenSpaceGuid.PcdSmmExceptionTestModeSupport|FALSE|BOOLEAN|0x00000021
 
 [PcdsFixedAtBuild]
@@ -209,7 +209,7 @@
   gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaSize|0x0|UINT32|0x30002006
 
   ## MS_CHANGE
-  ## If TRUE, the default Cpu Exception Handler in SMM will reboot 
+  ## If TRUE, the default Cpu Exception Handler in SMM will reboot
   # This handler state can be changed by using the Exception Test Protocol
   gUefiCpuPkgTokenSpaceGuid.PcdSmmExceptionRebootInsteadOfHaltDefault|TRUE|BOOLEAN|0x00000020
 
@@ -352,6 +352,13 @@
   # @Prompt Current boot is a power-on reset.
   gUefiCpuPkgTokenSpaceGuid.PcdIsPowerOnReset|FALSE|BOOLEAN|0x0000001B
 
+  # MU_CHANGE START Add PCD to make wakeup buffer reserved
+  ## Specifies that the wake-up buffer for AP startup should be permanently
+  # allocated as reserved.
+  # @Prompt Allocates CPU wake-up buffer as reserved.
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuApWakeupBufferReserved|FALSE|BOOLEAN|0x0000001F
+  # MU_CHANGE END
+
 [PcdsFixedAtBuild.X64, PcdsPatchableInModule.X64, PcdsDynamic.X64, PcdsDynamicEx.X64]
   ## Indicate access to non-SMRAM memory is restricted to reserved, runtime and ACPI NVS type after SmmReadyToLock.
   #  MMIO access is always allowed regardless of the value of this PCD.