OvmfPkg: Obtain SEV encryption mask with the new MemEncryptSevLib API

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

The early assembler code performs validation for some of the SEV-related
information, specifically the encryption bit position. The new
MemEncryptSevGetEncryptionMask() interface provides access to this
validated value.

To ensure that we always use a validated encryption mask for an SEV-ES
guest, update all locations that use CPUID to calculate the encryption
mask to use the new interface.

Also, clean up some call areas where extra masking was being performed
and where a function call was being used instead of the local variable
that was just set using the function.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Cc: Peter Grehan <grehan@freebsd.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <9de678c0d66443c6cc33e004a4cac0a0223c2ebc.1610045305.git.thomas.lendacky@amd.com>

OvmfPkg/PlatformPei/AmdSev.c

@@ -1,7 +1,7 @@
 /**@file
   Initialize Secure Encrypted Virtualization (SEV) support
 
-  Copyright (c) 2017, Advanced Micro Devices. All rights reserved.<BR>
+  Copyright (c) 2017 - 2020, Advanced Micro Devices. All rights reserved.<BR>
 
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
@@ -17,9 +17,7 @@
 #include <Library/MemoryAllocationLib.h>
 #include <Library/PcdLib.h>
 #include <PiPei.h>
-#include <Register/Amd/Cpuid.h>
 #include <Register/Amd/Msr.h>
-#include <Register/Cpuid.h>
 #include <Register/Intel/SmramSaveStateMap.h>
 
 #include "Platform.h"
@@ -116,7 +114,6 @@ AmdSevInitialize (
   VOID
   )
 {
-  CPUID_MEMORY_ENCRYPTION_INFO_EBX  Ebx;
   UINT64                            EncryptionMask;
   RETURN_STATUS                     PcdStatus;
 
@@ -127,15 +124,10 @@ AmdSevInitialize (
     return;
   }
 
-  //
-  // CPUID Fn8000_001F[EBX] Bit 0:5 (memory encryption bit position)
-  //
-  AsmCpuid (CPUID_MEMORY_ENCRYPTION_INFO, NULL, &Ebx.Uint32, NULL, NULL);
-  EncryptionMask = LShiftU64 (1, Ebx.Bits.PtePosBits);
-
   //
   // Set Memory Encryption Mask PCD
   //
+  EncryptionMask = MemEncryptSevGetEncryptionMask ();
   PcdStatus = PcdSet64S (PcdPteMemoryEncryptionAddressOrMask, EncryptionMask);
   ASSERT_RETURN_ERROR (PcdStatus);