OvmfPkg/IntelTdx: Add TdxHelperLibNull

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4243

TdxHelperLib provides below helper functions for a td-guest.
 - TdxHelperProcessTdHob
 - TdxHelperMeasureTdHob
 - TdxHelperMeasureCfvImage
 - TdxHelperBuildGuidHobForTdxMeasurement

TdxHelperLibNull is the NULL instance of TdxHelperLib.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/IntelTdx/TdxHelperLib/TdxHelperNull.c

@@ -0,0 +1,79 @@
+/** @file
+  NULL instance of TdxHelperLib
+
+  Copyright (c) 2022 - 2023, Intel Corporation. All rights reserved.<BR>
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Base.h>
+#include <PiPei.h>
+
+/**
+  In Tdx guest, some information need to be passed from host VMM to guest
+  firmware. For example, the memory resource, etc. These information are
+  prepared by host VMM and put in TdHob which is described in TdxMetadata.
+  TDVF processes the TdHob to accept memories.
+
+  @retval   EFI_SUCCESS   Successfully process the TdHob
+  @retval   Others        Other error as indicated
+**/
+EFI_STATUS
+EFIAPI
+TdxHelperProcessTdHob (
+  VOID
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+  In Tdx guest, TdHob is passed from host VMM to guest firmware and it contains
+  the information of the memory resource. From the security perspective before
+  it is consumed, it should be measured and extended.
+ *
+ * @retval EFI_SUCCESS Successfully measure the TdHob
+ * @retval Others      Other error as indicated
+ */
+EFI_STATUS
+EFIAPI
+TdxHelperMeasureTdHob (
+  VOID
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+ * In Tdx guest, Configuration FV (CFV) is treated as external input because it
+ * may contain the data provided by VMM. From the sucurity perspective Cfv image
+ * should be measured before it is consumed.
+ *
+ * @retval EFI_SUCCESS Successfully measure the CFV image
+ * @retval Others      Other error as indicated
+ */
+EFI_STATUS
+EFIAPI
+TdxHelperMeasureCfvImage (
+  VOID
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+  Build the GuidHob for tdx measurements which were done in SEC phase.
+  The measurement values are stored in WorkArea.
+
+  @retval EFI_SUCCESS  The GuidHob is built successfully
+  @retval Others       Other errors as indicated
+**/
+EFI_STATUS
+EFIAPI
+TdxHelperBuildGuidHobForTdxMeasurement (
+  VOID
+  )
+{
+  return EFI_UNSUPPORTED;
+}