MdePkg: Introduce basic Tdx functions in BaseLib

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429

Introduce basic Tdx functions in BaseLib:
 - TdCall ()
 - TdVmCall ()
 - TdIsEnabled ()

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

MdePkg/Library/BaseLib/IntelTdxNull.c

@@ -0,0 +1,83 @@
+/** @file
+
+  Null stub of TdxLib
+
+  Copyright (c) 2021, Intel Corporation. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Library/BaseLib.h>
+#include <Uefi/UefiBaseType.h>
+
+/**
+  The TDCALL instruction causes a VM exit to the Intel TDX module.  It is
+  used to call guest-side Intel TDX functions, either local or a TD exit
+  to the host VMM, as selected by Leaf.
+  Leaf functions are described at <https://software.intel.com/content/
+  www/us/en/develop/articles/intel-trust-domain-extensions.html>
+
+  @param[in]      Leaf        Leaf number of TDCALL instruction
+  @param[in]      Arg1        Arg1
+  @param[in]      Arg2        Arg2
+  @param[in]      Arg3        Arg3
+  @param[in,out]  Results  Returned result of the Leaf function
+
+  @return EFI_SUCCESS
+  @return Other           See individual leaf functions
+**/
+UINTN
+EFIAPI
+TdCall (
+  IN UINT64    Leaf,
+  IN UINT64    Arg1,
+  IN UINT64    Arg2,
+  IN UINT64    Arg3,
+  IN OUT VOID  *Results
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+  TDVMALL is a leaf function 0 for TDCALL. It helps invoke services from the
+  host VMM to pass/receive information.
+
+  @param[in]     Leaf        Number of sub-functions
+  @param[in]     Arg1        Arg1
+  @param[in]     Arg2        Arg2
+  @param[in]     Arg3        Arg3
+  @param[in]     Arg4        Arg4
+  @param[in,out] Results     Returned result of the sub-function
+
+  @return EFI_SUCCESS
+  @return Other           See individual sub-functions
+
+**/
+UINTN
+EFIAPI
+TdVmCall (
+  IN UINT64    Leaf,
+  IN UINT64    Arg1,
+  IN UINT64    Arg2,
+  IN UINT64    Arg3,
+  IN UINT64    Arg4,
+  IN OUT VOID  *Results
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+  Probe if TD is enabled.
+
+  @return TRUE    TD is enabled.
+  @return FALSE   TD is not enabled.
+**/
+BOOLEAN
+EFIAPI
+TdIsEnabled (
+  )
+{
+  return FALSE;
+}