MdePkg/BaseLib: Add support for the XGETBV instruction

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Under SEV-ES, a CPUID instruction requires the current value of the XCR0
register. In order to retrieve that value, the XGETBV instruction needs
to be executed.

Provide the necessary support to execute the XGETBV instruction.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

MdePkg/Library/BaseLib/X64/XGetBv.nasm

@@ -0,0 +1,34 @@
+;------------------------------------------------------------------------------
+;
+; Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+; Module Name:
+;
+;   XGetBv.Asm
+;
+; Abstract:
+;
+;   AsmXgetBv function
+;
+; Notes:
+;
+;------------------------------------------------------------------------------
+
+    DEFAULT REL
+    SECTION .text
+
+;------------------------------------------------------------------------------
+; UINT64
+; EFIAPI
+; AsmXGetBv (
+;   IN UINT32  Index
+;   );
+;------------------------------------------------------------------------------
+global ASM_PFX(AsmXGetBv)
+ASM_PFX(AsmXGetBv):
+    xgetbv
+    shl     rdx, 32
+    or      rax, rdx
+    ret
+