MdePkg/BaseLib: Add a new VMGEXIT instruction invocation for SVSM

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654

The SVSM specification relies on a specific register calling convention to
hold the parameters that are associated with the SVSM request. The SVSM is
invoked by requesting the hypervisor to run the VMPL0 VMSA of the guest
using the GHCB MSR Protocol or a GHCB NAE event.

Create a new version of the VMGEXIT instruction that will adhere to this
calling convention and load the SVSM function arguments into the proper
register before invoking the VMGEXIT instruction. On return, perform the
atomic exchange on the SVSM call pending value as specified in the SVSM
specification.

Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

MdePkg/Library/BaseLib/Ia32/VmgExitSvsm.nasm

@@ -0,0 +1,39 @@
+;------------------------------------------------------------------------------
+;
+; Copyright (C) 2024, Advanced Micro Devices, Inc. All rights reserved.<BR>
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+; Module Name:
+;
+;   VmgExitSvsm.Asm
+;
+; Abstract:
+;
+;   AsmVmgExitSvsm function
+;
+; Notes:
+;
+;------------------------------------------------------------------------------
+
+    DEFAULT REL
+    SECTION .text
+
+;------------------------------------------------------------------------------
+; UINT32
+; EFIAPI
+; AsmVmgExitSvsm (
+;   SVSM_CALL_DATA *SvsmCallData
+;   );
+;------------------------------------------------------------------------------
+global ASM_PFX(AsmVmgExitSvsm)
+ASM_PFX(AsmVmgExitSvsm):
+;
+; NASM doesn't support the vmmcall instruction in 32-bit mode and NASM versions
+; before 2.12 cannot translate the 64-bit "rep vmmcall" instruction into elf32
+; format. Given that VMGEXIT does not make sense on IA32, provide a stub
+; implementation that is identical to CpuBreakpoint(). In practice,
+; AsmVmgExitSvsm() should never be called on IA32.
+;
+    int  3
+    ret
+