OvmfPkg/VmgExitLib: Add support for MWAIT/MWAITX NAE events

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

Under SEV-ES, a MWAIT/MWAITX intercept generates a #VC exception.
VMGEXIT must be used to allow the hypervisor to handle this intercept.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c

@@ -858,6 +858,38 @@ MmioExit (
   return Status;
 }
 
+/**
+  Handle a MWAIT event.
+
+  Use the VMGEXIT instruction to handle a MWAIT event.
+
+  @param[in, out] Ghcb             Pointer to the Guest-Hypervisor Communication
+                                   Block
+  @param[in, out] Regs             x64 processor context
+  @param[in]      InstructionData  Instruction parsing context
+
+  @retval 0                        Event handled successfully
+  @return                          New exception value to propagate
+
+**/
+STATIC
+UINT64
+MwaitExit (
+  IN OUT GHCB                     *Ghcb,
+  IN OUT EFI_SYSTEM_CONTEXT_X64   *Regs,
+  IN     SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  DecodeModRm (Regs, InstructionData);
+
+  Ghcb->SaveArea.Rax = Regs->Rax;
+  GhcbSetRegValid (Ghcb, GhcbRax);
+  Ghcb->SaveArea.Rcx = Regs->Rcx;
+  GhcbSetRegValid (Ghcb, GhcbRcx);
+
+  return VmgExit (Ghcb, SVM_EXIT_MWAIT, 0, 0);
+}
+
 /**
   Handle a MONITOR event.
 
@@ -1534,6 +1566,10 @@ VmgExitHandleVc (
     NaeExit = MonitorExit;
     break;
 
+  case SVM_EXIT_MWAIT:
+    NaeExit = MwaitExit;
+    break;
+
   case SVM_EXIT_NPF:
     NaeExit = MmioExit;
     break;