OvmfPkg/VmgExitLib: Add support for RDTSCP NAE events

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

Under SEV-ES, a RDTSCP 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

@@ -883,6 +883,49 @@ WbinvdExit (
   return VmgExit (Ghcb, SVM_EXIT_WBINVD, 0, 0);
 }
 
+/**
+  Handle a RDTSCP event.
+
+  Use the VMGEXIT instruction to handle a RDTSCP 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
+RdtscpExit (
+  IN OUT GHCB                     *Ghcb,
+  IN OUT EFI_SYSTEM_CONTEXT_X64   *Regs,
+  IN     SEV_ES_INSTRUCTION_DATA  *InstructionData
+  )
+{
+  UINT64  Status;
+
+  DecodeModRm (Regs, InstructionData);
+
+  Status = VmgExit (Ghcb, SVM_EXIT_RDTSCP, 0, 0);
+  if (Status != 0) {
+    return Status;
+  }
+
+  if (!GhcbIsRegValid (Ghcb, GhcbRax) ||
+      !GhcbIsRegValid (Ghcb, GhcbRcx) ||
+      !GhcbIsRegValid (Ghcb, GhcbRdx)) {
+    return UnsupportedExit (Ghcb, Regs, InstructionData);
+  }
+  Regs->Rax = Ghcb->SaveArea.Rax;
+  Regs->Rcx = Ghcb->SaveArea.Rcx;
+  Regs->Rdx = Ghcb->SaveArea.Rdx;
+
+  return 0;
+}
+
 /**
   Handle a VMMCALL event.
 
@@ -1445,6 +1488,10 @@ VmgExitHandleVc (
     NaeExit = VmmCallExit;
     break;
 
+  case SVM_EXIT_RDTSCP:
+    NaeExit = RdtscpExit;
+    break;
+
   case SVM_EXIT_WBINVD:
     NaeExit = WbinvdExit;
     break;