[PATCH v2 13/15] OvmfPkg/VmgExitLib: Support nested #VCs

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

From: Tom Lendacky <thomas.lendacky@amd.com>

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3108

In order to be able to issue messages or make interface calls that cause
another #VC (e.g. GetLocalApicBaseAddress () issues RDMSR), add support
for nested #VCs.

In order to support nested #VCs, GHCB backup pages are required. If a #VC
is received while currently processing a #VC, a backup of the current GHCB
content is made. This allows the #VC handler to continue processing the
new #VC. Upon completion of the new #VC, the GHCB is restored from the
backup page. The #VC recursion level is tracked in the per-vCPU variable
area.

Support is added to handle up to one nested #VC (or two #VCs total). If
a second nested #VC is encountered, an ASSERT will be issued and the vCPU
will enter CpuDeadLoop ().

For SEC, the GHCB backup pages are reserved in the OvmfPkgX64.fdf memory
layout, with two new fixed PCDs to provide the address and size of the
backup area.

For PEI/DXE, the GHCB backup pages are allocated as boot services pages
using the memory allocation library.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 OvmfPkg/OvmfPkg.dec                                 |   2 +
 OvmfPkg/AmdSev/AmdSevX64.dsc                        |   1 +
 OvmfPkg/OvmfPkgX64.dsc                              |   1 +
 OvmfPkg/AmdSev/AmdSevX64.fdf                        |   3 +
 OvmfPkg/OvmfPkgX64.fdf                              |   3 +
 OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf        |  43 ++++++++
 OvmfPkg/Library/VmgExitLib/VmgExitLib.inf           |   4 +-
 OvmfPkg/Include/Library/MemEncryptSevLib.h          |  23 +++++
 OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.h       |  53 ++++++++++
 OvmfPkg/Library/VmgExitLib/PeiDxeVmgExitVcHandler.c | 103 ++++++++++++++++++
 OvmfPkg/Library/VmgExitLib/SecVmgExitVcHandler.c    | 109 ++++++++++++++++++++
 OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c       |  48 +++++----
 OvmfPkg/PlatformPei/AmdSev.c                        |  38 +++++--
 13 files changed, 404 insertions(+), 27 deletions(-)

diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index 50d7b27d941c..54804962ec02 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -304,6 +304,8 @@ [PcdsFixedAtBuild]
   ## The base address of the SEC GHCB page used by SEV-ES.
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBase|0|UINT32|0x40
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbSize|0|UINT32|0x41
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupBase|0|UINT32|0x44
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupSize|0|UINT32|0x45
 
   ## The base address and size of the SEV Launch Secret Area provisioned
   #  after remote attestation.  If this is set in the .fdf, the platform
diff --git a/OvmfPkg/AmdSev/AmdSevX64.dsc b/OvmfPkg/AmdSev/AmdSevX64.dsc
index d39436e7afc9..c4d93f39b9f1 100644
--- a/OvmfPkg/AmdSev/AmdSevX64.dsc
+++ b/OvmfPkg/AmdSev/AmdSevX64.dsc
@@ -236,6 +236,7 @@ [LibraryClasses.common.SEC]
 !else
   CpuExceptionHandlerLib|UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
 !endif
+  VmgExitLib|OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
 
 [LibraryClasses.common.PEI_CORE]
   HobLib|MdePkg/Library/PeiHobLib/PeiHobLib.inf
diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
index 799468e08e92..bfa9dd7cac1f 100644
--- a/OvmfPkg/OvmfPkgX64.dsc
+++ b/OvmfPkg/OvmfPkgX64.dsc
@@ -265,6 +265,7 @@ [LibraryClasses.common.SEC]
 !else
   CpuExceptionHandlerLib|UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
 !endif
+  VmgExitLib|OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
 
 [LibraryClasses.common.PEI_CORE]
   HobLib|MdePkg/Library/PeiHobLib/PeiHobLib.inf
diff --git a/OvmfPkg/AmdSev/AmdSevX64.fdf b/OvmfPkg/AmdSev/AmdSevX64.fdf
index e8fd4b8c7b89..c0098502aa90 100644
--- a/OvmfPkg/AmdSev/AmdSevX64.fdf
+++ b/OvmfPkg/AmdSev/AmdSevX64.fdf
@@ -62,6 +62,9 @@ [FD.MEMFD]
 0x00C000|0x001000
 gUefiOvmfPkgTokenSpaceGuid.PcdSevLaunchSecretBase|gUefiOvmfPkgTokenSpaceGuid.PcdSevLaunchSecretSize
 
+0x00D000|0x001000
+gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupBase|gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupSize
+
 0x010000|0x010000
 gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamBase|gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamSize
 
diff --git a/OvmfPkg/OvmfPkgX64.fdf b/OvmfPkg/OvmfPkgX64.fdf
index 1a2ef5bf2ae3..d519f8532822 100644
--- a/OvmfPkg/OvmfPkgX64.fdf
+++ b/OvmfPkg/OvmfPkgX64.fdf
@@ -85,6 +85,9 @@ [FD.MEMFD]
 0x00B000|0x001000
 gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase|gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaSize
 
+0x00C000|0x001000
+gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupBase|gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupSize
+
 0x010000|0x010000
 gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamBase|gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecPeiTempRamSize
 
diff --git a/OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf b/OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
new file mode 100644
index 000000000000..df14de3c21bc
--- /dev/null
+++ b/OvmfPkg/Library/VmgExitLib/SecVmgExitLib.inf
@@ -0,0 +1,43 @@
+## @file
+#  VMGEXIT Support Library.
+#
+#  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = SecVmgExitLib
+  FILE_GUID                      = dafff819-f86c-4cff-a70e-83161e5bcf9a
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = VmgExitLib|SEC
+
+#
+# The following information is for reference only and not required by the build tools.
+#
+#  VALID_ARCHITECTURES           = X64
+#
+
+[Sources.common]
+  VmgExitLib.c
+  VmgExitVcHandler.c
+  VmgExitVcHandler.h
+  SecVmgExitVcHandler.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+  UefiCpuPkg/UefiCpuPkg.dec
+
+[LibraryClasses]
+  BaseLib
+  BaseMemoryLib
+  DebugLib
+  PcdLib
+
+[FixedPcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupBase
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupSize
+
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitLib.inf b/OvmfPkg/Library/VmgExitLib/VmgExitLib.inf
index d003ac63173e..b3c3e56ecff8 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitLib.inf
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitLib.inf
@@ -12,7 +12,7 @@ [Defines]
   FILE_GUID                      = 0e923c25-13cd-430b-8714-ffe85652a97b
   MODULE_TYPE                    = BASE
   VERSION_STRING                 = 1.0
-  LIBRARY_CLASS                  = VmgExitLib
+  LIBRARY_CLASS                  = VmgExitLib|PEIM DXE_CORE DXE_DRIVER DXE_RUNTIME_DRIVER DXE_SMM_DRIVER UEFI_DRIVER
 
 #
 # The following information is for reference only and not required by the build tools.
@@ -23,6 +23,8 @@ [Defines]
 [Sources.common]
   VmgExitLib.c
   VmgExitVcHandler.c
+  VmgExitVcHandler.h
+  PeiDxeVmgExitVcHandler.c
 
 [Packages]
   MdePkg/MdePkg.dec
diff --git a/OvmfPkg/Include/Library/MemEncryptSevLib.h b/OvmfPkg/Include/Library/MemEncryptSevLib.h
index ec470b8d0363..99f15a7d1271 100644
--- a/OvmfPkg/Include/Library/MemEncryptSevLib.h
+++ b/OvmfPkg/Include/Library/MemEncryptSevLib.h
@@ -13,6 +13,29 @@
 
 #include <Base.h>
 
+//
+// Define the maximum number of #VCs allowed (e.g. the level of nesting
+// that is allowed => 2 allows for 1 nested #VCs). I this value is changed,
+// be sure to increase the size of
+//   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfSecGhcbBackupSize
+// in any FDF file using this PCD.
+//
+#define VMGEXIT_MAXIMUM_VC_COUNT   2
+
+//
+// Per-CPU data mapping structure
+//   Use UINT32 for cached indicators and compare to a specific value
+//   so that the hypervisor can't indicate a value is cached by just
+//   writing random data to that area.
+//
+typedef struct {
+  UINT32  Dr7Cached;
+  UINT64  Dr7;
+
+  UINTN   VcCount;
+  VOID    *GhcbBackupPages;
+} SEV_ES_PER_CPU_DATA;
+
 //
 // Internal structure for holding SEV-ES information needed during SEC phase
 // and valid only during SEC phase and early PEI during platform
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.h b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.h
new file mode 100644
index 000000000000..3a37cb04f616
--- /dev/null
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.h
@@ -0,0 +1,53 @@
+/** @file
+  X64 #VC Exception Handler functon header file.
+
+  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef __VMG_EXIT_VC_HANDLER_H__
+#define __VMG_EXIT_VC_HANDLER_H__
+
+#include <Base.h>
+#include <Uefi.h>
+#include <Library/VmgExitLib.h>
+
+/**
+  Handle a #VC exception.
+
+  Performs the necessary processing to handle a #VC exception.
+
+  @param[in, out]  Ghcb           Pointer to the GHCB
+  @param[in, out]  ExceptionType  Pointer to an EFI_EXCEPTION_TYPE to be set
+                                  as value to use on error.
+  @param[in, out]  SystemContext  Pointer to EFI_SYSTEM_CONTEXT
+
+  @retval  EFI_SUCCESS            Exception handled
+  @retval  EFI_UNSUPPORTED        #VC not supported, (new) exception value to
+                                  propagate provided
+  @retval  EFI_PROTOCOL_ERROR     #VC handling failed, (new) exception value to
+                                  propagate provided
+
+**/
+EFI_STATUS
+EFIAPI
+InternalVmgExitHandleVc (
+  IN OUT GHCB                *Ghcb,
+  IN OUT EFI_EXCEPTION_TYPE  *ExceptionType,
+  IN OUT EFI_SYSTEM_CONTEXT  SystemContext
+  );
+
+/**
+  Routine to allow ASSERT from within #VC.
+
+  @param[in, out]  SevEsData  Pointer to the per-CPU data
+
+**/
+VOID
+EFIAPI
+VmgExitIssueAssert (
+  IN OUT SEV_ES_PER_CPU_DATA  *SevEsData
+  );
+
+#endif
diff --git a/OvmfPkg/Library/VmgExitLib/PeiDxeVmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/PeiDxeVmgExitVcHandler.c
new file mode 100644
index 000000000000..fb4942df37ca
--- /dev/null
+++ b/OvmfPkg/Library/VmgExitLib/PeiDxeVmgExitVcHandler.c
@@ -0,0 +1,103 @@
+/** @file
+  X64 #VC Exception Handler functon.
+
+  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Base.h>
+#include <Uefi.h>
+#include <Library/BaseMemoryLib.h>
+#include <Library/MemEncryptSevLib.h>
+#include <Library/VmgExitLib.h>
+#include <Register/Amd/Msr.h>
+
+#include "VmgExitVcHandler.h"
+
+/**
+  Handle a #VC exception.
+
+  Performs the necessary processing to handle a #VC exception.
+
+  @param[in, out]  ExceptionType  Pointer to an EFI_EXCEPTION_TYPE to be set
+                                  as value to use on error.
+  @param[in, out]  SystemContext  Pointer to EFI_SYSTEM_CONTEXT
+
+  @retval  EFI_SUCCESS            Exception handled
+  @retval  EFI_UNSUPPORTED        #VC not supported, (new) exception value to
+                                  propagate provided
+  @retval  EFI_PROTOCOL_ERROR     #VC handling failed, (new) exception value to
+                                  propagate provided
+
+**/
+EFI_STATUS
+EFIAPI
+VmgExitHandleVc (
+  IN OUT EFI_EXCEPTION_TYPE  *ExceptionType,
+  IN OUT EFI_SYSTEM_CONTEXT  SystemContext
+  )
+{
+  MSR_SEV_ES_GHCB_REGISTER  Msr;
+  GHCB                      *Ghcb;
+  GHCB                      *GhcbBackup;
+  EFI_STATUS                VcRet;
+  BOOLEAN                   InterruptState;
+  SEV_ES_PER_CPU_DATA       *SevEsData;
+
+  InterruptState = GetInterruptState ();
+  if (InterruptState) {
+    DisableInterrupts ();
+  }
+
+  Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
+  ASSERT (Msr.GhcbInfo.Function == 0);
+  ASSERT (Msr.Ghcb != 0);
+
+  Ghcb = Msr.Ghcb;
+  GhcbBackup = NULL;
+
+  SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+  SevEsData->VcCount++;
+
+  //
+  // Check for maximum PEI/DXE #VC nesting.
+  //
+  if (SevEsData->VcCount > VMGEXIT_MAXIMUM_VC_COUNT) {
+    VmgExitIssueAssert (SevEsData);
+  } else if (SevEsData->VcCount > 1) {
+    //
+    // Nested #VC
+    //
+    if (SevEsData->GhcbBackupPages == NULL) {
+      VmgExitIssueAssert (SevEsData);
+    }
+
+    //
+    // Save the active GHCB to a backup page.
+    //   To access the correct backup page, increment the backup page pointer
+    //   based on the current VcCount.
+    //
+    GhcbBackup = (GHCB *) SevEsData->GhcbBackupPages;
+    GhcbBackup += (SevEsData->VcCount - 2);
+
+    CopyMem (GhcbBackup, Ghcb, sizeof (*Ghcb));
+  }
+
+  VcRet = InternalVmgExitHandleVc (Ghcb, ExceptionType, SystemContext);
+
+  if (GhcbBackup != NULL) {
+    //
+    // Restore the active GHCB from the backup page.
+    //
+    CopyMem (Ghcb, GhcbBackup, sizeof (*Ghcb));
+  }
+
+  SevEsData->VcCount--;
+
+  if (InterruptState) {
+    EnableInterrupts ();
+  }
+
+  return VcRet;
+}
diff --git a/OvmfPkg/Library/VmgExitLib/SecVmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/SecVmgExitVcHandler.c
new file mode 100644
index 000000000000..85853d334b35
--- /dev/null
+++ b/OvmfPkg/Library/VmgExitLib/SecVmgExitVcHandler.c
@@ -0,0 +1,109 @@
+/** @file
+  X64 #VC Exception Handler functon.
+
+  Copyright (C) 2020, Advanced Micro Devices, Inc. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Base.h>
+#include <Uefi.h>
+#include <Library/BaseMemoryLib.h>
+#include <Library/MemEncryptSevLib.h>
+#include <Library/VmgExitLib.h>
+#include <Register/Amd/Msr.h>
+
+#include "VmgExitVcHandler.h"
+
+/**
+  Handle a #VC exception.
+
+  Performs the necessary processing to handle a #VC exception.
+
+  @param[in, out]  ExceptionType  Pointer to an EFI_EXCEPTION_TYPE to be set
+                                  as value to use on error.
+  @param[in, out]  SystemContext  Pointer to EFI_SYSTEM_CONTEXT
+
+  @retval  EFI_SUCCESS            Exception handled
+  @retval  EFI_UNSUPPORTED        #VC not supported, (new) exception value to
+                                  propagate provided
+  @retval  EFI_PROTOCOL_ERROR     #VC handling failed, (new) exception value to
+                                  propagate provided
+
+**/
+EFI_STATUS
+EFIAPI
+VmgExitHandleVc (
+  IN OUT EFI_EXCEPTION_TYPE  *ExceptionType,
+  IN OUT EFI_SYSTEM_CONTEXT  SystemContext
+  )
+{
+  MSR_SEV_ES_GHCB_REGISTER  Msr;
+  GHCB                      *Ghcb;
+  GHCB                      *GhcbBackup;
+  EFI_STATUS                VcRet;
+  BOOLEAN                   InterruptState;
+  SEV_ES_PER_CPU_DATA       *SevEsData;
+
+  InterruptState = GetInterruptState ();
+  if (InterruptState) {
+    DisableInterrupts ();
+  }
+
+  Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
+  ASSERT (Msr.GhcbInfo.Function == 0);
+  ASSERT (Msr.Ghcb != 0);
+
+  Ghcb = Msr.Ghcb;
+  GhcbBackup = NULL;
+
+  SevEsData = (SEV_ES_PER_CPU_DATA *) (Ghcb + 1);
+  SevEsData->VcCount++;
+
+  //
+  // Check for maximum SEC #VC nesting.
+  //
+  if (SevEsData->VcCount > VMGEXIT_MAXIMUM_VC_COUNT) {
+    VmgExitIssueAssert (SevEsData);
+  } else if (SevEsData->VcCount > 1) {
+    UINTN  GhcbBackupSize;
+
+    //
+    // Be sure that the proper amount of pages are allocated
+    //
+    GhcbBackupSize = (VMGEXIT_MAXIMUM_VC_COUNT - 1) * sizeof (*Ghcb);
+    if (GhcbBackupSize > FixedPcdGet32 (PcdOvmfSecGhcbBackupSize)) {
+      //
+      // Not enough SEC backup pages allocated.
+      //
+      VmgExitIssueAssert (SevEsData);
+    }
+
+    //
+    // Save the active GHCB to a backup page.
+    //   To access the correct backup page, increment the backup page pointer
+    //   based on the current VcCount.
+    //
+    GhcbBackup = (GHCB *) FixedPcdGet32 (PcdOvmfSecGhcbBackupBase);
+    GhcbBackup += (SevEsData->VcCount - 2);
+
+    CopyMem (GhcbBackup, Ghcb, sizeof (*Ghcb));
+  }
+
+  VcRet = InternalVmgExitHandleVc (Ghcb, ExceptionType, SystemContext);
+
+  if (GhcbBackup != NULL) {
+    //
+    // Restore the active GHCB from the backup page.
+    //
+    CopyMem (Ghcb, GhcbBackup, sizeof (*Ghcb));
+  }
+
+  SevEsData->VcCount--;
+
+  if (InterruptState) {
+    EnableInterrupts ();
+  }
+
+  return VcRet;
+}
diff --git a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
index 5149ab2bc989..ce577e4677eb 100644
--- a/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
+++ b/OvmfPkg/Library/VmgExitLib/VmgExitVcHandler.c
@@ -9,11 +9,14 @@
 #include <Base.h>
 #include <Uefi.h>
 #include <Library/BaseMemoryLib.h>
+#include <Library/MemEncryptSevLib.h>
 #include <Library/VmgExitLib.h>
 #include <Register/Amd/Msr.h>
 #include <Register/Intel/Cpuid.h>
 #include <IndustryStandard/InstructionParsing.h>
 
+#include "VmgExitVcHandler.h"
+
 //
 // Instruction execution mode definition
 //
@@ -126,18 +129,6 @@ UINT64
   SEV_ES_INSTRUCTION_DATA  *InstructionData
   );
 
-//
-// Per-CPU data mapping structure
-//   Use UINT32 for cached indicators and compare to a specific value
-//   so that the hypervisor can't indicate a value is cached by just
-//   writing random data to that area.
-//
-typedef struct {
-  UINT32  Dr7Cached;
-  UINT64  Dr7;
-} SEV_ES_PER_CPU_DATA;
-
-
 /**
   Return a pointer to the contents of the specified register.
 
@@ -1546,6 +1537,7 @@ Dr7ReadExit (
 
   Performs the necessary processing to handle a #VC exception.
 
+  @param[in, out]  Ghcb           Pointer to the GHCB
   @param[in, out]  ExceptionType  Pointer to an EFI_EXCEPTION_TYPE to be set
                                   as value to use on error.
   @param[in, out]  SystemContext  Pointer to EFI_SYSTEM_CONTEXT
@@ -1559,14 +1551,13 @@ Dr7ReadExit (
 **/
 EFI_STATUS
 EFIAPI
-VmgExitHandleVc (
+InternalVmgExitHandleVc (
+  IN OUT GHCB                *Ghcb,
   IN OUT EFI_EXCEPTION_TYPE  *ExceptionType,
   IN OUT EFI_SYSTEM_CONTEXT  SystemContext
   )
 {
-  MSR_SEV_ES_GHCB_REGISTER  Msr;
   EFI_SYSTEM_CONTEXT_X64    *Regs;
-  GHCB                      *Ghcb;
   NAE_EXIT                  NaeExit;
   SEV_ES_INSTRUCTION_DATA   InstructionData;
   UINT64                    ExitCode, Status;
@@ -1575,12 +1566,7 @@ VmgExitHandleVc (
 
   VcRet = EFI_SUCCESS;
 
-  Msr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);
-  ASSERT (Msr.GhcbInfo.Function == 0);
-  ASSERT (Msr.Ghcb != 0);
-
   Regs = SystemContext.SystemContextX64;
-  Ghcb = Msr.Ghcb;
 
   VmgInit (Ghcb, &InterruptState);
 
@@ -1670,3 +1656,25 @@ VmgExitHandleVc (
 
   return VcRet;
 }
+
+/**
+  Routine to allow ASSERT from within #VC.
+
+  @param[in, out]  SevEsData  Pointer to the per-CPU data
+
+**/
+VOID
+EFIAPI
+VmgExitIssueAssert (
+  IN OUT SEV_ES_PER_CPU_DATA  *SevEsData
+  )
+{
+  //
+  // Progress will be halted, so set VcCount to allow for ASSERT output
+  // to be seen.
+  //
+  SevEsData->VcCount = 0;
+
+  ASSERT (FALSE);
+  CpuDeadLoop ();
+}
diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
index 954d53eba4e8..dddffdebda4b 100644
--- a/OvmfPkg/PlatformPei/AmdSev.c
+++ b/OvmfPkg/PlatformPei/AmdSev.c
@@ -33,12 +33,17 @@ AmdSevEsInitialize (
   VOID
   )
 {
-  VOID              *GhcbBase;
-  PHYSICAL_ADDRESS  GhcbBasePa;
-  UINTN             GhcbPageCount, PageCount;
-  RETURN_STATUS     PcdStatus, DecryptStatus;
-  IA32_DESCRIPTOR   Gdtr;
-  VOID              *Gdt;
+  UINT8                *GhcbBase;
+  PHYSICAL_ADDRESS     GhcbBasePa;
+  UINTN                GhcbPageCount;
+  UINT8                *GhcbBackupBase;
+  UINT8                *GhcbBackupPages;
+  UINTN                GhcbBackupPageCount;
+  SEV_ES_PER_CPU_DATA  *SevEsData;
+  UINTN                PageCount;
+  RETURN_STATUS        PcdStatus, DecryptStatus;
+  IA32_DESCRIPTOR      Gdtr;
+  VOID                 *Gdt;
 
   if (!MemEncryptSevEsIsEnabled ()) {
     return;
@@ -84,6 +89,27 @@ AmdSevEsInitialize (
     "SEV-ES is enabled, %lu GHCB pages allocated starting at 0x%p\n",
     (UINT64)GhcbPageCount, GhcbBase));
 
+  //
+  // Allocate #VC recursion backup pages. The number of backup pages needed is
+  // one less than the maximum VC count.
+  //
+  GhcbBackupPageCount = mMaxCpuCount * (VMGEXIT_MAXIMUM_VC_COUNT - 1);
+  GhcbBackupBase = AllocatePages (GhcbBackupPageCount);
+  ASSERT (GhcbBackupBase != NULL);
+
+  GhcbBackupPages = GhcbBackupBase;
+  for (PageCount = 1; PageCount < GhcbPageCount; PageCount += 2) {
+    SevEsData =
+      (SEV_ES_PER_CPU_DATA *)(GhcbBase + EFI_PAGES_TO_SIZE (PageCount));
+    SevEsData->GhcbBackupPages = GhcbBackupPages;
+
+    GhcbBackupPages += EFI_PAGE_SIZE * (VMGEXIT_MAXIMUM_VC_COUNT - 1);
+  }
+
+  DEBUG ((DEBUG_INFO,
+    "SEV-ES is enabled, %lu GHCB backup pages allocated starting at 0x%p\n",
+    (UINT64)GhcbBackupPageCount, GhcbBackupBase));
+
   AsmWriteMsr64 (MSR_SEV_ES_GHCB, GhcbBasePa);
 
   //
-- 
2.30.0