diff --git a/OvmfPkg/SmmAccess/SmmAccess2Dxe.c b/OvmfPkg/SmmAccess/SmmAccess2Dxe.c
index e098f6f15f..3691a6cd1f 100644
--- a/OvmfPkg/SmmAccess/SmmAccess2Dxe.c
+++ b/OvmfPkg/SmmAccess/SmmAccess2Dxe.c
@@ -145,6 +145,13 @@ SmmAccess2DxeEntryPoint (
 
   InitQ35TsegMbytes ();
   GetStates (&mAccess2.LockState, &mAccess2.OpenState);
+
+  //
+  // SmramAccessLock() depends on "mQ35SmramAtDefaultSmbase"; init the latter
+  // just before exposing the former via EFI_SMM_ACCESS2_PROTOCOL.Lock().
+  //
+  InitQ35SmramAtDefaultSmbase ();
+
   return gBS->InstallMultipleProtocolInterfaces (&ImageHandle,
                 &gEfiSmmAccess2ProtocolGuid, &mAccess2,
                 NULL);
diff --git a/OvmfPkg/SmmAccess/SmmAccess2Dxe.inf b/OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
index 7ced6b4e7f..d86381d0fb 100644
--- a/OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
+++ b/OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
@@ -49,6 +49,7 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
 
 [Pcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdQ35SmramAtDefaultSmbase
   gUefiOvmfPkgTokenSpaceGuid.PcdQ35TsegMbytes
 
 [Depex]
diff --git a/OvmfPkg/SmmAccess/SmmAccessPei.c b/OvmfPkg/SmmAccess/SmmAccessPei.c
index d67850651c..c8bbc17e90 100644
--- a/OvmfPkg/SmmAccess/SmmAccessPei.c
+++ b/OvmfPkg/SmmAccess/SmmAccessPei.c
@@ -372,6 +372,12 @@ SmmAccessPeiEntryPoint (
   CopyMem (GuidHob, &SmramMap[DescIdxSmmS3ResumeState],
     sizeof SmramMap[DescIdxSmmS3ResumeState]);
 
+  //
+  // SmramAccessLock() depends on "mQ35SmramAtDefaultSmbase"; init the latter
+  // just before exposing the former via PEI_SMM_ACCESS_PPI.Lock().
+  //
+  InitQ35SmramAtDefaultSmbase ();
+
   //
   // We're done. The next step should succeed, but even if it fails, we can't
   // roll back the above BuildGuidHob() allocation, because PEI doesn't support
diff --git a/OvmfPkg/SmmAccess/SmmAccessPei.inf b/OvmfPkg/SmmAccess/SmmAccessPei.inf
index d73a029cc7..1698c4ce6c 100644
--- a/OvmfPkg/SmmAccess/SmmAccessPei.inf
+++ b/OvmfPkg/SmmAccess/SmmAccessPei.inf
@@ -54,6 +54,7 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
 
 [Pcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdQ35SmramAtDefaultSmbase
   gUefiOvmfPkgTokenSpaceGuid.PcdQ35TsegMbytes
 
 [Ppis]
diff --git a/OvmfPkg/SmmAccess/SmramInternal.c b/OvmfPkg/SmmAccess/SmramInternal.c
index 09657d0f9b..0b07dc667b 100644
--- a/OvmfPkg/SmmAccess/SmramInternal.c
+++ b/OvmfPkg/SmmAccess/SmramInternal.c
@@ -21,6 +21,12 @@
 //
 UINT16 mQ35TsegMbytes;
 
+//
+// The value of PcdQ35SmramAtDefaultSmbase is saved into this variable at
+// module startup.
+//
+STATIC BOOLEAN mQ35SmramAtDefaultSmbase;
+
 /**
   Save PcdQ35TsegMbytes into mQ35TsegMbytes.
 **/
@@ -32,6 +38,17 @@ InitQ35TsegMbytes (
   mQ35TsegMbytes = PcdGet16 (PcdQ35TsegMbytes);
 }
 
+/**
+  Save PcdQ35SmramAtDefaultSmbase into mQ35SmramAtDefaultSmbase.
+**/
+VOID
+InitQ35SmramAtDefaultSmbase (
+  VOID
+  )
+{
+  mQ35SmramAtDefaultSmbase = PcdGetBool (PcdQ35SmramAtDefaultSmbase);
+}
+
 /**
   Read the MCH_SMRAM and ESMRAMC registers, and update the LockState and
   OpenState fields in the PEI_SMM_ACCESS_PPI / EFI_SMM_ACCESS2_PROTOCOL object,
@@ -125,6 +142,14 @@ SmramAccessLock (
   PciOr8 (DRAMC_REGISTER_Q35 (MCH_ESMRAMC), MCH_ESMRAMC_T_EN);
   PciOr8 (DRAMC_REGISTER_Q35 (MCH_SMRAM),   MCH_SMRAM_D_LCK);
 
+  //
+  // Close & lock the SMRAM at the default SMBASE, if it exists.
+  //
+  if (mQ35SmramAtDefaultSmbase) {
+    PciWrite8 (DRAMC_REGISTER_Q35 (MCH_DEFAULT_SMBASE_CTL),
+      MCH_DEFAULT_SMBASE_LCK);
+  }
+
   GetStates (LockState, OpenState);
   if (*OpenState || !*LockState) {
     return EFI_DEVICE_ERROR;
diff --git a/OvmfPkg/SmmAccess/SmramInternal.h b/OvmfPkg/SmmAccess/SmramInternal.h
index 74d962b4ec..a4d8827adf 100644
--- a/OvmfPkg/SmmAccess/SmramInternal.h
+++ b/OvmfPkg/SmmAccess/SmramInternal.h
@@ -38,6 +38,14 @@ InitQ35TsegMbytes (
   VOID
   );
 
+/**
+  Save PcdQ35SmramAtDefaultSmbase into mQ35SmramAtDefaultSmbase.
+**/
+VOID
+InitQ35SmramAtDefaultSmbase (
+  VOID
+  );
+
 /**
   Read the MCH_SMRAM and ESMRAMC registers, and update the LockState and
   OpenState fields in the PEI_SMM_ACCESS_PPI / EFI_SMM_ACCESS2_PROTOCOL object,