diff --git a/OvmfPkg/Include/Library/PlatformInitLib.h b/OvmfPkg/Include/Library/PlatformInitLib.h
index 57b18b94d9..ce5af42e09 100644
--- a/OvmfPkg/Include/Library/PlatformInitLib.h
+++ b/OvmfPkg/Include/Library/PlatformInitLib.h
@@ -37,6 +37,7 @@ typedef struct {
 
   UINT64               PcdPciMmio64Base;
   UINT64               PcdPciMmio64Size;
+  BOOLEAN              PcdPciMmio64Override;
   UINT32               PcdPciMmio32Base;
   UINT32               PcdPciMmio32Size;
   UINT64               PcdPciIoBase;
diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
index 1f987f2efd..44b2e0d24f 100644
--- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
+++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
@@ -556,7 +556,8 @@ PlatformGetFirstNonAddress (
       break;
     case EFI_SUCCESS:
       if (FwCfgPciMmio64Mb <= 0x1000000) {
-        PlatformInfoHob->PcdPciMmio64Size = LShiftU64 (FwCfgPciMmio64Mb, 20);
+        PlatformInfoHob->PcdPciMmio64Size     = LShiftU64 (FwCfgPciMmio64Mb, 20);
+        PlatformInfoHob->PcdPciMmio64Override = TRUE;
         break;
       }
 
@@ -795,8 +796,10 @@ PlatformDynamicMmioWindow (
   AddrSpace = LShiftU64 (1, PlatformInfoHob->PhysMemAddressWidth);
   MmioSpace = LShiftU64 (1, PlatformInfoHob->PhysMemAddressWidth - 3);
 
-  if ((PlatformInfoHob->PcdPciMmio64Size < MmioSpace) &&
-      (PlatformInfoHob->PcdPciMmio64Base + MmioSpace < AddrSpace))
+  if (PlatformInfoHob->PcdPciMmio64Override) {
+    DEBUG ((DEBUG_INFO, "%a: using fwcfg override for mmio window\n", __func__));
+  } else if ((PlatformInfoHob->PcdPciMmio64Size < MmioSpace) &&
+             (PlatformInfoHob->PcdPciMmio64Base + MmioSpace < AddrSpace))
   {
     DEBUG ((DEBUG_INFO, "%a: using dynamic mmio window\n", __func__));
     DEBUG ((DEBUG_INFO, "%a:   Addr Space 0x%Lx (%Ld GB)\n", __func__, AddrSpace, RShiftU64 (AddrSpace, 30)));