diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
index 47dc9c5437..f32df937f9 100644
--- a/OvmfPkg/PlatformPei/MemDetect.c
+++ b/OvmfPkg/PlatformPei/MemDetect.c
@@ -33,6 +33,7 @@ Module Name:
 #include <Library/ResourcePublicationLib.h>
 #include <Library/MtrrLib.h>
 #include <Library/QemuFwCfgLib.h>
+#include <Library/QemuFwCfgSimpleParserLib.h>
 
 #include "Platform.h"
 #include "Cmos.h"
@@ -336,7 +337,7 @@ GetFirstNonAddress (
 {
   UINT64               FirstNonAddress;
   UINT64               Pci64Base, Pci64Size;
-  CHAR8                MbString[7 + 1];
+  UINT32               FwCfgPciMmio64Mb;
   EFI_STATUS           Status;
   FIRMWARE_CONFIG_ITEM FwCfgItem;
   UINTN                FwCfgSize;
@@ -379,25 +380,30 @@ GetFirstNonAddress (
 
   //
   // See if the user specified the number of megabytes for the 64-bit PCI host
-  // aperture. The number of non-NUL characters in MbString allows for
-  // 9,999,999 MB, which is approximately 10 TB.
+  // aperture. Accept an aperture size up to 16TB.
   //
   // As signaled by the "X-" prefix, this knob is experimental, and might go
   // away at any time.
   //
-  Status = QemuFwCfgFindFile ("opt/ovmf/X-PciMmio64Mb", &FwCfgItem,
-             &FwCfgSize);
-  if (!EFI_ERROR (Status)) {
-    if (FwCfgSize >= sizeof MbString) {
-      DEBUG ((EFI_D_WARN,
-        "%a: ignoring malformed 64-bit PCI host aperture size from fw_cfg\n",
-        __FUNCTION__));
-    } else {
-      QemuFwCfgSelectItem (FwCfgItem);
-      QemuFwCfgReadBytes (FwCfgSize, MbString);
-      MbString[FwCfgSize] = '\0';
-      Pci64Size = LShiftU64 (AsciiStrDecimalToUint64 (MbString), 20);
+  Status = QemuFwCfgParseUint32 ("opt/ovmf/X-PciMmio64Mb", FALSE,
+             &FwCfgPciMmio64Mb);
+  switch (Status) {
+  case EFI_UNSUPPORTED:
+  case EFI_NOT_FOUND:
+    break;
+  case EFI_SUCCESS:
+    if (FwCfgPciMmio64Mb <= 0x1000000) {
+      Pci64Size = LShiftU64 (FwCfgPciMmio64Mb, 20);
+      break;
     }
+    //
+    // fall through
+    //
+  default:
+    DEBUG ((DEBUG_WARN,
+      "%a: ignoring malformed 64-bit PCI host aperture size from fw_cfg\n",
+      __FUNCTION__));
+    break;
   }
 
   if (Pci64Size == 0) {
diff --git a/OvmfPkg/PlatformPei/PlatformPei.inf b/OvmfPkg/PlatformPei/PlatformPei.inf
index 19f2424981..e72ef7963d 100644
--- a/OvmfPkg/PlatformPei/PlatformPei.inf
+++ b/OvmfPkg/PlatformPei/PlatformPei.inf
@@ -60,6 +60,7 @@
   PeimEntryPoint
   QemuFwCfgLib
   QemuFwCfgS3Lib
+  QemuFwCfgSimpleParserLib
   MtrrLib
   MemEncryptSevLib
   PcdLib