diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
index e890e36408..d451989f31 100644
--- a/OvmfPkg/PlatformPei/MemDetect.c
+++ b/OvmfPkg/PlatformPei/MemDetect.c
@@ -14,6 +14,7 @@ Module Name:
 // The package level header files this module uses
 //
 #include <IndustryStandard/E820.h>
+#include <IndustryStandard/I440FxPiix4.h>
 #include <IndustryStandard/Q35MchIch9.h>
 #include <PiPei.h>
 
@@ -42,6 +43,8 @@ STATIC UINT32 mS3AcpiReservedMemorySize;
 
 STATIC UINT16 mQ35TsegMbytes;
 
+UINT32 mQemuUc32Base;
+
 VOID
 Q35TsegMbytesInitialization (
   VOID
@@ -98,6 +101,54 @@ Q35TsegMbytesInitialization (
 }
 
 
+VOID
+QemuUc32BaseInitialization (
+  VOID
+  )
+{
+  UINT32 LowerMemorySize;
+  UINT32 Uc32Size;
+
+  if (mXen) {
+    return;
+  }
+
+  if (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
+    //
+    // On q35, the 32-bit area that we'll mark as UC, through variable MTRRs,
+    // starts at PcdPciExpressBaseAddress. The platform DSC is responsible for
+    // setting PcdPciExpressBaseAddress such that describing the
+    // [PcdPciExpressBaseAddress, 4GB) range require a very small number of
+    // variable MTRRs (preferably 1 or 2).
+    //
+    ASSERT (FixedPcdGet64 (PcdPciExpressBaseAddress) <= MAX_UINT32);
+    mQemuUc32Base = (UINT32)FixedPcdGet64 (PcdPciExpressBaseAddress);
+    return;
+  }
+
+  ASSERT (mHostBridgeDevId == INTEL_82441_DEVICE_ID);
+  //
+  // On i440fx, start with the [LowerMemorySize, 4GB) range. Make sure one
+  // variable MTRR suffices by truncating the size to a whole power of two,
+  // while keeping the end affixed to 4GB. This will round the base up.
+  //
+  LowerMemorySize = GetSystemMemorySizeBelow4gb ();
+  Uc32Size = GetPowerOfTwo32 ((UINT32)(SIZE_4GB - LowerMemorySize));
+  mQemuUc32Base = (UINT32)(SIZE_4GB - Uc32Size);
+  //
+  // Assuming that LowerMemorySize is at least 1 byte, Uc32Size is at most 2GB.
+  // Therefore mQemuUc32Base is at least 2GB.
+  //
+  ASSERT (mQemuUc32Base >= BASE_2GB);
+
+  if (mQemuUc32Base != LowerMemorySize) {
+    DEBUG ((DEBUG_VERBOSE, "%a: rounded UC32 base from 0x%x up to 0x%x, for "
+      "an UC32 size of 0x%x\n", __FUNCTION__, LowerMemorySize, mQemuUc32Base,
+      Uc32Size));
+  }
+}
+
+
 /**
   Iterate over the RAM entries in QEMU's fw_cfg E820 RAM map that start outside
   of the 32-bit address range.
@@ -688,11 +739,11 @@ QemuInitializeRam (
     ASSERT_EFI_ERROR (Status);
 
     //
-    // Set memory range from the "top of lower RAM" (RAM below 4GB) to 4GB as
-    // uncacheable
+    // Set the memory range from the start of the 32-bit MMIO area (32-bit PCI
+    // MMIO aperture on i440fx, PCIEXBAR on q35) to 4GB as uncacheable.
     //
-    Status = MtrrSetMemoryAttribute (LowerMemorySize,
-               SIZE_4GB - LowerMemorySize, CacheUncacheable);
+    Status = MtrrSetMemoryAttribute (mQemuUc32Base, SIZE_4GB - mQemuUc32Base,
+               CacheUncacheable);
     ASSERT_EFI_ERROR (Status);
   }
 }
diff --git a/OvmfPkg/PlatformPei/Platform.c b/OvmfPkg/PlatformPei/Platform.c
index 0876316eef..3ba2459872 100644
--- a/OvmfPkg/PlatformPei/Platform.c
+++ b/OvmfPkg/PlatformPei/Platform.c
@@ -191,7 +191,8 @@ MemMapInitialization (
       ASSERT (PciExBarBase <= MAX_UINT32 - SIZE_256MB);
       PciBase = (UINT32)(PciExBarBase + SIZE_256MB);
     } else {
-      PciBase = (TopOfLowRam < BASE_2GB) ? BASE_2GB : TopOfLowRam;
+      ASSERT (TopOfLowRam <= mQemuUc32Base);
+      PciBase = mQemuUc32Base;
     }
 
     //
@@ -650,6 +651,8 @@ InitializePlatform (
 
   PublishPeiMemory ();
 
+  QemuUc32BaseInitialization ();
+
   InitializeRamRegions ();
 
   if (mXen) {
diff --git a/OvmfPkg/PlatformPei/Platform.h b/OvmfPkg/PlatformPei/Platform.h
index 81af8b7148..2f3cebcd3a 100644
--- a/OvmfPkg/PlatformPei/Platform.h
+++ b/OvmfPkg/PlatformPei/Platform.h
@@ -62,6 +62,11 @@ GetSystemMemorySizeBelow4gb (
   VOID
   );
 
+VOID
+QemuUc32BaseInitialization (
+  VOID
+  );
+
 VOID
 InitializeRamRegions (
   VOID
@@ -114,4 +119,6 @@ extern UINT32 mMaxCpuCount;
 
 extern UINT16 mHostBridgeDevId;
 
+extern UINT32 mQemuUc32Base;
+
 #endif // _PLATFORM_PEI_H_INCLUDED_