diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
index 639e33cb28..ad20531ceb 100644
--- a/OvmfPkg/OvmfPkgIa32X64.dsc
+++ b/OvmfPkg/OvmfPkgIa32X64.dsc
@@ -543,7 +543,11 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Base|0x0
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Size|0x0
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Base|0x0
+!ifdef $(CSM_ENABLE)
+  gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x0
+!else
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x800000000
+!endif
 
   gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut|0
 
diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
index 69a3497c2c..0542ac2235 100644
--- a/OvmfPkg/OvmfPkgX64.dsc
+++ b/OvmfPkg/OvmfPkgX64.dsc
@@ -542,7 +542,11 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Base|0x0
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio32Size|0x0
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Base|0x0
+!ifdef $(CSM_ENABLE)
+  gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x0
+!else
   gUefiOvmfPkgTokenSpaceGuid.PcdPciMmio64Size|0x800000000
+!endif
 
   gEfiMdePkgTokenSpaceGuid.PcdPlatformBootTimeOut|0