diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index d59508db56..cc2a4909af 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -122,6 +122,32 @@
   gUefiOvmfPkgTokenSpaceGuid.PcdGuidedExtractHandlerTableSize|0x0|UINT32|0x1a
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDecompressionScratchEnd|0x0|UINT32|0x1f
 
+  ## Pcd8259LegacyModeMask defines the default mask value for platform. This
+  #  value is determined.
+  #  1) If platform only support pure UEFI, value should be set to 0xFFFF or
+  #     0xFFFE; Because only clock interrupt is allowed in legacy mode in pure
+  #     UEFI platform.
+  #  2) If platform install CSM and use thunk module:
+  #     a) If thunk call provided by CSM binary requires some legacy interrupt
+  #        support, the corresponding bit should be opened as 0.
+  #        For example, if keyboard interfaces provided CSM binary use legacy
+  #        keyboard interrupt in 8259 bit 1, then the value should be set to
+  #        0xFFFC.
+  #     b) If all thunk call provied by CSM binary do not require legacy
+  #        interrupt support, value should be set to 0xFFFF or 0xFFFE.
+  #
+  #  The default value of legacy mode mask could be changed by
+  #  EFI_LEGACY_8259_PROTOCOL->SetMask(). But it is rarely need change it
+  #  except some special cases such as when initializing the CSM binary, it
+  #  should be set to 0xFFFF to mask all legacy interrupt. Please restore the
+  #  original legacy mask value if changing is made for these special case.
+  gUefiOvmfPkgTokenSpaceGuid.Pcd8259LegacyModeMask|0xFFFF|UINT16|0x3
+
+  ## Pcd8259LegacyModeEdgeLevel defines the default edge level for legacy
+  #  mode's interrrupt controller.
+  #  For the corresponding bits, 0 = Edge triggered and 1 = Level triggered.
+  gUefiOvmfPkgTokenSpaceGuid.Pcd8259LegacyModeEdgeLevel|0x0000|UINT16|0x5
+
 [PcdsDynamic, PcdsDynamicEx]
   gUefiOvmfPkgTokenSpaceGuid.PcdEmuVariableEvent|0|UINT64|2
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashVariablesEnable|FALSE|BOOLEAN|0x10