diff --git a/OvmfPkg/Library/XenRealTimeClockLib/XenRealTimeClockLib.c b/OvmfPkg/Library/XenRealTimeClockLib/XenRealTimeClockLib.c
index 30c2583131..6d7f452a75 100644
--- a/OvmfPkg/Library/XenRealTimeClockLib/XenRealTimeClockLib.c
+++ b/OvmfPkg/Library/XenRealTimeClockLib/XenRealTimeClockLib.c
@@ -80,6 +80,9 @@ EpochToEfiTime (
   @retval EFI_SUCCESS           The operation completed successfully.
   @retval EFI_INVALID_PARAMETER Time is NULL.
   @retval EFI_DEVICE_ERROR      The time could not be retrieved due to hardware error.
+  @retval EFI_UNSUPPORTED       This call is not supported by this platform at the time the call is made.
+                                The platform should describe this runtime service as unsupported at runtime
+                                via an EFI_RT_PROPERTIES_TABLE configuration table.
 
 **/
 EFI_STATUS
@@ -109,6 +112,9 @@ LibGetTime (
   @retval EFI_SUCCESS           The operation completed successfully.
   @retval EFI_INVALID_PARAMETER A time field is out of range.
   @retval EFI_DEVICE_ERROR      The time could not be set due to hardware error.
+  @retval EFI_UNSUPPORTED       This call is not supported by this platform at the time the call is made.
+                                The platform should describe this runtime service as unsupported at runtime
+                                via an EFI_RT_PROPERTIES_TABLE configuration table.
 
 **/
 EFI_STATUS
@@ -128,9 +134,13 @@ LibSetTime (
   @param  Time                  The current alarm setting.
 
   @retval EFI_SUCCESS           The alarm settings were returned.
-  @retval EFI_INVALID_PARAMETER Any parameter is NULL.
+  @retval EFI_INVALID_PARAMETER Enabled is NULL.
+  @retval EFI_INVALID_PARAMETER Pending is NULL.
+  @retval EFI_INVALID_PARAMETER Time is NULL.
   @retval EFI_DEVICE_ERROR      The wakeup time could not be retrieved due to a hardware error.
-  @retval EFI_UNSUPPORTED       A wakeup timer is not supported on this platform.
+  @retval EFI_UNSUPPORTED       This call is not supported by this platform at the time the call is made.
+                                The platform should describe this runtime service as unsupported at runtime
+                                via an EFI_RT_PROPERTIES_TABLE configuration table.
 
 **/
 EFI_STATUS
@@ -152,9 +162,13 @@ LibGetWakeupTime (
 
   @retval EFI_SUCCESS           If Enable is TRUE, then the wakeup alarm was enabled. If
                                 Enable is FALSE, then the wakeup alarm was disabled.
-  @retval EFI_INVALID_PARAMETER A time field is out of range.
+  @retval EFI_INVALID_PARAMETER Enabled is NULL.
+  @retval EFI_INVALID_PARAMETER Pending is NULL.
+  @retval EFI_INVALID_PARAMETER Time is NULL.
   @retval EFI_DEVICE_ERROR      The wakeup time could not be set due to a hardware error.
-  @retval EFI_UNSUPPORTED       A wakeup timer is not supported on this platform.
+  @retval EFI_UNSUPPORTED       This call is not supported by this platform at the time the call is made.
+                                The platform should describe this runtime service as unsupported at runtime
+                                via an EFI_RT_PROPERTIES_TABLE configuration table.
 
 **/
 EFI_STATUS