This is used in KiUserModeCallout instead of KiServiceExit2. The latter is broken, leaks non-volatile registers and will need to be modified to handle an exception frame, which we don't need/have here. It will also use sysret instead of iret and is generally simpler/faster.
Eventually it would be desirable to skip the entire trap frame setup and do everything in KiCallUserMode. This requires some cleanup and special handling for user APC delivery.