RtlXSaveS

void __fastcall RtlXSaveS(_QWORD *a1, UINT64 a2){
  if( (KUSER_SHARED_DATA.XState.ControlFlags & 2) != 0 )
    XSaveSHelper((INT64)a1, a2 & a1[65]);
  else
    RtlXSave((XSAVE_AREA *)a1, a2);
}

Referenced by:

KeContextFromKframes
KeSaveExtendedAndSupervisorState
KeSaveSupervisorState
PspGetContext