You sound like you’re living in the weeds, friend.
What’s MPK? And by UID I assume you’re not talking about the system level user ID but some kind of processor-level process ID?
I don’t know the details of the MPK. So I consider it as some kind of function that maps {process PID, DLL} => Set of UID. And by UID, I AM talking about the system level user ID. Remember that this feature is a processor level feature. So it has to be transparent to the OS (well at least, to the OS Scheduler). Hence the output of this feature should be understandable to the OS kernel. Or so I hope as the implementation details are vague till now.