perf: is it possible to userspace rdpmc but only on a certain core type

[Vince Weaver]

Hello

so we've been working on PAPI support for Intel Top-Down events, which
let's say does "exciting" things involving the rdpmc instruction.

One issue we are having is that on a hybrid machine (Raptor Lake in this 
case with performance/efficiency cores) there is no top-down support
for the E-cores, and it will gpf/segfault if you try to rdpmc the top-down 
events.

Obviously PAPI would like to avoid this, and somehow only run the rdpmc 
from userspace if scheduled on a P-core.

Is there any way to atomically do this?  Somehow detect what core we are 
on and atomically execute a userspace instruction before a core-reschedule 
can happen?

Or barring that, any other way to handle this in a way that won't crash 
without having to have the users have to bind to a core any time they want 
to run PAPI?

Thanks

Vince Weaver
vincent.weaver@xxxxxxxxx