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Message-ID: <ac391d4c-af02-f52b-2ac2-be874272f773@maine.edu>
Date: Fri, 17 Jan 2025 17:04:54 -0500 (EST)
From: Vince Weaver <vincent.weaver@...ne.edu>
To: peterz@...radead.org
cc: Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, 
    Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>, 
    Alexander Shishkin <alexander.shishkin@...ux.intel.com>, 
    Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>, 
    "Liang, Kan" <kan.liang@...ux.intel.com>, linux-kernel@...r.kernel.org
Subject: perf: is it possible to userspace rdpmc but only on a certain core
 type

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@...ne.edu

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