linux-kernel - Re: perf: rdpmc bug when viewing all procs on remote cpu

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Date:   Fri, 11 Jan 2019 16:52:22 -0500 (EST)
From:   Vince Weaver <vincent.weaver@...ne.edu>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: perf: rdpmc bug when viewing all procs on remote cpu

On Thu, 10 Jan 2019, Vince Weaver wrote:

> On Thu, 10 Jan 2019, Vince Weaver wrote:
> 
> > On Thu, 10 Jan 2019, Vince Weaver wrote:
> > 
> > > However if you create an all-process attached to CPU event:
> > > 	perf_event_open(attr, -1, X, -1, 0);
> > > the mmap event index is set as if this were a valid event and so the rdpmc
> > > succeeds even though it shouldn't (we're trying to read an event value
> > > on a remote cpu with a local rdpmc).

so on further looking at the code, it doesn't appear that rdpmc events are 
explicitly marked as unavailable in the attach-cpu or attach-pid case, 
it's just by luck the check for PERF_EVENT_STATE_ACTIVE catches most of 
the cases?

should an explicit check be added to zero out userpg->index in cases where 
the event being measured is running on a different core?

Vince