linux-kernel - Re: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX operation is on

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Date:   Wed, 15 Feb 2017 10:16:17 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] perf/x86/intel/pt: Fail event creation if VMX
 operation is on

Em Wed, Feb 15, 2017 at 03:09:25PM +0200, Alexander Shishkin escreveu:
> Arnaldo Carvalho de Melo <acme@...nel.org> writes:
> 
> > Em Wed, Feb 15, 2017 at 10:34:58AM +0200, Alexander Shishkin escreveu:
> >> Arnaldo Carvalho de Melo <acme@...nel.org> writes:
> >> > Em Tue, Feb 14, 2017 at 03:24:16PM +0200, Alexander Shishkin escreveu:
> >> > Yeah, I saw that as well, and Andi told me about this limitation, so,
> >> > for quite a while now, everytime I need to test PT on the only machine I
> >> > have with it, I have to stop my kvm sessions :-\
> >
> >> > Thanks for working on this!
> >
> >> Well, we can't make the limitation go away. You'll still have to stop
> >> kvms to get any 'meaningful' PT data.
> >
> > If we can't make the limitation go away we need to at least warn users
> > instead of let 'perf record' sit there doing nothing and then at the end
> > return as if everything went well only to when trying to use 'perf
> > script' nothing will appear.
> 
> I tend to like Peter's idea about PERF_RECORD_AUX with a PAUSED flag to
> indicate this.

Ok, you mean that I will then be able to start a intel pt session while
VMs are there, which will sample nothing, but then, with this session
still in place, as soon as I stop all VMs, samples will start appearing
on the buffer, then, at report/script/whatever analysis tool start, I'll
be able to show the blind spots and say that happened because something
prevented PT records from being produced?

Would be good to have some indication of _what_ prevented sampling, to
not have to say

  "hey, something banned PT records for a while, maybe VMs?"

Best thing I could do would be to correlated that with PERF_RECORD_MMAP
for suspecting pathnames.

- Arnaldo