linux-kernel - Re: [PATCH 2/2] perf, x86: Don't allow unusual PEBS raw flags

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Date:	Mon, 6 May 2013 20:52:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/2] perf, x86: Don't allow unusual PEBS raw flags

On Mon, May 06, 2013 at 07:44:19PM +0200, Stephane Eranian wrote:
> On Thu, May 2, 2013 at 9:37 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Apr 24, 2013 at 04:04:54PM -0700, Andi Kleen wrote:
> >> From: Andi Kleen <ak@...ux.intel.com>
> >>
> >> The PEBS documentation in the Intel SDM 18.6.1.1 states:
> >>
> >> """
> >> PEBS events are only valid when the following fields of IA32_PERFEVTSELx are all
> >> zero: AnyThread, Edge, Invert, CMask.
> >> """
> >>
> >> Since we had problems with this earlier, don't allow cmask, any, edge, invert
> >> as raw events, except for the ones explicitly listed as pebs_aliases.
> >
> > If its a simple matter of crap in crap out without affecting anything else we
> > shouldn't do anything.
> >
> The problem here is that you are sampling an instruction which did not cause
> the event you are measuring. Remember that using cmask, changes the
> nature of what's being measured (from event to cycles).

Yeah.. I don't see the problem though. If you're using cmask and the like
you're supposed to know wth you're doing; which includes knowing your cpu and
what it thinks of such an event.

The only reason to disallow events is if they (badly) interact with other
counters.
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