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

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Message-ID: <20130507064804.GE17705@gmail.com>
Date:	Tue, 7 May 2013 08:48:05 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.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


* Stephane Eranian <eranian@...gle.com> wrote:

> On Mon, May 6, 2013 at 8:52 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 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.
>
> But that implies that you'd know that on Intel precise mode uses PEBS 
> and that PEBS does not take cmask events. That seems to contradict the 
> philosophy of perf_events where the kernel does the work for you.

Also, this code only runs when the event is set up, so a bit of sanity 
checking can only help, right?

Thanks,

	Ingo
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