linux-kernel - Re: [PATCH 01/31] perf, x86: Add PEBSv2 record support

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Date:	Fri, 28 Sep 2012 11:28:20 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 01/31] perf, x86: Add PEBSv2 record support

On Fri, 2012-09-28 at 10:54 +0200, Stephane Eranian wrote:
> On Fri, Sep 28, 2012 at 10:43 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > On Thu, 2012-09-27 at 21:31 -0700, Andi Kleen wrote:
> >> +               if (event->attr.precise_ip > 1 && x86_pmu.intel_cap.pebs_format < 2) {
> >
> > Shouldn't that be: && x86_pmu.intel_cap.pebs_trap, like most other sites
> > instead? Or didn't they flip the trap capability on Haswell?
> 
> On Haswell, you get the event_ip which points to the sampled
> instruction, i.e., the off-by-one
> error can be avoided by using that value instead of pebs.rip. The nice
> side effect is that you
> free the LBR and minimize the overhead (no fixups). Therfore the LBR
> filter can have any
> setting when combined with PEBS, thus we do not need to check for
> compatibility nor force
> any setting for the LBR filter.

Yes I got that, but what good is that trap capability flag if they don't
use it? Them adding a second u64 to the format to report it seems to
suggest their trap capability is pointless, but nowhere has this been
explained.
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