linux-kernel - Re: [RFC][PATCH 2/6] perf, arch: Rework perf_event

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Message-ID: <20111121224343.GA7862@mudshark.cambridge.arm.com>
Date:	Mon, 21 Nov 2011 22:43:43 +0000
From:	Will Deacon <will.deacon@....com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"mingo@...e.hu" <mingo@...e.hu>, William Cohen <wcohen@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Cree <mcree@...on.net.nz>,
	Deng-Cheng Zhu <dengcheng.zhu@...il.com>,
	Anton Blanchard <anton@...ba.org>,
	Eric B Munson <emunson@...bm.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Richard Kuo <rkuo@...eaurora.org>,
	Stephane Eranian <eranian@...gle.com>,
	Arun Sharma <asharma@...com>, Vince Weaver <vince@...ter.net>
Subject: Re: [RFC][PATCH 2/6] perf, arch: Rework perf_event_index()

On Mon, Nov 21, 2011 at 08:35:55PM +0000, Peter Zijlstra wrote:
> On Mon, 2011-11-21 at 20:31 +0000, Will Deacon wrote:
> > On Mon, Nov 21, 2011 at 07:18:10PM +0000, Peter Zijlstra wrote:
> > > But does ARM have a read PMU counter from userspace
> > > instruction/capability?
> > 
> > In ARMv7, you can enable user access to the performance counters but the
> > access is R/W so I don't think it's something we want to do (could interfere
> > with another task doing system-wide profiling).
> 
> Yeah, write access is pushing it a bit.. 

Perhaps we could disable it while per-cpu events are running, although I
think this will probably just lead to SIGILL central for anybody trying to
use the counters in userspace.

> > > Lacking that its all moot of course. If it does, it would be nice to
> > > have an ARM version of patch 6.
> > 
> > Well we'd need a way to get around the all-or-nothing user access to the PMU
> > and also the fact that we don't always have a user-readable clocksource.
> > 
> > Either way, the event counters are indexed from 0 on ARMv7 so we should
> > use perf_event_idx_default.
> 
> Ok, done. Thanks!

Brill.

Will
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