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

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Message-ID: <1321962710.5148.34.camel@twins>
Date:	Tue, 22 Nov 2011 12:51:50 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Will Deacon <will.deacon@....com>
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>,
	"ostrikov@...dia.com" <ostrikov@...dia.com>
Subject: Re: [RFC][PATCH 2/6] perf, arch: Rework perf_event_index()

On Tue, 2011-11-22 at 11:47 +0000, Will Deacon wrote:
> On Tue, Nov 22, 2011 at 11:26:20AM +0000, Peter Zijlstra wrote:
> > On Mon, 2011-11-21 at 22:43 +0000, Will Deacon wrote:
> > > 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.
> > 
> > One possibility would be to do as I did in patch 4, except ARM has it
> > disabled by default and the folks who think they know WTF they're doing
> > can enable it or so.
> 
> The problem is that everybody thinks they know WTF they're doing!

> But you know that the first thing people will do is zero the registers.

*groan*, fair enough ;-)

> > Also, for those ARMs that do have a user readable clock, you could
> > support the new time_{mult,shift,offset} from patch 5.
> 
> The user-readable clock will first appear in Cortex-A15, so the code for
> that still needs to hit mainline before I can look at doing this in perf.

OK, I had interpreted your "we don't always have" to be slightly more
common than just A15.
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