linux-kernel - Re: [PATCH -tip] perf, x86: P4 PMU - Add missing read of a counter before test

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Date:	Thu, 24 Mar 2011 17:33:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Lin Ming <ming.m.lin@...el.com>, Don Zickus <dzickus@...hat.com>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -tip] perf, x86: P4 PMU - Add missing read of a counter
 before test


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> On 03/24/2011 11:48 AM, Ingo Molnar wrote:
> > 
> > * Cyrill Gorcunov <gorcunov@...il.com> wrote:
> > 
> >> Don, I've added yours SOB, ok? (The patch is attached to avoid
> >> space/tabs problem
> >> due to web-mail client)
> > 
> > The patch lacks a proper description about the motivation and effects of the 
> > patch.
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> Ingo, does this one looks better?
> 
> ---
> From: Don Zickus <dzickus@...hat.com>
> Subject: [PATCH -tip] perf, x86: P4 PMU - Add missing read of MSR register to catch unflagged overflows
> 
> The read of a proper MSR register was missed so instead of a counter the 
> configration register is tested (it has ARCH_P4_UNFLAGGED_BIT always cleared) 
> and unflagged overflows never have been catched. Fix it by reading a proper 
> MSR register.

So what effect does this have on the regular perf user? Please try to describe 
the real-life effect of the bug/problem fixed here.

Thanks,

	Ingo
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