linux-kernel - Re: [PATCH v2] perf/x86/intel: ignore CondChgd bit to avoid false NMI handling

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Message-ID: <20140701081413.GJ6758@twins.programming.kicks-ass.net>
Date:	Tue, 1 Jul 2014 10:14:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"HATAYAMA, Daisuke" <d.hatayama@...fujitsu.com>, hpa@...or.com,
	Don Zickus <dzickus@...hat.com>, matt@...sole-pimps.org,
	acme@...nel.org, mingo@...hat.com, paulus@...ba.org,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] perf/x86/intel: ignore CondChgd bit to avoid false
 NMI handling

On Mon, Jun 30, 2014 at 03:22:24PM -0700, Andi Kleen wrote:
> > 
> > I'm also interested in the behaviour of CondChgd bit on Ivy Bridge processors.
> 
> The intended meaning of CondChgd is that a hardware debugger has taken over the
> PMU. It shouldn't really be set in other circumstances.

The SDM utterly and totally fails to mention this, please fix that.

> I think right now for perf it would be best to just ignore it.
> 
> In theory could stop using the PMU, but if some BIOS set it it would
> completely disable perf there. So better to just ignore it.

The even better option would be to kill off SMM mode, all that ever does
is create problems.
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