linux-kernel - Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift

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Date:	Thu, 18 Nov 2010 15:52:03 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Robert Richter <robert.richter@....com>, ying.huang@...el.com,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift

On Thu, Nov 18, 2010 at 11:11:18PM +0300, Cyrill Gorcunov wrote:
> On Thu, Nov 18, 2010 at 03:08:07PM -0500, Don Zickus wrote:
> > On Thu, Nov 18, 2010 at 01:51:44PM -0600, Jason Wessel wrote:
> > > > So the problem is when the nmi watchdog is enabled, the perf event is
> > > > 'active' and thus tries to read the counter value.  Because it is always
> > > > zero, perf just assumes the counter overflowed and the NMI is his.
> > > >
> > > > Not sure how to fix it yet, other than include the logic that detects we
> > > > are on a guest and disable perf??
> > > >
> > > >   
> > > 
> > > I highly doubt we want to disable perf.   I would rather use the source
> > > and fix the nmi emulation in KVM/Qemu after we hear back the results
> > 
> > Well I think Peter does not have a positive opinion about emulating perf
> > inside a guest.  Nor are the KVM folks having much success in doing so.
> > 
> > Just to clarify, perf counter emulation is _not_ implemented in kvm.
> > Therefore disabling perf in the guest makes sense until someone gets
> > around to actually writing the emulation code for perf in a guest. :-)
> > 
> > Cheers,
> > Don
> 
> ok, Don, but you mentioned there are false alarms on real P4 machine, right?

Yeah, there are two problems.  One is using kgdb tests on kvm guests.  The
other is using kgdb tests on a bare metal p4 machine.

Cheers,
Don

> 
>  Cyrill
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