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

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Date:	Thu, 18 Nov 2010 23:39:48 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Don Zickus <dzickus@...hat.com>,
	Robert Richter <robert.richter@....com>
Cc:	Jason Wessel <jason.wessel@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, 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 11:28:50PM +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
> 
>  Don, Robert,
> 
>  I still have suspicious on ours 'pending' nmi handler. Look what I mean --
> (keep in mind that p4 has a a way more counters than others).
> 

 To be precise -- it seems this scenario may force the back-to-back
nmi handler to eat unknown nmi.

 Cyrill
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