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Date: Thu, 18 Nov 2010 23:39:48 +0300
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Don Zickus <dzickus@redhat.com>, Robert Richter <robert.richter@amd.com>
Cc: Jason Wessel <jason.wessel@windriver.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        ying.huang@intel.com, Andi Kleen <andi@firstfloor.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift
Message-ID: <20101118203948.GE6028@lenovo>
References: <20101112172755.GR4823@redhat.com> <20101116184325.GB4823@redhat.com> <4CE2E3C3.6060800@windriver.com> <20101118080516.GJ32621@elte.hu> <4CE52048.5080802@windriver.com> <1290086232.2109.1507.camel@laptop> <20101118193247.GF18100@redhat.com> <4CE583D0.8050407@windriver.com> <20101118200807.GC8131@redhat.com> <20101118202850.GD6028@lenovo>
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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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