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

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Date:	Thu, 18 Nov 2010 16:16:38 -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 Fri, Nov 19, 2010 at 12:01:59AM +0300, Cyrill Gorcunov wrote:
> On Thu, Nov 18, 2010 at 03:52:03PM -0500, Don Zickus wrote:
> ...
> > > 
> > > 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
> >
> 
>  ok, thanks, Jason just confirmed it too. I thing if we run with kgdb armed
> it (kgdb) should obtain nmi handler first and perf only after.
> 
>  actually I'm not sure why p4 hangs here while core passes the test, most

because p4 reads the wrong high register which comes back zero.  This will
always set overflow=1, thus swallowing all NMI if something like the nmi
watchdog has perf active.

Cheers,
Don
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