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Date: Wed, 7 Sep 2011 13:21:17 -0400
From: Don Zickus <dzickus@redhat.com>
To: Avi Kivity <avi@redhat.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>,
        Peter Zijlstra <peterz@infradead.org>,
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        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking
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On Wed, Sep 07, 2011 at 08:09:37PM +0300, Avi Kivity wrote:
> On 09/07/2011 07:52 PM, Don Zickus wrote:
> >>
> >>  May I ask how?  Detecting a back-to-back NMI?
> >
> >Pretty boring actually.  Currently we execute an NMI handler until one of
> >them returns handled.  Then we stop.  This may cause us to miss an NMI in
> >the case of multiple NMIs at once.  Now we are changing it to execute
> >_all_ the handlers to make sure we didn't miss one.
> 
> That's going to be pretty bad for kvm - those handlers become a lot
> more expensive since they involve reading MSRs.  Even worse if we
> start using NMIs as a wakeup for pv spinlocks as provided by this
> patchset.

Oh.

> 
> >But then the downside
> >here is we accidentally handle an NMI that was latched.  This would cause
> >a 'Dazed on confused' message as that NMI was already handled by the
> >previous NMI.
> >
> >We are working on an algorithm to detect this condition and flag it
> >(nothing complicated).  But it may never be perfect.
> >
> >On the other hand, what else are we going to do with an edge-triggered
> >shared interrupt line?
> >
> 
> How about, during NMI, save %rip to a per-cpu variable.  Handle just
> one cause.  If, on the next NMI, we hit the same %rip, assume
> back-to-back NMI has occured and now handle all causes.

I had a similar idea a couple of months ago while debugging a continuous
flow of back-to-back NMIs from a stress-test perf application and I
couldn't get it to work.  But let me try it again, because it does make
sense as an optimization.

Thanks,
Don