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Date: Tue, 13 Sep 2011 14:40:44 -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:
> >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.

So I got around to implementing this and it seems to work great.  The back
to back NMIs are detected properly using the %rip and that info is passed to
the NMI notifier.  That info is used to determine if only the first
handler to report 'handled' is executed or _all_ the handlers are
executed.

I think all the 'unknown' NMIs I generated with various perf runs have
disappeared.  I'll post a new version of my nmi notifier rewrite soon.

Thanks for the great suggestion Avi!

Cheers,
Don