linux-kernel - Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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Date:	Tue, 13 Sep 2011 14:40:44 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH 08/13] xen/pvticketlock: disable interrupts while blocking

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
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