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

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Date:	Wed, 7 Sep 2011 13:21:17 -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:
> 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
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