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

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Message-ID: <4E67A551.4000502@redhat.com>
Date:	Wed, 07 Sep 2011 20:09:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Don Zickus <dzickus@...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 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.

> 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.

-- 
error compiling committee.c: too many arguments to function

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