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    SubjectRe: [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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