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    SubjectRe: [V2 PATCH 0/6] x86, NMI: give NMI handler a face-lift
    On Thu, Nov 18, 2010 at 04:02:31PM -0500, Don Zickus wrote:
    > > > Don, Robert,
    > > >
    > > > I still have suspicious on ours 'pending' nmi handler. Look what I mean --
    > > > (keep in mind that p4 has a a way more counters than others).
    > > >
    > >
    > > To be precise -- it seems this scenario may force the back-to-back
    > > nmi handler to eat unknown nmi.
    > That was the point of the change to do exactly that.

    I missed the word, I meant to eat 'real' unknown nmi, not those
    generated by counters and stand 'in-fly', so that unknown_nmi_error()
    will be eaten. what is worse the NMI generated by kgdb may be eaten as
    well and treated as being 'pending' one, though there is quite a small
    probability of such situation I believe.

    > The problem is/was when you go to check to see if the period expired in
    > x86_perf_event_set_period(), you refresh the perf counter. The next step
    > is to see if the event period has expired, if so disabled the 'active'
    > bit.
    > However, there is a race between when you refresh the counter to when you
    > actually disable it, such that you may cause the counter to overflow again
    > and thus generate another NMI. The whole ->running thing was implemented
    > by Robert to try and check for that condition and eat the NMI as we have
    > no intention of handling it (because it is bogus).
    > The alternative is to use another rdmsrl to actually see if we trigger
    > another NMI. This was deemed a performance hit for such a small case.
    > Cheers,
    > Don

    yeah, I recall now, thanks for refresh Don!


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