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    SubjectRe: [Xen-devel] [GIT PULL] Fix lost interrupt race in Xen event channels
    >>> On 25.08.10 at 12:04, Daniel Stodden <daniel.stodden@citrix.com> wrote:
    > On Wed, 2010-08-25 at 03:52 -0400, Jan Beulich wrote:
    >> >>> On 24.08.10 at 23:35, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
    >> > We worked out the root cause was that it was incorrectly treating Xen
    >> > events as level rather than edge triggered interrupts, which works fine
    >> > unless you're handling one interrupt, the interrupt gets migrated to
    >> > another cpu and then re-raised. This ends up losing the interrupt
    >> > because the edge-triggering of the second interrupt is lost.
    >>
    >> While this description would seem plausible at the first glance, it
    >> doesn't match up with unmask_evtchn() already taking care of
    >> exactly this case. Or are you implicitly saying that this code is
    >> broken in some way (if so, how, and shouldn't it then be that
    >> code that needs fixing, or removing if you want to stay with the
    >> edge handling)?
    >
    > Not broken, but a different problem. The unmask 'resend' only catches
    > the edge lost if the event was raised while it was still masked. But
    > level irq doesn't have to save PENDING state. In the Xen event migration
    > case the edge isn't lost, but the upcall will drop the invocation when
    > the handler is found inprogress on the previous cpu.

    Hmm, indeed. But that problem must have existed in all post-2.6.18
    kernels then... And that shouldn't be a problem with fasteoi, as that
    one calls ->eoi() even when INPROGRESS was set (other than level,
    which calls unmask only when it wasn't set).

    >> I do however agree that using handle_level_irq() is problematic
    >> (see
    > http://lists.xensource.com/archives/html/xen-devel/2010-04/msg01178.html),
    >> but as said there I think using the fasteoi logic is preferable. No
    >> matter whether using edge or level, the ->end() method will
    >> never be called (whereas fasteoi calls ->eoi(), which would
    >> just need to be vectored to the same function as ->end()).
    >> Without end_pirq() ever called, you can't let Xen know of
    >> bad PIRQs (so that it can disable them instead of continuing
    >> to call the [now shortcut] handler in the owning domain).
    >
    > Not an opinion, just confused: Isn't all that dealt with in
    > chip->disable?

    With disable_pirq() being empty (at least in the branches I
    looked at)?

    Jan



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