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    SubjectRe: [PATCH 3/4] kvm: Extend irqfd to support level interrupts
    On Sun, Jun 24, 2012 at 03:59:27PM -0600, Alex Williamson wrote:
    > On Sun, 2012-06-24 at 18:49 +0300, Michael S. Tsirkin wrote:
    > > On Sun, Jun 24, 2012 at 09:18:38AM -0600, Alex Williamson wrote:
    > > > > > @@ -242,7 +299,8 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
    > > > > >
    > > > > > ret = 0;
    > > > > > list_for_each_entry(tmp, &kvm->irqfds.items, list) {
    > > > > > - if (irqfd->eventfd != tmp->eventfd)
    > > > > > + if (irqfd->eventfd != tmp->eventfd &&
    > > > > > + irqfd->eventfd != tmp->eoi_eventfd)
    > > > > > continue;
    > > > >
    > > > > So we allow duplicate irqfd with differing eoifd (or edge-triggered and
    > > > > level-triggered irqfd on the same context).
    > > > >
    > > > > (why the check in the first place? just so we can have a reliable
    > > > > deassign or is it avoiding a deeper problem?)
    > > >
    > > > I really wasn't sure to what extent we wanted to prevent duplicates. My
    > > > guess was that we don't want to have an irqfd trigger more than one
    > > > thing. That seems to be what the current code does. I don't see any
    > > > problems with multiple irqfds triggering the same eventfd though. I
    > > > only added a test that a new irqfd can't be triggered by an existing
    > > > eoi_eventfd as that could make a nasty loop.
    > >
    > > How would that make a loop? You can have the same thing
    > > with e.g. ioeventfd - why isn't it a problem there?
    >
    > eoi_eventfd1 -> irqfd2 [eoi] eoi_eventfd2 -> irqfd1 [eoi] eoi_eventfd1 ->...

    Sorry I don't understand.
    What does this [eoi] mean? How is eoi eventfd different from ioeventfd?

    >
    > Yes, in reality we'd need to search fds from all the interfaces and come
    > up with some grossly complicated truth table of what's allowed and
    > what's not. The original code didn't go to that kind of extreme, so I
    > just added something that seemed like a reasonable case
    > we'd want to prevent. Thanks,
    >
    > Alex


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