Re: [PATCH v8 0/6] kvm: level irqfd support

[Michael S. Tsirkin]

On Thu, Aug 16, 2012 at 06:34:52AM -0600, Alex Williamson wrote:
> > > > So I'm inclined to say source IDs are a requirement for shared
> > > > interrupts.
> > > 
> > > Can yo show a specific example that breaks?
> > > I don't think it can exist.
> > 
> > Only the edge vs level interaction if we define the policy above for
> > de-assert.
> 
> Hmm, there is still a race w/ level.  If we have a number of
> level-deassert-irqfds making use of the same gsi and sourceid and we
> individually de-assert and notify, a re-assert could get lost if it
> happens before all of the de-asserts have finished.
> We either need
> separate sourceids or we need to do a single de-assert followed by
> multiple notifies.  Right?  Thanks,
> 
> Alex

Good catch, I agree, we need a single deassert.

I think I see how to implement this without reference counting and
stuff.  So we chain all auto-deassert irqfds for a given GSI together,
and have a single ack notifier.  When list becomes empty, remove the ack
notifier.

It's actually a good thing to do anyway, too many ack notifiers
would slow unrelated GSIs down.

-- 
MST
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