linux-kernel - Re: [PATCH 3/4] kvm: Extend irqfd to support level interrupts

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Message-ID: <20120624230251.GE17046@redhat.com>
Date:	Mon, 25 Jun 2012 02:02:51 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [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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