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

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Date:	Mon, 25 Jun 2012 23:13:56 +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 Mon, Jun 25, 2012 at 10:17:14AM -0600, Alex Williamson wrote:
> On Mon, 2012-06-25 at 02:02 +0300, Michael S. Tsirkin wrote:
> > 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?
> 
> [eoi] is simply the guest doing an EOI write.

So you trigger irq, later guest does an eoi etc. Why is this a loop?

>  There's some interaction
> required from the guest which could rate limit the loop.  I'm not really
> following your comparison to an ioeventfd.  If the question is can you
> create loops with an ioeventfd, I imagine the answer is probably so.

I think the answer is no.

>  I
> certainly don't plan on adding code to test every fd in use by a vm and
> validate it can't do crazy things, so if even this minor sanity test
> could prevent useful things, I'll happily remove it.  Thanks,
> 
> Alex
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