linux-kernel - Re: [PATCH v2 0/6] kvm: level triggered irqfd support

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Date:	Thu, 28 Jun 2012 11:42:36 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	avi@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v2 0/6] kvm: level triggered irqfd support

On Wed, Jun 27, 2012 at 08:33:40AM -0600, Alex Williamson wrote:
> On Wed, 2012-06-27 at 12:58 +0300, Michael S. Tsirkin wrote:
> > On Tue, Jun 26, 2012 at 11:08:52PM -0600, Alex Williamson wrote:
> > > Ok, let's see how this flies.  I actually quite like this, so be
> > > gentle tearing it apart ;)
> > > 
> > > I just couldn't bring myself to contort KVM_IRQFD into something
> > > that either sets up an irqfd or specifies a nearly unrelated EOI
> > > eventfd.  The solution I've come up with, that also avoids exposing
> > > irq_source_ids to userspace, is to work through the irqfd.  If we
> > > setup a level irqfd, we can optionally associate an eoifd with
> > > the same irq_source_id, by passing the irqfd.  To do this, we just
> > > need to create a new reference counted object for the source ID
> > > so we don't run into problems ordering release.  This means we
> > > end up with a KVM_EOIFD ioctl that has both general usefulness and
> > > can still tie into an irqfd.
> > 
> > I like this API.
> 
> Yay!  I'll work on the bugs you've spotted.
> 
> > > In patch 6/6 I also include an alternate de-assert mechanism via an
> > > irqfd with the opposite polarity.  I don't currently use this, but
> > > it's pretty trivial and at least available in archives now.
> > 
> > The nasty bit about that is that if you assert twice then
> > deassert once it's not clear what should happen.
> > But yea, it does not hurt to put them in the archives.
> 
> It's no different that KVM_IRQ_LINE in that sense.  It's not an
> accumulator, it's a set state.  Thanks,
> 
> Alex

Yes but eventfd semantics are that of an accumulator so this
does not match what a normal eventfd does very well.
Anyway, as it's not required now we can argue about it
when we get to it.

-- 
MST
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