linux-kernel - Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

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Message-ID: <20120905151523.GD11058@redhat.com>
Date:	Wed, 5 Sep 2012 18:15:23 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, gleb@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

On Wed, Sep 05, 2012 at 06:12:04PM +0300, Avi Kivity wrote:
> On 09/05/2012 06:09 PM, Michael S. Tsirkin wrote:
> > On Wed, Sep 05, 2012 at 05:51:53PM +0300, Michael S. Tsirkin wrote:
> >> On Wed, Sep 05, 2012 at 05:35:43PM +0300, Avi Kivity wrote:
> >> > On 08/22/2012 03:41 AM, Michael S. Tsirkin wrote:
> >> > >> 
> >> > >> I assumed you were pointing out the level vs edge interaction.  If we
> >> > >> call that a userspace bug, I can just drop this.  Thanks,
> >> > >> 
> >> > >> Alex
> >> > > 
> >> > > level is userspace bug I think :)
> >> > 
> >> > I don't see how it's a bug.  Suppose we have a vfio device that shares a
> >> > gsi with an emulated device.  The emulated device naturally uses
> >> > KVM_IRQ_LINE (it has no need to re-sample on ADN), while vfio naturally
> >> > has to use irqfd.
> >> 
> >> Absolutely. But vfio needs to use irqfd with the new flag.
> >> Using existing irqfd for level is a bug.
> >> 
> >> > Note one would expect that each irqfd gets its own irq source id, since
> >> > they are all independent level sources.  The reason they don't is that
> >> > we shut them down anyway and let the sources re-trigger (it is more
> >> > accurate to say that they have no irq source id, but that would just
> >> > muddle the implementation).
> >> > 
> >> > Alex, if the conclusion is that we do need this patch, then please add a
> >> > comment explaining why we can share the source id among all irqfd users.
> > 
> > Something along the lines of
> > 
> > /* 
> >    For resample irqfds, level is a logical OR of all inputs;
> >    to support this, track state for RESAMPLE irqfds separately
> >    from userspace. We do not need to track state for each input since
> >    they are all deasserted at the same time, before resampling.
> >    */
> 
> Well the comment style is wrong.

Ouch.

> To expand a little more, irqfd only sends assert events, so assigning
> the level is equivalent to an OR.  Clearing an resampling simply builds
> the state again.
> 
> btw, there can be other irq source IDs if the lines are shared with the
> PIT or kvm assigned devices.

Nod.

> -- 
> error compiling committee.c: too many arguments to function
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