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

Michael S. Tsirkin
2012-09-05T15:14:07Z

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