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SubjectRe: [PATCH v8 0/6] kvm: level irqfd support
On Thu, Aug 16, 2012 at 07:29:40PM +0300, Avi Kivity wrote:
> On 08/15/2012 10:22 PM, Michael S. Tsirkin wrote:
> > On Wed, Aug 15, 2012 at 11:36:31AM -0600, Alex Williamson wrote:
> >> On Wed, 2012-08-15 at 17:28 +0300, Michael S. Tsirkin wrote:
> >> > On Fri, Aug 10, 2012 at 04:37:08PM -0600, Alex Williamson wrote:
> >> > > v8:
> >> > >
> >> > > Trying a new approach. Nobody seems to like the internal IRQ
> >> > > source ID object and the interactions it implies between irqfd
> >> > > and eoifd, so let's get rid of it. Instead, simply expose
> >> > > IRQ source IDs to userspace. This lets the user be in charge
> >> > > of freeing them or hanging onto a source ID for later use.
> >> >
> >> > In the end it turns out source ID is an optimization for shared
> >> > interrupts, isn't it? Can't we apply the optimization transparently to
> >> > the user? E.g. if we have some spare source IDs, allocate them, if we
> >> > run out, use a shared source ID?
> >>
> >> Let's think about shared source IDs a bit more. I think it's wrong that
> >> irqfd uses KVM_USERSPACE_IRQ_SOURCE_ID, but I'm questioning whether all
> >> irqfd users can share a source ID. We do not get the logical OR of all
> >> users by putting them on the same source ID, we get "last set wins".
> >> KVM_USERSPACE_IRQ_SOURCE_ID is used for multiple inputs because the
> >> logical OR happens in userspace. How would we not starve a user if we
> >> define KVM_IRQFD_SOURCE_ID? What am I missing?
> >
> > That all irqfds are deasserted on EOI anyway. So there's no point
> > to do a logical OR.
> >
> >
>
> What if a level irqfd shares a line with a KVM_IRQ_LINE ioctl? Then an
> EOI can de-assert the irqfd source, but the line is kept high by the
> last KVM_IRQ_LINE invocation.

Exactly. So 1 ID for userspace and 1 for irqfd.

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


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