Re: How is irq delivered in kvm?

[Avi Kivity <avi@redhat.com>]

On 04/21/2011 05:23 PM, Nguyen Thai Ngoc Duy wrote:
> On Thu, Apr 21, 2011 at 9:11 PM, Avi Kivity<avi@redhat.com>  wrote:
> >  On 04/21/2011 04:49 PM, Nguyen Thai Ngoc Duy wrote:
> >>
> >>  Hi,
> >>
> >>  I have a specialized e1000 device driver that expects to receive a
> >>  single frame per interrupt, no more. It's by design and very hard to
> >>  change (and it does not serve IP traffic). -net socket or tap can
> >>  sometimes deliver more than one frame in a row and blow up the driver
> >>  in turn. I'd like to experiment with tap/socket to only call
> >>  qemu_send_packet..() once and leave pending frames in queue until next
> >>  time, with hope that guest will have time to process the frame.
> >
> >  I don't understand how the driver can expect that.  The card is free to
> >  deliver multiple packets per interrupt.  Are you counting on fast timing to
> >  process the packet before the next packet arrives?
>
> Yes. It uses ethernet as transport to "clone" memory from one machine
> to another in a short, precalculated amount of time.

Well, anything time related will misbehave under virtualization.

> >  If you restrict the number of buffers you provide to the card to exactly
> >  one, you'll get one packet per interrupts (and dropped packets).
> >
> >>  The problem is I'm new to kvm and not sure how the main loop is run.
> >>  Will there be guest execution time between two tap/socket polls, how
> >>  long is it? Or is guest run in parallel with the event loop and
> >>  qemu_set_irq() somehow signals guest immediately?
> >
> >  The latter, it's in parallel.
> >
> >  Are you using qemu-kvm or qemu?  qemu-kvm will deliver better interrupt
> >  performance.
>
> I'm using qemu-kvm. What function is used to deliver the interrupt
> then, kvm_inject_irq?

No, kvm_set_irq_level().


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