On Mon, Aug 13, 2012 at 02:30:49PM +0300, Avi Kivity wrote:
> On 08/13/2012 02:12 PM, Gleb Natapov wrote:
> > On Mon, Aug 13, 2012 at 02:03:51PM +0300, Avi Kivity wrote:
> >> On 08/13/2012 02:01 PM, Gleb Natapov wrote:
> >> >>
> >> >> Actually this is overkill. Suppose we do an apicid->vcpu translation
> >> >> cache? Then we retain O(1) behaviour, no need for a huge cache.
> >> >>
> >> > Not sure I follow.
> >>
> >> Unicast MSIs and IPIs can be speeded up by looking up the vcpu using the
> >> apic id, using a static lookup table (only changed when the guest
> >> updates apicid or a vcpu is inserted).
> >>
> > To check that MSI/IPI is unicast you need to check a lot of things: delivery
> > mode, shorthand, dest mode, vector. In short everything but level. This
> > is exactly what kvm_irq_delivery_to_apic() is doing. Caching apicid->vcpu
> > is not enough, caching (delivery mode, shorthand, dest mode,
> > vector)->vcpu is enough and this is exactly what the patch does for irq
> > routing entries.
>
>
> apicid is checked in a loop, the others aren't.
Along with dest_id.
> apicid is
> unpredicatable; the others are.
What do you mean "unpredicatable"?
>
> I think we should use apicid loopup exclusively. It doesn't accelerate
> everything, but most things, and is common to all unicast interrupts
> except PIC (and we can also precompute the target vcpu for PIC, too).
>
We can change kvm_irq_delivery_to_apic() to avoid the loop if interrupt
is physical, non broadcast, non low prio. Do whatever it does now
otherwise. You think we do not need cache in such case?
--
Gleb.
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