On Mon, Aug 13, 2012 at 02:29:31PM +0300, Gleb Natapov wrote:
> On Mon, Aug 13, 2012 at 02:22:14PM +0300, Michael S. Tsirkin wrote:
> > On Mon, Aug 13, 2012 at 02:12:41PM +0300, 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.
> >
> > At least for MSI I think it is simple. Here's the relevant code from
> > my old patch:
> >
> > +static bool kvm_msi_is_multicast(unsigned dest, int dest_mode)
> > +{
> > + if (dest_mode == 0)
> > + /* Physical mode. */
> > + return dest == 0xff;
> > + else
> > + /* Logical mode. */
> > + return dest & (dest - 1);
> > +}
> >
> MSI does not have shorthand, so it is simpler but the code above does
> work for APIC_DFR_CLUSTER as far as I can tell and it does not check
> lowest prio, which is not multicast, but should bot be cached.
>
It also a little bit pessimistic for logical mode. Dest may have more
than one bit set, but be delivered to only one cpu.
--
Gleb.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
