Re: [PATCH v4] KVM: Introduce direct MSI message injection for in-kernel irqchips

[Avi Kivity <avi@redhat.com>]

On 04/04/2012 11:38 AM, Michael S. Tsirkin wrote:
> > 
> > > 
> > > A performance note: delivering an interrupt needs to search all vcpus
> > > for an APIC ID match.  The previous plan was to cache (or pre-calculate)
> > > this lookup in the irq routing table.  Now it looks like we'll need a
> > > separate cache for this.
> > 
> > As this is non-existent until today, we don't regress here. And it can
> > still be added on top later on, transparently.
>
> I always worry about hash collisions and the cost of
> calculating good hash functions.
>
> We could instead return an index in the cache on injection, maintain in
> userspace and use it for fast path on the next injection.

Ahem, that is almost the existing routing table to a T.

> Will make it easy to use an array index instead of a hash here,
> and fallback to a slower ID lookup on mismatch.

Need a free ioctl so we can reuse IDs.

> Until we do have this fast path we can just fill this value with zeros,
> so kernel patch (almost) does not need to change for this -
> just the header.

Partially implemented interfaces invite breakage.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.