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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 04, 2012 at 11:44:23AM +0300, Avi Kivity wrote:
> 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.

No, it could be kernel controlled not userspace controlled. We get both
and address and an index:

if (table[u.i].addr == u.addr && table[u.i].data == u.data) {
	return table[u.i].id;
}

u.i = find_lru_idx(&table);
table[u.i].addr = u.addr;
table[u.i].data = u.data;
table[u.i].id = find_id(u.addr, u.data);
return table[u.i].id;


> > 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.

Hmm true. OK scrap this idea then, it's not clear
whether we are going to optimize this anyway.

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