Re: [PATCH RFC 0/8] virtio: add guest MSI-X support

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

On Mon, Apr 27, 2009 at 05:37:25PM +0200, Christian Borntraeger wrote:
> > As  number of virtqueues <= number of vectors,
> > we could pre-allocate all vectors that host supports, but this seems
> > a bit drastic as an MSI-X device could support up to 2K vectors.
> > 
> > > In fact, the transport has to have a way of getting the number of virtqeues
> > > because find_vq returns ENOENT on invalid index numbers.
> > > 
> > > Christian
> > 
> > So again, I think this is an upper bound supported by host. Right?
> 
> Not the upper bound, but the real available virtqueues. (With current qemu
> 3 for virtio-net, 2 for virtio-console etc.)

Here's what I mean by upper bound: guest and host number of
virtqueues might be different: the host might support
control virtqueue like in virtio-net, but guest might be an older
version and not use it.

> Since I use a different transport (drivers/s390/kvm/kvm_virtio.c), my
> motiviation is to keep the virtio interface as generic as possible. I
> dont really like the new interface, but I cannot give you silver
> bullet technical reasons - its more a gut feeling. The interface would
> work with lguest and s390.
> 
> Anyway. Avis suggestion to decouple MSI count and virtqueue count looks
> like a promising approach. 

So generally, we add request_vectors which gives us the number of
available MSI vectors and then find_vq gets a vector #?
Does this sound better?

-- 
MST