Re: [Qemu-devel] [PATCH 0/4] virtio: Move host features to backends

[Michael S. Tsirkin]

On Mon, Apr 20, 2015 at 10:54:11AM +0200, Cornelia Huck wrote:
> On Fri, 17 Apr 2015 15:00:45 +0100
> Peter Maydell <address@hidden> wrote:
> 
> > On 17 April 2015 at 14:43, Cornelia Huck <address@hidden> wrote:
> > > On Fri, 17 Apr 2015 20:13:42 +0800
> > > Shannon Zhao <address@hidden> wrote:
> > >
> > > [Some questions may be silly, but I'm not familiar with the virtio-mmio
> > > code]
> > >
> > >> The reason to do this is that the virtio-net-device can't expose host
> > >> features to guest while using virtio-mmio. So the performance is low.
> > >
> > > So how does virtio-mmio expose any host features?
> > 
> > The features are properties of the backend, not the transport.
> > So for devices where we didn't set these up as "properties
> > exist on the backend and the compatibility transport+backend
> > wrapper devices just forward those properties to the backend",
> > you can't set the properties. We got this right for some of
> > the backends (eg blk) but not all of them, I think.
> 
> The reason why blk is ok is that it adds the feature bits in its
> ->get_features() callback. net expects the feature bits already present
> and removes not supported ones and therefore requires
> statically-defined bits somewhere.
> 
> If we move the feature bits to virtio-net and virtio-scsi, it should
> work for virtio-mmio - but the feature bit propagation from the device
> into the transport becomes a bit useless.
> 
> Could net and scsi add the feature bits dynamically in their
> ->get_features() callback instead? This should work for virtio-mmio as
> well afaics.
> 
> In the end, we should probably end up with the same mechanism for all
> device types.

I think I would also prefer that the host features live in the
generic virtio device structure. This would make it possible
e.g. to validate guest features on vm load in generic code.

-- 
MST