On Fri, 17 Apr 2015 15:00:45 +0100 Peter Maydell <peter.mayd...@linaro.org> wrote:
> On 17 April 2015 at 14:43, Cornelia Huck <cornelia.h...@de.ibm.com> wrote: > > On Fri, 17 Apr 2015 20:13:42 +0800 > > Shannon Zhao <shannon.z...@linaro.org> 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.
