On Tue, 28 May 2024 17:32:26 +0200 Stefano Garzarella <sgarz...@redhat.com> wrote:
> >1) The uses is explicitly asking for a vhost device and giving the user > >a non vhost device is not an option. > > I didn't get this point :-( can you elaborate? I was thinking along the lines: QEMU gets told what devices to provision, and that includes things like what virtio features, and what kind of a backend. In this example, the default for vsock-vhost is no VIRTIO_F_RING_PACKED, but if we tell QEMU to create a vsock-vhost device with the feature VIRTIO_F_RING_PACKED, things go south in a not nice way. Given that vhost not supporting VIRTIO_F_RING_PACKED as of today is a fact of life we must accept, there are multiple ways how such a situation can be handled. For instance vhost-net is handling this by the device not offering the VIRTIO_F_RING_PACKED feature. This is at least what I think I have observed, but I would not mind somebody confirming it. But for the sake of the argument, let us look at other options. The straightforward one would be to not realize the device, because we can't provide what we have been asked to provide. And this actually makes me think about migration! What would happen, were we to eventually introduce, packed to vhost and vhost net, and then attempt to migrate between a host that has this new feature and host that has not. I guess things would pretty much blow up in a very unpleasant way! Then for some devices, at least in theory, it might be possible to abandon not the feature but the backend. Along the lines we were asked to provide the feature X with backend Y but since backend Y does not support that feature and backed Z does, we will determistically go with backend Z. But IMHO this is a purely theoretical consideration, and we shall not go this way. In any case if we are asked to provide with properties such that we can't actually do that, something has to go out of the window: either some of the properties, or the entire device. Regards, Halil
