On Wed, Jun 10, 2020 at 11:21:50AM +0800, Jason Wang wrote: > > On 2020/6/10 上午2:07, Michael S. Tsirkin wrote: > > > +/* > > > + * Default vhost_get_features() feature bits for existing device types > > > that do > > > + * not define their own. > > > + * > > > + * This is a workaround for existing device types, do not use this in > > > new vhost > > > + * device types. Explicitly define a list of feature bits instead. > > > + * > > > + * The following feature bits are excluded because libvhost-user device > > > + * backends did not advertise them for a long time. Therefore we cannot > > > detect > > > + * their presence. Instead we assume they are always supported by the > > > device > > > + * backend: > > > + * VIRTIO_F_NOTIFY_ON_EMPTY > > > + * VIRTIO_F_ANY_LAYOUT > > > + * VIRTIO_F_VERSION_1 > > > + * VIRTIO_RING_F_INDIRECT_DESC > > > + * VIRTIO_RING_F_EVENT_IDX > > Weird. I remember that it's common for vhost-user not to set > > VIRTIO_RING_F_INDIRECT_DESC - they have huge queues so > > don't need it and inline descriptors give them better > > performance. > > > > So what's going on here? > > > I guess one reason is to support live migration between vhost-user and > vhost-net. > > Thanks >
But how can we force-enable features backend doesn't want to enable? This may or may not break backends ... I would rather just be strict and ask backends to fix their feature bits. See user_feature_bits in hw/net/vhost-net.c which supports all these features. -- MST
