On Thu, Dec 13, 2018 at 11:41:06AM +0800, Yongji Xie wrote: > On Thu, 13 Dec 2018 at 10:58, Jason Wang <jasow...@redhat.com> wrote: > > > > > > On 2018/12/12 下午5:18, Yongji Xie wrote: > > >>>> Ok, then we can simply forbid increasing the avail_idx in this case? > > >>>> > > >>>> Basically, it's a question of whether or not it's better to done it in > > >>>> the level of virtio instead of vhost. I'm pretty sure if we expose > > >>>> sufficient information, it could be done without touching vhost-user. > > >>>> And we won't deal with e.g migration and other cases. > > >>>> > > >>> OK, I get your point. That's indeed an alternative way. But this > > >>> feature seems > > >>> to be only useful to vhost-user backend. > > >> I admit I could not think of a use case other than vhost-user. > > >> > > >> > > >>> I'm not sure whether it make sense to > > >>> touch virtio protocol for this feature. > > >> Some possible advantages: > > >> > > >> - Feature could be determined and noticed by user or management layer. > > >> > > >> - There's no need to invent ring layout specific protocol to record in > > >> flight descriptors. E.g if my understanding is correct, for this series > > >> and for the example above, it still can not work for packed virtqueue > > >> since descriptor id is not sufficient (descriptor could be overwritten > > >> by used one). You probably need to have a (partial) copy of descriptor > > >> ring for this. > > >> > > >> - No need to deal with migration, all information was in guest memory. > > >> > > > Yes, we have those advantages. But seems like handle this in vhost-user > > > level could be easier to be maintained in production environment. We can > > > support old guest. And the bug fix will not depend on guest kernel > > > updating. > > > > > > Yes. But the my main concern is the layout specific data structure. If > > it could be done through a generic structure (can it?), it would be > > fine. Otherwise, I believe we don't want another negotiation about what > > kind of layout that backend support for reconnect. > > > > Yes, the current layout in shared memory didn't support packed virtqueue > because > the information of one descriptor in descriptor ring will not be > available once device fetch it. > > I also thought about a generic structure before. But I failed... So I > tried another way > to acheive that in this series. In QEMU side, we just provide a shared > memory to backend > and we didn't define anything for this memory. In backend side, they > should know how to > use those memory to record inflight I/O no matter what kind of > virtqueue they used. > Thus, If we updates virtqueue for new virtio spec in the feature, we > don't need to touch > QEMU and guest. What do you think about it? > > Thanks, > Yongji
I think that's a good direction to take, yes. Backends need to be very careful about the layout, with versioning etc. -- MST
