On Fri, Dec 14, 2018 at 12:36:01PM +0800, Jason Wang wrote: > > On 2018/12/13 下午10:56, Michael S. Tsirkin wrote: > > 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. > > > I'm not sure this could be done 100% transparent to qemu. E.g you need to > deal with reset I think and you need to carefully choose the size of the > region. Which means you need negotiate the size, layout through backend.
I am not sure I follow. The point is all this state is internal to the backend. QEMU does not care at all - it just helps a little by hanging on to it. > And > need to deal with migration with them. Good catch. There definitely is an issue in that you can not migrate with backend being disconnected: migration needs to flush the backend and we can't when it's disconnected. This needs to be addressed. I think it's cleanest to just defer migration until backend does reconnect. Backend cross version migration is all messed up in vhost user, I agree. There was a plan to fix it that was never executed unfortunately. Maxime, do you still plan to look into it? > This is another sin of splitting > virtio dataplane from qemu anyway. > > > Thanks It wasn't split as such - dpdk was never a part of qemu. We just enabled it without fuse hacks. -- MST
