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Re: [Qemu-devel] [PATCH for-4.0 0/6] vhost-user-blk: Add support for bac

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH for-4.0 0/6] vhost-user-blk: Add support for backend reconnecting
Date: Fri, 14 Dec 2018 12:36:01 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

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 <address@hidden> 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?

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. And need to deal with migration with them. This is another sin of splitting virtio dataplane from qemu anyway.


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