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Re: [Qemu-devel] [PATCH v4 for-4.0 4/7] libvhost-user: Support tracking

From: Jason Wang
Subject: Re: [Qemu-devel] [PATCH v4 for-4.0 4/7] libvhost-user: Support tracking inflight I/O in shared memory
Date: Fri, 18 Jan 2019 11:59:50 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 2019/1/18 上午11:32, Yongji Xie wrote:
On Thu, 17 Jan 2019 at 17:57, Jason Wang <address@hidden> wrote:

On 2019/1/15 下午10:51, Yongji Xie wrote:
Well, this may work but here're my points:

1) The code want to recover from backed crash by introducing extra space
to store inflight data, but it still depends on the backend to set/get
the inflight state

2) Since the backend could be killed at any time, the backend must have
the ability to recover from the partial inflight state

So it looks to me 1) tends to be self-contradictory and 2) tends to be
recursive. The above lines show how tricky could the code looks like.

Solving this at vhost-user level through at backend is probably wrong.
It's time to consider the support from virtio itself.

I agree that supporting this in virtio level may be better. For
example, resubmitting inflight I/O once DEVICE_NEEDS_RESET is set in
Stefan's proposal. But I still think QEMU should be able to provide
this ability too. Supposed that one vhost-user backend need to support
multiple VMs. We can't enable reconnect ability until all VMs' guest
driver support the new feature. It's limited.

That's the way virtio evolves.

   But if QEMU have the
ability to store inflight buffer, the backend could at least have a
chance to support this case.

The problem is, you need a careful designed protocol described somewhere
That's what we should discuss in detail in this series.

Well, I ask some questions for this patch, but it looks like they were still not answered. No?

(is vhost-user.txt a good place for this?). And this work will be
(partial) duplicated for the future support from virtio spec itself.

I think the duplicated code is to maintain the inflight descriptor
list which should be in backend. That's not main work in this series.
And backend could choose to include it or not.

You need to have a documentation to describe the protocol. Otherwise, it would be very hard for other backend to implement.



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