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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: Wed, 12 Dec 2018 15:47:04 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1

On 2018/12/12 下午2:41, Yongji Xie wrote:
On Wed, 12 Dec 2018 at 12:07, Jason Wang <address@hidden> wrote:

On 2018/12/12 上午11:21, Yongji Xie wrote:
On Wed, 12 Dec 2018 at 11:00, Jason Wang <address@hidden> wrote:
On 2018/12/12 上午10:48, Yongji Xie wrote:
On Mon, 10 Dec 2018 at 17:32, Jason Wang <address@hidden> wrote:
On 2018/12/6 下午9:59, Michael S. Tsirkin wrote:
On Thu, Dec 06, 2018 at 09:57:22PM +0800, Jason Wang wrote:
On 2018/12/6 下午2:35,address@hidden  wrote:
From: Xie Yongji<address@hidden>

This patchset is aimed at supporting qemu to reconnect
vhost-user-blk backend after vhost-user-blk backend crash or

The patch 1 tries to implenment the sync connection for
"reconnect socket".

The patch 2 introduces a new message VHOST_USER_SET_VRING_INFLIGHT
to support offering shared memory to backend to record
its inflight I/O.

The patch 3,4 are the corresponding libvhost-user patches of
patch 2. Make libvhost-user support VHOST_USER_SET_VRING_INFLIGHT.

The patch 5 supports vhost-user-blk to reconnect backend when
connection closed.

The patch 6 tells qemu that we support reconnecting now.

To use it, we could start qemu with:

qemu-system-x86_64 \
             -chardev socket,id=char0,path=/path/vhost.socket,reconnect=1,wait \
             -device vhost-user-blk-pci,chardev=char0 \

and start vhost-user-blk backend with:

vhost-user-blk -b /path/file -s /path/vhost.socket

Then we can restart vhost-user-blk at any time during VM running.
I wonder whether or not it's better to handle this at the level of virtio
protocol itself instead of vhost-user level. E.g expose last_avail_idx to
driver might be sufficient?

Another possible issue is, looks like you need to deal with different kinds
of ring layouts e.g packed virtqueues.

I'm not sure I understand your comments here.
All these would be guest-visible extensions.
Looks not, it only introduces a shared memory between qemu and
vhost-user backend?

Possible for sure but how is this related to
a patch supporting transparent reconnects?
I might miss something. My understanding is that we support transparent
reconnects, but we can't deduce an accurate last_avail_idx and this is
what capability this series try to add. To me, this series is functional
equivalent to expose last_avail_idx (or avail_idx_cons) in available
ring. So the information is inside guest memory, vhost-user backend can
access it and update it directly. I believe this is some modern NIC did
as well (but index is in MMIO area of course).

Hi Jason,

If my understand is correct, it might be not enough to only expose
Because we would not process descriptors in the same order in which they have
been made available sometimes. If so, we can't get correct inflight
I/O information
from available ring.
You can get this with the help of the both used ring and last_avail_idx
I believe. Or maybe you can give us an example?

A simple example, we assume ring size is 8:

1. guest fill avail ring

avail ring: 0 1 2 3 4 5 6 7
used ring:

2. vhost-user backend complete 4,5,6,7 and fill used ring

avail ring: 0 1 2 3 4 5 6 7
used ring: 4 5 6 7

3. guest fill avail ring again

avail ring: 4 5 6 7 4 5 6 7
used ring: 4 5 6 7

4. vhost-user backend crash

The inflight descriptors 0, 1, 2, 3 lost.


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.



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