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

[Jason Wang <jasowang@redhat.com>]

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,elohimes@gmail.com  wrote:
>>> From: Xie Yongji<xieyongji@baidu.com>
>>>
>>> This patchset is aimed at supporting qemu to reconnect
>>> vhost-user-blk backend after vhost-user-blk backend crash or
>>> restart.
>>>
>>> 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.
>>
>> Thanks
> 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).

Thanks