Re: [Qemu-devel] vhost-user graceful connect/disconnect

[Wei Wang <wei.w.wang@intel.com>]

On 01/09/2018 12:09 AM, Stefan Hajnoczi wrote:
> On Mon, Jan 08, 2018 at 07:22:37PM +0800, Wei Wang wrote:
>> On 01/05/2018 11:49 PM, Stefan Hajnoczi wrote:
>>> On Thu, Jan 04, 2018 at 07:15:38PM +0800, Wei Wang wrote:
>>>> On 01/04/2018 06:47 PM, Stefan Hajnoczi wrote:
>>>>> On Thu, Dec 21, 2017 at 06:01:29AM -0500, Marc-André Lureau wrote:
>>>>>
>>>>> I'm not going to prototype this yet, I'm working on virtio-vhost-user
>>>>> first, but eventually I might get back to -object vhost-user(-backend).
>>>>>
>>>> Hi Stefan, are you implementing the guest slave and vhost-pci driver (we've
>>>> posted to the dpdk mailinglist) as well? and do you have an estimation when
>>>> would the prototype be ready?
>>> I'm implementing the "[RFC virtio-dev] vhost-user-slave: add vhost-user
>>> slave device type" device in QEMU and DPDK in order to show how the
>>> ideas we've discussed work.
>>>
>>> Here is the VIRTIO spec link again:
>>> https://stefanha.github.io/virtio/vhost-user-slave.html#x1-2830007
>> There are four virtqueues documented in the spec, would two suffice? Request
>> and Response can be distinguished by VHOST_USER_REPLY_MASK.
> That's a good idea and I'll do it for the master-to-slave virtqueue.
>
> Unfortunately virtqueues are asymmetric so a single virtqueue cannot
> support the slave-to-master message flow (VHOST_USER_SET_SLAVE_REQ_FD):
> 1. Master puts an empty buffer onto vq.
> 2. Slave takes buffer and fills in a request.  The buffer is now used.
> 3. Master pops the used buffer but there is no way to reply back to the
>     slave!


Essentially, I think two virtqueues should be sufficient for 
guest-to-host and host-to-guest communication.

Host-to-guest virtqueue: used for master request/response messages 
passed to the guest slave (driver). The buffer can be primed by the 
guest driver (like virtio-net rxq).
Guest-to-host virtqueue: used for guest slave request/response messages, 
like virtio-net txq, it doesn't need to prime buffers in advance.


Best,
Wei