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

From: Wei Wang
Subject: Re: [Qemu-devel] vhost-user graceful connect/disconnect
Date: Fri, 12 Jan 2018 14:53:43 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

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:
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

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.


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