Re: [Qemu-devel] [PATCH v3 2/2] vhost user: Add RARP injection for legacy guest

[Thibaut Collet]

If I correctly understand how vhost user / virtio works the solution
proposed by Michael is OK:
 - Rings to exchange data between host and guest are allocated by the guest.
 - As soon as the guest add rings in a queue (for RX or TX) a kick is
done on the eventfd associated to the queue
 - On a live migration (as for a first startup), the guest virtio-net
pre-allocates rings for each queue and so send a kick on the eventfd
(for RX and TX)

So even if the guest is only doing receiving, there is a kick on any
queues. --> Vhost-user backend knows when send the rarp in any
conditions without involving QEMU.

Michael, could you confirm that my analysis is correct?

On Fri, Jun 12, 2015 at 9:55 AM, Jason Wang <address@hidden> wrote:
>
>
> On 06/11/2015 08:13 PM, Michael S. Tsirkin wrote:
>> On Thu, Jun 11, 2015 at 02:10:48PM +0200, Thibaut Collet wrote:
>>> I am not sure to understand your remark:
>>>
>>>> It needs to be sent when backend is activated by guest kick
>>>> (in case of virtio 1, it's possible to use DRIVER_OK for this).
>>>> This does not happen when VM still runs on source.
>>> Could you confirm rarp can be sent by backend when the
>>> VHOST_USER_SET_VRING_KICK message is received by the backend ?
>> No - the time to send pakets is when you start processing
>> the rings.
>>
>> And the time to do that is when you detect a kick on
>> an eventfd, not when said fd is set.
>>
>
> Probably not. What if guest is only doing receiving? In this case, you
> won't detect any kick if you don't send the rarp first.