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

[Thibaut Collet <thibaut.collet@6wind.com>]

After a resume operation the guest always kicks the backend for each
virtual queues.
A live migration does a suspend operation on the old host and a resume
operation on the new host. So the backend has a kick after migration.

I have checked this point with a legacy guest (redhat 6-5 with kernel
version 2.6.32-431.29.2) and the kick occurs after migration or
resume.

Jason have you an example of legacy guest that will not kick the
virtual queue after a resume ?

On Mon, Jun 15, 2015 at 10:44 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Jun 15, 2015 at 03:43:13PM +0800, Jason Wang wrote:
>>
>>
>> On 06/12/2015 10:28 PM, Michael S. Tsirkin wrote:
>> > On Fri, Jun 12, 2015 at 03:55:33PM +0800, Jason Wang 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?
>> > Clarification: the kick can be on any VQs.
>> > In your example, guest kicks after adding receive buffers.
>>
>> Yes, but refill only happens on we are lacking of receive buffers. It is
>> not guaranteed to happen just after migration, we may have still have
>> enough rx buffers for device to receive.
>
> I think we also kick the backend after migration, do we not?
> Further, DRIVER_OK can be used as a signal to start backend too.
>
>> >
>> >> In this case, you
>> >> won't detect any kick if you don't send the rarp first.