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

[Jason Wang <jasowang@redhat.com>]

On 06/15/2015 08:12 PM, Thibaut Collet wrote:
> 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 ?

I must miss something but migration should be transparent to guest.
Could you show me the code that guest does the kick after migration?

>
> 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.