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

[Jason Wang <jasowang@redhat.com>]

On 06/16/2015 03:24 PM, Thibaut Collet wrote:
> If my understanding is correct, on a resume operation, we have the
> following callback trace:
> 1. virtio_pci_restore function that calls all restore call back of
> virtio devices
> 2. virtnet_restore that calls try_fill_recv function for each virtual queues
> 3. try_fill_recv function kicks the virtual queue (through
> virtqueue_kick function)

Yes, but this happens only after pm resume not migration. Migration is
totally transparent to guest.

>
>
> On Tue, Jun 16, 2015 at 7:29 AM, Jason Wang <jasowang@redhat.com> wrote:
>>
>> 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.