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

[Thibaut Collet]

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)


On Tue, Jun 16, 2015 at 7:29 AM, Jason Wang <address@hidden> 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 <address@hidden> 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.
>