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

[Jason Wang]

On 06/24/2015 07:05 PM, Michael S. Tsirkin wrote:
> On Wed, Jun 24, 2015 at 04:31:15PM +0800, Jason Wang wrote:
>>
>> On 06/23/2015 01:49 PM, Michael S. Tsirkin wrote:
>>> On Tue, Jun 23, 2015 at 10:12:17AM +0800, Jason Wang wrote:
>>>>>
>>>>> On 06/18/2015 11:16 PM, Thibaut Collet wrote:
>>>>>>> On Tue, Jun 16, 2015 at 10:05 AM, Jason Wang <address@hidden> wrote:
>>>>>>>>> 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.
>>>>>>>>>
>>>>>>> Hi Jason,
>>>>>>>
>>>>>>> After a deeper look in the migration code of QEMU a resume event is
>>>>>>> always sent when the live migration is finished.
>>>>>>> On a live migration we have the following callback trace:
>>>>>>> 1. The VM on the new host is set to the state RUN_STATE_INMIGRATE, the
>>>>>>> autostart boolean to 1  and calls the qemu_start_incoming_migration
>>>>>>> function (see function main of vl.c)
>>>>>>> .....
>>>>>>> 2. call of process_incoming_migration function in
>>>>>>> migration/migration.c file whatever the way to do the live migration
>>>>>>> (tcp:, fd:, unix:, exec: ...)
>>>>>>> 3. call of process_incoming_migration_co function in 
>>>>>>> migration/migration.c
>>>>>>> 4. call of vm_start function in vl.c (otherwise the migrated VM stay
>>>>>>> in the pause state, the autostart boolean is set to 1 by the main
>>>>>>> function in vl.c)
>>>>>>> 5. call of vm_start function that sets the VM is the RUN_STATE_RUNNING 
>>>>>>> state.
>>>>>>> 6. call of qapi_event_send_resume function that ends a resume event to 
>>>>>>> the VM
>>>>> AFAIK, this function sends resume event to qemu monitor not VM.
>>>>>
>>>>>>> So when a live migration is ended:
>>>>>>> 1. a resume event is sent to the guest
>>>>>>> 2. On the reception of this resume event the virtual queue are kicked
>>>>>>> by the guest
>>>>>>> 3. Backend vhost user catches this kick and can emit a RARP to guest
>>>>>>> that does not support GUEST_ANNOUNCE
>>>>>>>
>>>>>>> This solution, as solution based on detection of DRIVER_OK status
>>>>>>> suggested by Michael, allows backend to send the RARP to legacy guest
>>>>>>> without involving QEMU and add ioctl to vhost-user.
>>>>> A question here is did vhost-user code pass status to the backend? If
>>>>> not, how can userspace backend detect DRIVER_OK?
>>> Sorry, I must have been unclear.
>>> vhost core calls VHOST_NET_SET_BACKEND on DRIVER_OK.
>>> Unfortunately vhost user currently translates it to VHOST_USER_NONE.
>> Looks like VHOST_NET_SET_BACKEND was only used for tap backend.
>>
>>> As a work around, I think kicking ioeventfds once you get
>>> VHOST_NET_SET_BACKEND will work.
>> Maybe just a eventfd_set() in vhost_net_start(). But is this
>> "workaround" elegant enough to be documented? Is it better to do this
>> explicitly with a new feature?
> If you are going to do this anyway, there are a couple of other changes
> we should do, in particular, decide what we want to do with control vq.
>

If I understand correctly, you mean VIRTIO_NET_CTRL_MQ and
VIRTIO_NET_CTRL_GUEST_OFFLOADS? Looks like both of these were broken.
Need more thought, maybe new kinds of requests.