Re: [Qemu-devel] [Question] why need to start all queues in vhost_net_start

[Longpeng (Mike)]

On 2017/11/16 16:54, Jason Wang wrote:

> 
> 
> On 2017年11月16日 13:53, Longpeng (Mike) wrote:
>> On 2017/11/15 23:54, Longpeng(Mike) wrote:
>>> 2017-11-15 23:05 GMT+08:00 Jason Wang<address@hidden>:
>>>> On 2017年11月15日 22:55, Longpeng(Mike) wrote:
>>>>> Hi guys,
>>>>>
>>>>> We got a BUG report from our testers yesterday, the testing scenario was
>>>>> migrating a VM (Windows guest, *4 vcpus*, 4GB, vhost-user net: *7
>>>>> queues*).
>>>>>
>>>>> We found the cause reason, and we'll report the BUG or send a fix patch
>>>>> to upstream if necessary( we haven't test the upstream yet, sorry... ).
>>>> Could you explain this a little bit more?
>>>>
>>>>> We want to know why the vhost_net_start() must start*total queues*  ( in
>>>>> our
>>>>> VM there're 7 queues ) but not*the queues that current used*  ( in our VM,
>>>>> guest
>>>>> only uses the first 4 queues because it's limited by the number of vcpus)
>>>>> ?
>>>>>
>>>>> Looking forward to your help, thx:)
>>>> Since the codes have been there for years and works well for kernel
>>>> datapath. You should really explain what's wrong.
>>>>
>>> OK.:)
>>>
>>> In our scenario,  the Windows's virtio-net driver only use the first 4
>>> queues and it
>>> *only set desc/avail/used table for the first 4 queues*, so in QEMU
>>> the desc/avail/
>>> used of the last 3 queues are ZERO,  but unfortunately...
>>> '''
>>> vhost_net_start
>>>    for (i = 0; i < total_queues; i++)
>>>      vhost_net_start_one
>>>        vhost_dev_start
>>>          vhost_virtqueue_start
>>> '''
>>> In vhost_virtqueue_start(), it will calculate the HVA of
>>> desc/avail/used table, so for last
>>> 3 queues, it will use ZERO as the GPA to calculate the HVA, and then
>>> send the results
>>> to the user-mode backend ( we use*vhost-user*  ) by 
>>> vhost_virtqueue_set_addr().
>>>
>>> When the EVS get these address, it will update a*idx*  which will be
>>> treated as  vq's
>>> last_avail_idx when virtio-net stop ( pls see vhost_virtqueue_stop() ).
>>>
>>> So we get the following result after virtio-net stop:
>>>    the desc/avail/used of the last 3 queues's vqs are all ZERO, but these 
>>> vqs's
>>>    last_avail_idx is NOT ZERO.
>>>
>>> At last, virtio_load() reports an error:
>>> '''
>>>    if (!vdev->vq[i].vring.desc && vdev->vq[i].last_avail_idx) { // <--
>>> will be TRUE
>>>        error_report("VQ %d address 0x0 "
>>>                           "inconsistent with Host index 0x%x",
>>>                           i, vdev->vq[i].last_avail_idx);
>>>              return -1;
>>>     }
>>> '''
>>>
>>> BTW, the problem won't appear if use Linux guest, because the Linux 
>>> virtio-net
>>> driver will set all 7 queues's desc/avail/used tables. And the problem
>>> won't appear
>>> if the VM use vhost-net, because vhost-net won't update*idx*  in SET_ADDR 
>>> ioctl.
> 
> Just to make sure I understand here, I thought Windows guest + vhost_net hit
> this issue?
> 


Windows guest + vhost-user hit.
Windows guest + vhost-net is fine.

'''
In vhost_virtqueue_start(), it will calculate the HVA of
desc/avail/used tables, so for last
3 queues, it will use ZERO as the GPA to calculate the HVA, and then
send the results
to the user-mode backend ( we use *vhost-user* ) by vhost_virtqueue_set_addr().
'''
I think this is the root cause, it is strange, right ?

> Thanks
> 
>>>
>>> Sorry for my pool English, Maybe I could describe the problem in Chinese 
>>> for you
>>> in private if necessary.
>>>
>>>
>>>> Thanks
>> -- Regards, Longpeng(Mike)
> 
> 
> .
> 


-- 
Regards,
Longpeng(Mike)