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

[Longpeng (Mike)]

On 2017/11/16 20:04, Jason Wang wrote:

> 
> 
> On 2017年11月16日 17:32, Longpeng (Mike) wrote:
>> Hi Jason,
>>
>> On 2017/11/16 17:13, Jason Wang wrote:
>>
>>>
>>> On 2017年11月16日 17:01, Gonglei (Arei) wrote:
>>>> No, Windows guest + vhost-user/DPDK.
>>>>
>>>> BTW pls see virtio spec in :
>>>>
>>>> "If VIRTIO_NET_F_MQ is negotiated, each of receiveq1. . .receiveqN that 
>>>> will
>>>> be used SHOULD be populated
>>>> with receive buffers."
>>>>
>>>> It is not mandatory that all queues must be initialized.
>>> I think not, since it said we should fill receive buffers for each queue 
>>> which
>>> means we should initialize all queues. May Michael can clarify on this.
>>>
>>
>> I think this doesn't matter, but QEMU should consider this scenario...
>>
>> For example, if one queues isn't initialized (Windows guest), the 
>> vring.avail=0,
>> so vq->desc_phys=0, then vq->desc='a avail HVA'(which is the start addr of
>> pc.ram).
>>
>>      vq->desc_size = s = l = virtio_queue_get_desc_size(vdev, idx);
>>      vq->desc_phys = a = virtio_queue_get_desc_addr(vdev, idx);
>>      vq->desc = vhost_memory_map(dev, a, &l, 0);
>>      if (!vq->desc || l != s) {
>>          r = -ENOMEM;
>>          goto fail_alloc_desc;
>>      }
>>      .....
>>      r = vhost_virtqueue_set_addr(dev, vq, vhost_vq_index, dev->log_enabled);
>>      if (r < 0) {
>>          r = -errno;
>>          goto fail_alloc;
>>      }
>>
>> Then the HVA is send to the vhost-user.
>>
>> I think this is wrong, because the '0' here means guest driver doesn't init 
>> this
>> queues, it should not be used to calculate the HVA for this vq.
> 
> Yes, workaround is not hard if windows driver won't use the left 3 queues any
> more. But we should have a complete solution. The main problem is when vhost
> need to be started. For legacy device, there's no easy way to detect whether 
> or
> not a specific virtqueue is ready to be used. For modern device, we can 
> probably
> do this through queue_enable (but this is not implemented in current code).
> 


Can we initialize the vring.desc/vring.avail/vring.used to -1, then we can
detect whether or not a specific vq is ready to be used by check the addr of its
tables is -1 or not ?

Initialize to 0 is not suitable, because we don't know whether the guest set GPA
0 or the guest not initialize this vq.

> Thanks
> 
>>
>>> Thanks
>>>
>>>> Thanks,
>>>> -Gonglei
>>>>
>>>
>>>
>>>
>>> .
>>>
>>
> 
> 
> .
> 


-- 
Regards,
Longpeng(Mike)