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

[Jason Wang <jasowang@redhat.com>]

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).

Thanks

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