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

From: Longpeng(Mike)
Subject: Re: [Qemu-devel] [Question] why need to start all queues in vhost_net_start
Date: Wed, 15 Nov 2017 23:54:01 +0800

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...
  for (i = 0; i < total_queues; i++)
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.

Sorry for my pool English, Maybe I could describe the problem in Chinese for you
in private if necessary.

> Thanks


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