Re: [Qemu-devel] [PATCH 1/1] vhost: set the correct queue index in case of migration with multiqueue

[Jason Wang]

On 10/20/2015 09:33 PM, Michael S. Tsirkin wrote:
> On Mon, Oct 19, 2015 at 02:59:27PM +0200, Thibaut Collet wrote:
>> When a live migration is started the log address to mark dirty pages is 
>> provided
>> to the vhost backend through the vhost_dev_set_log function.
>> This function is called for each queue pairs but the queue index is wrongly 
>> set:
>> always set to the first queue pair. Then vhost backend lost descriptor 
>> addresses
>> of the queue pairs greater than 1 and behaviour of the vhost backend is
>> unpredictable.
>>
>> The queue index is computed by taking account of the vq_index (to retrieve 
>> the
>> queue pair index) and calling the vhost_get_vq_index method of the backend.
>>
>> Signed-off-by: Thibaut Collet <address@hidden>
> Thanks!
> The code in question was added by:
>     commit a9f98bb5ebe6fb1869321dcc58e72041ae626ad8
>     Author: Jason Wang <address@hidden>
>     Date:   Wed Jan 30 19:12:35 2013 +0800
>
>         vhost: multiqueue support
>
> Jason, could you comment on whether this makes sense please?

It makes sense.

> If yes - this is an old bug, and we need this on stable,
> do we not?

But not an old bug, only vhost-user has this issue. So no need for stable.

>
> Maybe we should refactor vhost_virtqueue_set_addr to
> make it call vhost_get_vq_index internally automatically.
> All callers do this anyway.
> This can be a patch on top.

Yes, this looks cleaner.

>
>> ---
>>  hw/virtio/vhost.c | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index feeaaa4..de29968 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -656,13 +656,14 @@ static int vhost_dev_set_features(struct vhost_dev 
>> *dev, bool enable_log)
>>  
>>  static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>>  {
>> -    int r, t, i;
>> +    int r, t, i, idx;
>>      r = vhost_dev_set_features(dev, enable_log);
>>      if (r < 0) {
>>          goto err_features;
>>      }
>>      for (i = 0; i < dev->nvqs; ++i) {
>> -        r = vhost_virtqueue_set_addr(dev, dev->vqs + i, i,
>> +        idx = dev->vhost_ops->vhost_get_vq_index(dev, dev->vq_index + i);
>> +        r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
>>                                       enable_log);
>>          if (r < 0) {
>>              goto err_vq;
>> @@ -671,7 +672,8 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool 
>> enable_log)
>>      return 0;
>>  err_vq:
>>      for (; i >= 0; --i) {
>> -        t = vhost_virtqueue_set_addr(dev, dev->vqs + i, i,
>> +        idx = dev->vhost_ops->vhost_get_vq_index(dev, dev->vq_index + i);
>> +        t = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx,
>>                                       dev->log_enabled);
>>          assert(t >= 0);
>>      }
>> -- 
>> 2.1.4