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

[Thibaut Collet <thibaut.collet@6wind.com>]

On Mon, Oct 19, 2015 at 5:41 PM, Michael S. Tsirkin <mst@redhat.com> 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 <thibaut.collet@6wind.com>
>
> This needs some thought to make sure we don't break the kernel vhost.

For kernel vhost my patch does nothing has vhost_get_vq_index method
for vhost kernel subtract dev->vq_index (that was just added before)
and idx is still equal to i.

>
> I queued this temporarily to enable your testing but I think it would be
> preferable to make vhost_virtqueue_set_addr for vhost_user call
> vhost_get_vq_index internally.
>

If I call the vhost_get_vq_index internally by vhost_user when
vhost_virtqueue_set_addr is called I will break the
vhost_virtqueue_start: this function calls the vhost_get_vq_index
function for vhost user and vhost kernel to initializes the queue.

>
>
>> ---
>>  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