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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 19, 2015 at 06:41:38PM +0200, Thibaut Collet wrote:
> 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.

So drop vhost_get_vq_index from there as well then?


> >
> >
> >> ---
> >>  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