Re: [PATCH 3/3] mlx5_vdpa: defer clear_virtqueues to until DRIVER_OK

[Si-Wei Liu]

On 2/8/2021 7:37 PM, Jason Wang wrote:
> On 2021/2/6 下午8:29, Si-Wei Liu wrote:
> > While virtq is stopped,  get_vq_state() is supposed to
> > be  called to  get  sync'ed  with  the latest internal
> > avail_index from device. The saved avail_index is used
> > to restate  the virtq  once device is started.  Commit
> > b35ccebe3ef7 introduced the clear_virtqueues() routine
> > to  reset  the saved  avail_index,  however, the index
> > gets cleared a bit earlier before get_vq_state() tries
> > to read it. This would cause consistency problems when
> > virtq is restarted, e.g. through a series of link down
> > and link up events. We  could  defer  the  clearing of
> > avail_index  to  until  the  device  is to be started,
> > i.e. until  VIRTIO_CONFIG_S_DRIVER_OK  is set again in
> > set_status().
> >
> > Fixes: b35ccebe3ef7 ("vdpa/mlx5: Restore the hardware used index 
> > after change map")
> > Signed-off-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>
> > ---
> >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
> > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index aa6f8cd..444ab58 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1785,7 +1785,6 @@ static void mlx5_vdpa_set_status(struct 
> > vdpa_device *vdev, u8 status)
> >       if (!status) {
> >           mlx5_vdpa_info(mvdev, "performing device reset\n");
> >           teardown_driver(ndev);
> > -        clear_virtqueues(ndev);
> >           mlx5_vdpa_destroy_mr(&ndev->mvdev);
> >           ndev->mvdev.status = 0;
> >           ++mvdev->generation;
> > @@ -1794,6 +1793,7 @@ static void mlx5_vdpa_set_status(struct 
> > vdpa_device *vdev, u8 status)
> >         if ((status ^ ndev->mvdev.status) & VIRTIO_CONFIG_S_DRIVER_OK) {
> >           if (status & VIRTIO_CONFIG_S_DRIVER_OK) {
> > +            clear_virtqueues(ndev);
>
>
> Rethink about this. As mentioned in another thread, this in fact 
> breaks set_vq_state().  (See vhost_virtqueue_start() -> 
> vhost_vdpa_set_vring_base() in qemu codes).
I assume that the clearing for vhost-vdpa would be done via (qemu code),

vhost_dev_start()->vhost_vdpa_dev_start()->vhost_vdpa_call(status | 
VIRTIO_CONFIG_S_DRIVER_OK)

which is _after_ vhost_virtqueue_start() gets called to restore the 
avail_idx to h/w in vhost_dev_start(). What am I missing here?

-Siwei


> The issue is that the avail idx is forgot, we need keep it.
>
> Thanks
>
>
> >               err = setup_driver(ndev);
> >               if (err) {
> >                   mlx5_vdpa_warn(mvdev, "failed to setup driver\n");