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

[Si-Wei Liu]

On 2/7/2021 9:48 PM, Eli Cohen wrote:
> On Sat, Feb 06, 2021 at 04:29:24AM -0800, 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().
>
> Not sure I understand the scenario. You are talking about reset of the
> device followed by up/down events on the interface. How can you trigger
> this?
Currently it's not possible to trigger link up/down events with upstream 
QEMU due lack of config/control interrupt implementation. And live 
migration could be another scenario that cannot be satisfied because of 
inconsistent queue state. They share the same root of cause as captured 
here.

-Siwei

> > 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);
> >   			err = setup_driver(ndev);
> >   			if (err) {
> >   				mlx5_vdpa_warn(mvdev, "failed to setup driver\n");
> > --
> > 1.8.3.1