netdev - Re: [PATCH v2 3/3] vdpa/mlx5: defer clear_virtqueues to until DRIVER

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Date:   Tue, 16 Feb 2021 17:21:48 +0200
From:   Eli Cohen <elic@...dia.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
CC:     <mst@...hat.com>, <jasowang@...hat.com>,
        <linux-kernel@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] vdpa/mlx5: defer clear_virtqueues to until
 DRIVER_OK

On Thu, Feb 11, 2021 at 09:33:14AM +0200, Eli Cohen wrote:
> On Wed, Feb 10, 2021 at 01:48:00PM -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().
> > 
> > Fixes: b35ccebe3ef7 ("vdpa/mlx5: Restore the hardware used index after change map")
> > Signed-off-by: Si-Wei Liu <si-wei.liu@...cle.com>
> > Acked-by: Jason Wang <jasowang@...hat.com>
> 
> Acked-by: Eli Cohen <elic@...dia.com>
> 

I take it back. I think we don't need to clear the indexes at all. In
case we need to restore indexes we'll get the right values through
set_vq_state(). If we suspend the virtqueue due to VM being suspended,
qemu will query first and will provide the the queried value. In case of
VM reboot, it will provide 0 in set_vq_state().

I am sending a patch that addresses both reboot and suspend.

> > ---
> >  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 7c1f789..ce6aae8 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1777,7 +1777,6 @@ 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;
> > @@ -1786,6 +1785,7 @@ 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
> >