linux-kernel - Re: [PATCH] vdpa/mlx5: Use write memory barrier after updating CQ index

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Date:   Wed, 9 Dec 2020 01:46:22 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, lulu@...hat.com
Subject: Re: [PATCH] vdpa/mlx5: Use write memory barrier after updating CQ
 index

On Wed, Dec 09, 2020 at 08:02:30AM +0200, Eli Cohen wrote:
> On Tue, Dec 08, 2020 at 04:45:04PM -0500, Michael S. Tsirkin wrote:
> > On Sun, Dec 06, 2020 at 12:57:19PM +0200, Eli Cohen wrote:
> > > Make sure to put write memory barrier after updating CQ consumer index
> > > so the hardware knows that there are available CQE slots in the queue.
> > > 
> > > Failure to do this can cause the update of the RX doorbell record to get
> > > updated before the CQ consumer index resulting in CQ overrun.
> > > 
> > > Change-Id: Ib0ae4c118cce524c9f492b32569179f3c1f04cc1
> > > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
> > > Signed-off-by: Eli Cohen <elic@...dia.com>
> > 
> > Aren't both memory writes?
> 
> Not sure what exactly you mean here.

Both updates are CPU writes into RAM that hardware then reads
using DMA.

> > And given that, isn't dma_wmb() sufficient here?
> 
> I agree that dma_wmb() is more appropriate here.
> 
> > 
> > 
> > > ---
> > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index 1f4089c6f9d7..295f46eea2a5 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -478,6 +478,11 @@ static int mlx5_vdpa_poll_one(struct mlx5_vdpa_cq *vcq)
> > >  static void mlx5_vdpa_handle_completions(struct mlx5_vdpa_virtqueue *mvq, int num)
> > >  {
> > >  	mlx5_cq_set_ci(&mvq->cq.mcq);
> > > +
> > > +	/* make sure CQ cosumer update is visible to the hardware before updating
> > > +	 * RX doorbell record.
> > > +	 */
> > > +	wmb();
> > >  	rx_post(&mvq->vqqp, num);
> > >  	if (mvq->event_cb.callback)
> > >  		mvq->event_cb.callback(mvq->event_cb.private);
> > > -- 
> > > 2.27.0
> >