Re: [dpdk-dev] [PATCH 2/2] vdpa/mlx5: retry VAR allocation during vDPA restart

["Xueming(Steven) Li" <xuemingl@nvidia.com>]

On Wed, 2021-10-13 at 12:06 +0200, Maxime Coquelin wrote:
> 
> On 9/23/21 10:17, Xueming Li wrote:
> > VAR is the device memory space for the virtio queues doorbells, qemu
> > could mmap it to directly to speed up doorbell push.
> > 
> > On a busy system, Qemu takes time to release VAR resources during driver
> > shutdown. If vdpa restarted quickly, the VAR allocation failed with
> > error 28 since the VAR is singleton resource per device.
> > 
> > This patch adds retry mechanism for VAR allocation.
> > 
> > Signed-off-by: Xueming Li <xuemingl@nvidia.com>
> > Reviewed-by: Matan Azrad <matan@nvidia.com>
> > ---
> >   drivers/vdpa/mlx5/mlx5_vdpa.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vdpa/mlx5/mlx5_vdpa.c b/drivers/vdpa/mlx5/mlx5_vdpa.c
> > index 6d17d7a6f3..991739e984 100644
> > --- a/drivers/vdpa/mlx5/mlx5_vdpa.c
> > +++ b/drivers/vdpa/mlx5/mlx5_vdpa.c
> > @@ -693,7 +693,14 @@ mlx5_vdpa_dev_probe(struct rte_device *dev)
> >   	if (attr.num_lag_ports == 0)
> >   		priv->num_lag_ports = 1;
> >   	priv->ctx = ctx;
> > -	priv->var = mlx5_glue->dv_alloc_var(ctx, 0);
> > +	for (retry = 0; retry < 7; retry++) {
> > +		priv->var = mlx5_glue->dv_alloc_var(ctx, 0);
> > +		if (priv->var != NULL)
> > +			break;
> > +		DRV_LOG(WARNING, "Failed to allocate VAR, retry %d.\n", retry);
> > +		/* Wait Qemu release VAR during vdpa restart, 0.1 sec based. */
> > +		usleep(100000U << retry);
> > +	}
> >   	if (!priv->var) {
> >   		DRV_LOG(ERR, "Failed to allocate VAR %u.", errno);
> >   		goto error;
> > 
> 
> That looks fragile, but at least we have a warning we can rely on.
> Shouldn't we have a way to wait for Qemu to release the resources at
> vdpa driver shutdown time?

If dpdk-vdpa get killed and restart, qemu will shutdown device and
unmap the resources independently.

> 
> Also as on patch 1, please add a Fixes tag it you want it to be
> backported.

Agree to backport, but not a fix, I'll add cc:stable@dpdk.org, the
patch will be noticed by maintainer, thanks for the suggestion!

> 
> Regards,
> Maxime
>