RE: [PATCH] vdpa: conditionally fill max max queue pair for stats

[Eli Cohen]

> From: Jason Wang <jasowang@xxxxxxxxxx>
> Sent: Wednesday, 7 September 2022 9:01
> To: mst@xxxxxxxxxx; jasowang@xxxxxxxxxx; Eli Cohen <elic@xxxxxxxxxx>;
> virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: [PATCH] vdpa: conditionally fill max max queue pair for stats
> 
> For the device without multiqueue feature, we will read 0 as
> max_virtqueue_pairs from the config.
If this is the case for other vdpa vendor drivers, shouldn't we fix it there? After all,
config->max_virtqueue_pairs should always show valid values.

> So if we fill
> VDPA_ATTR_DEV_NET_CFG_MAX_VQP with the value we read from the
> config
> we will confuse the user.
> 
> Fixing this by only filling the value when multiqueue is offered by
> the device so userspace can assume 1 when the attr is not provided.
> 
> Fixes: 13b00b135665c("vdpa: Add support for querying vendor statistics")
> Cc: Eli Cohen <elic@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
>  drivers/vdpa/vdpa.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> index c06c02704461..bc328197263f 100644
> --- a/drivers/vdpa/vdpa.c
> +++ b/drivers/vdpa/vdpa.c
> @@ -894,7 +894,6 @@ static int vdpa_fill_stats_rec(struct vdpa_device
> *vdev, struct sk_buff *msg,
>  {
>  	struct virtio_net_config config = {};
>  	u64 features;
> -	u16 max_vqp;
>  	u8 status;
>  	int err;
> 
> @@ -905,15 +904,15 @@ static int vdpa_fill_stats_rec(struct vdpa_device
> *vdev, struct sk_buff *msg,
>  	}
>  	vdpa_get_config_unlocked(vdev, 0, &config, sizeof(config));
> 
> -	max_vqp = __virtio16_to_cpu(true, config.max_virtqueue_pairs);
> -	if (nla_put_u16(msg, VDPA_ATTR_DEV_NET_CFG_MAX_VQP,
> max_vqp))
> -		return -EMSGSIZE;
> -
>  	features = vdev->config->get_driver_features(vdev);
>  	if (nla_put_u64_64bit(msg,
> VDPA_ATTR_DEV_NEGOTIATED_FEATURES,
>  			      features, VDPA_ATTR_PAD))
>  		return -EMSGSIZE;
> 
> +	err = vdpa_dev_net_mq_config_fill(vdev, msg, features, &config);
> +	if (err)
> +		return err;
> +

So that means that you can't read statistics when MQ is not supported. Is this worth sacrificing?

>  	if (nla_put_u32(msg, VDPA_ATTR_DEV_QUEUE_INDEX, index))
>  		return -EMSGSIZE;
> 
> --
> 2.25.1