Re: [PATCH net-next v10 2/3] xen networking: add basic XDP support for xen-netfront

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Mon, 22 Jun 2020 12:21:11 +0300 Denis Kirjanov <kda@linux-powerpc.org> wrote:

> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 482c6c8..1b9f49e 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
[...]
> @@ -560,6 +572,65 @@ static u16 xennet_select_queue(struct net_device *dev, struct sk_buff *skb,
>  	return queue_idx;
>  }
>  
> +static int xennet_xdp_xmit_one(struct net_device *dev, struct xdp_frame *xdpf)
> +{
> +	struct netfront_info *np = netdev_priv(dev);
> +	struct netfront_stats *tx_stats = this_cpu_ptr(np->tx_stats);
> +	unsigned int num_queues = dev->real_num_tx_queues;
> +	struct netfront_queue *queue = NULL;
> +	struct xen_netif_tx_request *tx;
> +	unsigned long flags;
> +	int notify;
> +
> +	queue = &np->queues[smp_processor_id() % num_queues];
> +
> +	spin_lock_irqsave(&queue->tx_lock, flags);

Why are you taking a lock per packet (xdp_frame)?

> +
> +	tx = xennet_make_first_txreq(queue, NULL,
> +				     virt_to_page(xdpf->data),
> +				     offset_in_page(xdpf->data),
> +				     xdpf->len);
> +
> +	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&queue->tx, notify);
> +	if (notify)
> +		notify_remote_via_irq(queue->tx_irq);
> +
> +	u64_stats_update_begin(&tx_stats->syncp);
> +	tx_stats->bytes += xdpf->len;
> +	tx_stats->packets++;
> +	u64_stats_update_end(&tx_stats->syncp);
> +
> +	xennet_tx_buf_gc(queue);
> +
> +	spin_unlock_irqrestore(&queue->tx_lock, flags);

Is the irqsave/irqrestore variant really needed here?


> +	return 0;
> +}
> +
> +static int xennet_xdp_xmit(struct net_device *dev, int n,
> +			   struct xdp_frame **frames, u32 flags)
> +{
> +	int drops = 0;
> +	int i, err;
> +
> +	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
> +		return -EINVAL;
> +
> +	for (i = 0; i < n; i++) {
> +		struct xdp_frame *xdpf = frames[i];
> +
> +		if (!xdpf)
> +			continue;
> +		err = xennet_xdp_xmit_one(dev, xdpf);
> +		if (err) {
> +			xdp_return_frame_rx_napi(xdpf);
> +			drops++;
> +		}
> +	}
> +
> +	return n - drops;
> +}



-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer