Re: [PATCH net-next 21/33] virtio_net: add XDP frame size in two code paths

[Jason Wang <jasowang@redhat.com>]

On 2020/4/23 上午12:09, Jesper Dangaard Brouer wrote:
> The virtio_net driver is running inside the guest-OS. There are two
> XDP receive code-paths in virtio_net, namely receive_small() and
> receive_mergeable(). The receive_big() function does not support XDP.
>
> In receive_small() the frame size is available in buflen. The buffer
> backing these frames are allocated in add_recvbuf_small() with same
> size, except for the headroom, but tailroom have reserved room for
> skb_shared_info. The headroom is encoded in ctx pointer as a value.
>
> In receive_mergeable() the frame size is more dynamic. There are two
> basic cases: (1) buffer size is based on a exponentially weighted
> moving average (see DECLARE_EWMA) of packet length. Or (2) in case
> virtnet_get_headroom() have any headroom then buffer size is
> PAGE_SIZE. The ctx pointer is this time used for encoding two values;
> the buffer len "truesize" and headroom. In case (1) if the rx buffer
> size is underestimated, the packet will have been split over more
> buffers (num_buf info in virtio_net_hdr_mrg_rxbuf placed in top of
> buffer area). If that happens the XDP path does a xdp_linearize_page
> operation.
>
> Cc: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>   drivers/net/virtio_net.c |   15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 11f722460513..1df3676da185 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -689,6 +689,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
>   		xdp.data_end = xdp.data + len;
>   		xdp.data_meta = xdp.data;
>   		xdp.rxq = &rq->xdp_rxq;
> +		xdp.frame_sz = buflen;
>   		orig_data = xdp.data;
>   		act = bpf_prog_run_xdp(xdp_prog, &xdp);
>   		stats->xdp_packets++;
> @@ -797,10 +798,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   	int offset = buf - page_address(page);
>   	struct sk_buff *head_skb, *curr_skb;
>   	struct bpf_prog *xdp_prog;
> -	unsigned int truesize;
> +	unsigned int truesize = mergeable_ctx_to_truesize(ctx);
>   	unsigned int headroom = mergeable_ctx_to_headroom(ctx);
> -	int err;
>   	unsigned int metasize = 0;
> +	unsigned int frame_sz;
> +	int err;
>   
>   	head_skb = NULL;
>   	stats->bytes += len - vi->hdr_len;
> @@ -821,6 +823,11 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   		if (unlikely(hdr->hdr.gso_type))
>   			goto err_xdp;
>   
> +		/* Buffers with headroom use PAGE_SIZE as alloc size,
> +		 * see add_recvbuf_mergeable() + get_mergeable_buf_len()
> +		 */
> +		frame_sz = headroom ? PAGE_SIZE : truesize;
> +
>   		/* This happens when rx buffer size is underestimated
>   		 * or headroom is not enough because of the buffer
>   		 * was refilled before XDP is set. This should only
> @@ -834,6 +841,8 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   						      page, offset,
>   						      VIRTIO_XDP_HEADROOM,
>   						      &len);
> +			frame_sz = PAGE_SIZE;


Should this be PAGE_SIZE -  SKB_DATA_ALIGN(sizeof(struct skb_shared_info))?


> +
>   			if (!xdp_page)
>   				goto err_xdp;
>   			offset = VIRTIO_XDP_HEADROOM;
> @@ -850,6 +859,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   		xdp.data_end = xdp.data + (len - vi->hdr_len);
>   		xdp.data_meta = xdp.data;
>   		xdp.rxq = &rq->xdp_rxq;
> +		xdp.frame_sz = frame_sz;


Maybe we can easily check by

xdp.frame_sz = (xdp_page == page) ? truesize : ...

Thanks


>   
>   		act = bpf_prog_run_xdp(xdp_prog, &xdp);
>   		stats->xdp_packets++;
> @@ -924,7 +934,6 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
>   	}
>   	rcu_read_unlock();
>   
> -	truesize = mergeable_ctx_to_truesize(ctx);
>   	if (unlikely(len > truesize)) {
>   		pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
>   			 dev->name, len, (unsigned long)ctx);
>
>