Re: [PATCH net-next v3 21/33] virtio_net: add XDP frame size in two code paths

[Jason Wang <jasowang@redhat.com>]

On 2020/5/8 下午7:10, 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.
>
> V3: Adjust frame_sz in receive_mergeable() case, spotted by Jason Wang.
>
> The code is really hard to follow,


Yes, I plan to rework to make it more easier for reviewers.


>   so some hints to reviewers.
> The receive_mergeable() case gets frames that were allocated in
> add_recvbuf_mergeable() which uses headroom=virtnet_get_headroom(),
> and 'buf' ptr is advanced this headroom.  The headroom can only
> be 0 or VIRTIO_XDP_HEADROOM, as virtnet_get_headroom is really
> simple:
>
>    static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
>    {
> 	return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
>    }
>
> As frame_sz is an offset size from xdp.data_hard_start, reviewers
> should notice how this is calculated in receive_mergeable():
>
>    int offset = buf - page_address(page);
>    [...]
>    data = page_address(xdp_page) + offset;
>    xdp.data_hard_start = data - VIRTIO_XDP_HEADROOM + vi->hdr_len;
>
> The calculated offset will always be VIRTIO_XDP_HEADROOM when
> reaching this code.  Thus, xdp.data_hard_start will be page-start
> address plus vi->hdr_len.  Given this xdp.frame_sz need to be
> reduced with vi->hdr_len size.
>
> IMHO a followup patch should cleanup this code to make it easier
> to maintain and understand, but it is outside the scope of this
> patchset.
>
> Cc: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>


Acked-by: Jason Wang <jasowang@redhat.com>

Thanks


> ---
>   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..9e1b5d748586 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;
> +
>   			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 - vi->hdr_len;
>   
>   		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);
>
>