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Message-ID: <CACGkMEsdwApvHFTWc4biY-AvwQf9sPuonYFU_Zdn2=wa3cHBjg@mail.gmail.com>
Date:   Thu, 20 Apr 2023 14:23:48 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        virtualization@...ts.linux-foundation.org, bpf@...r.kernel.org
Subject: Re: [PATCH net-next v2 11/14] virtio_net: introduce receive_small_xdp()

On Tue, Apr 18, 2023 at 2:53 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> The purpose of this patch is to simplify the receive_small().
> Separate all the logic of XDP of small into a function.
>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 165 +++++++++++++++++++++++----------------
>  1 file changed, 99 insertions(+), 66 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index a4bb25f39f12..34220f5f27d1 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -941,6 +941,98 @@ static struct page *xdp_linearize_page(struct receive_queue *rq,
>         return NULL;
>  }
>
> +static struct sk_buff *receive_small_xdp(struct net_device *dev,
> +                                        struct virtnet_info *vi,
> +                                        struct receive_queue *rq,
> +                                        struct bpf_prog *xdp_prog,
> +                                        void *buf,
> +                                        unsigned int xdp_headroom,
> +                                        unsigned int len,
> +                                        unsigned int *xdp_xmit,
> +                                        struct virtnet_rq_stats *stats)
> +{
> +       unsigned int header_offset = VIRTNET_RX_PAD + xdp_headroom;
> +       unsigned int headroom = vi->hdr_len + header_offset;
> +       struct virtio_net_hdr_mrg_rxbuf *hdr = buf + header_offset;
> +       struct page *page = virt_to_head_page(buf);
> +       struct page *xdp_page;
> +       unsigned int buflen;
> +       struct xdp_buff xdp;
> +       struct sk_buff *skb;
> +       unsigned int delta = 0;
> +       unsigned int metasize = 0;
> +       void *orig_data;
> +       u32 act;
> +
> +       if (unlikely(hdr->hdr.gso_type))
> +               goto err_xdp;
> +
> +       buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> +               SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +
> +       if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
> +               int offset = buf - page_address(page) + header_offset;
> +               unsigned int tlen = len + vi->hdr_len;
> +               int num_buf = 1;
> +
> +               xdp_headroom = virtnet_get_headroom(vi);
> +               header_offset = VIRTNET_RX_PAD + xdp_headroom;
> +               headroom = vi->hdr_len + header_offset;
> +               buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> +                       SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> +               xdp_page = xdp_linearize_page(rq, &num_buf, page,
> +                                             offset, header_offset,
> +                                             &tlen);
> +               if (!xdp_page)
> +                       goto err_xdp;
> +
> +               buf = page_address(xdp_page);
> +               put_page(page);
> +               page = xdp_page;
> +       }
> +
> +       xdp_init_buff(&xdp, buflen, &rq->xdp_rxq);
> +       xdp_prepare_buff(&xdp, buf + VIRTNET_RX_PAD + vi->hdr_len,
> +                        xdp_headroom, len, true);
> +       orig_data = xdp.data;
> +
> +       act = virtnet_xdp_handler(xdp_prog, &xdp, dev, xdp_xmit, stats);
> +
> +       switch (act) {
> +       case VIRTNET_XDP_RES_PASS:
> +               /* Recalculate length in case bpf program changed it */
> +               delta = orig_data - xdp.data;
> +               len = xdp.data_end - xdp.data;
> +               metasize = xdp.data - xdp.data_meta;
> +               break;
> +
> +       case VIRTNET_XDP_RES_CONSUMED:
> +               goto xdp_xmit;
> +
> +       case VIRTNET_XDP_RES_DROP:
> +               goto err_xdp;
> +       }
> +
> +       skb = build_skb(buf, buflen);
> +       if (!skb)
> +               goto err;
> +
> +       skb_reserve(skb, headroom - delta);
> +       skb_put(skb, len);
> +       if (metasize)
> +               skb_metadata_set(skb, metasize);
> +
> +       return skb;
> +
> +err_xdp:
> +       stats->xdp_drops++;
> +err:
> +       stats->drops++;
> +       put_page(page);
> +xdp_xmit:
> +       return NULL;
> +}
> +
>  static struct sk_buff *receive_small(struct net_device *dev,
>                                      struct virtnet_info *vi,
>                                      struct receive_queue *rq,
> @@ -957,9 +1049,6 @@ static struct sk_buff *receive_small(struct net_device *dev,
>         unsigned int buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
>                               SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>         struct page *page = virt_to_head_page(buf);
> -       unsigned int delta = 0;
> -       struct page *xdp_page;
> -       unsigned int metasize = 0;
>
>         len -= vi->hdr_len;
>         stats->bytes += len;
> @@ -979,57 +1068,10 @@ static struct sk_buff *receive_small(struct net_device *dev,
>         rcu_read_lock();
>         xdp_prog = rcu_dereference(rq->xdp_prog);
>         if (xdp_prog) {
> -               struct virtio_net_hdr_mrg_rxbuf *hdr = buf + header_offset;
> -               struct xdp_buff xdp;
> -               void *orig_data;
> -               u32 act;
> -
> -               if (unlikely(hdr->hdr.gso_type))
> -                       goto err_xdp;
> -
> -               if (unlikely(xdp_headroom < virtnet_get_headroom(vi))) {
> -                       int offset = buf - page_address(page) + header_offset;
> -                       unsigned int tlen = len + vi->hdr_len;
> -                       int num_buf = 1;
> -
> -                       xdp_headroom = virtnet_get_headroom(vi);
> -                       header_offset = VIRTNET_RX_PAD + xdp_headroom;
> -                       headroom = vi->hdr_len + header_offset;
> -                       buflen = SKB_DATA_ALIGN(GOOD_PACKET_LEN + headroom) +
> -                                SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> -                       xdp_page = xdp_linearize_page(rq, &num_buf, page,
> -                                                     offset, header_offset,
> -                                                     &tlen);
> -                       if (!xdp_page)
> -                               goto err_xdp;
> -
> -                       buf = page_address(xdp_page);
> -                       put_page(page);
> -                       page = xdp_page;
> -               }
> -
> -               xdp_init_buff(&xdp, buflen, &rq->xdp_rxq);
> -               xdp_prepare_buff(&xdp, buf + VIRTNET_RX_PAD + vi->hdr_len,
> -                                xdp_headroom, len, true);
> -               orig_data = xdp.data;
> -
> -               act = virtnet_xdp_handler(xdp_prog, &xdp, dev, xdp_xmit, stats);
> -
> -               switch (act) {
> -               case VIRTNET_XDP_RES_PASS:
> -                       /* Recalculate length in case bpf program changed it */
> -                       delta = orig_data - xdp.data;
> -                       len = xdp.data_end - xdp.data;
> -                       metasize = xdp.data - xdp.data_meta;
> -                       break;
> -
> -               case VIRTNET_XDP_RES_CONSUMED:
> -                       rcu_read_unlock();
> -                       goto xdp_xmit;
> -
> -               case VIRTNET_XDP_RES_DROP:
> -                       goto err_xdp;
> -               }
> +               skb = receive_small_xdp(dev, vi, rq, xdp_prog, buf, xdp_headroom,
> +                                       len, xdp_xmit, stats);
> +               rcu_read_unlock();
> +               return skb;
>         }
>         rcu_read_unlock();
>
> @@ -1037,25 +1079,16 @@ static struct sk_buff *receive_small(struct net_device *dev,
>         skb = build_skb(buf, buflen);
>         if (!skb)
>                 goto err;
> -       skb_reserve(skb, headroom - delta);
> +       skb_reserve(skb, headroom);
>         skb_put(skb, len);
> -       if (!xdp_prog) {
> -               buf += header_offset;
> -               memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
> -       } /* keep zeroed vnet hdr since XDP is loaded */
> -
> -       if (metasize)
> -               skb_metadata_set(skb, metasize);
>
> +       buf += header_offset;
> +       memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);

So we end up with some code duplication between receive_small() and
receive_small_xdp() on building skbs. Is this intended?

For mergeable, we're probably fine since XDP_PASS doesn't need to
get_buf(). But for the path of small, I wonder if it's better to keep
the original code for building skb.

Thanks

>         return skb;
>
> -err_xdp:
> -       rcu_read_unlock();
> -       stats->xdp_drops++;
>  err:
>         stats->drops++;
>         put_page(page);
> -xdp_xmit:
>         return NULL;
>  }
>
> --
> 2.32.0.3.g01195cf9f
>

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