Re: [PATCH v2 4/9] virtio_net: build xdp_buff with multi buffers

Jason Wang
2022-12-27T06:47:22Z

在 2022/12/20 22:14, Heng Qi 写道:
> Support xdp for multi buffer packets in mergeable mode.
>
> Putting the first buffer as the linear part for xdp_buff,
> and the rest of the buffers as non-linear fragments to struct
> skb_shared_info in the tailroom belonging to xdp_buff.
>
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> drivers/net/virtio_net.c | 78 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 78 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 08f209d7b0bf..8fc3b1841d92 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -931,6 +931,84 @@ static struct sk_buff *receive_big(struct net_device *dev,
> return NULL;
> }
>
> +/* TODO: build xdp in big mode */
> +static int virtnet_build_xdp_buff_mrg(struct net_device *dev,
> + struct virtnet_info *vi,
> + struct receive_queue *rq,
> + struct xdp_buff *xdp,
> + void *buf,
> + unsigned int len,
> + unsigned int frame_sz,
> + u16 *num_buf,
> + unsigned int *xdp_frags_truesize,
> + struct virtnet_rq_stats *stats)
> +{
> + unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
> + struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> + unsigned int truesize, headroom;
> + struct skb_shared_info *shinfo;
> + unsigned int xdp_frags_truesz = 0;
> + unsigned int cur_frag_size;
> + struct page *page;
> + skb_frag_t *frag;
> + int offset;
> + void *ctx;
> +
> + xdp_init_buff(xdp, frame_sz, &rq->xdp_rxq);
> + xdp_prepare_buff(xdp, buf - VIRTIO_XDP_HEADROOM,
> + VIRTIO_XDP_HEADROOM + vi->hdr_len, len - vi->hdr_len, true);
> +
> + if (*num_buf > 1) {
> + shinfo = xdp_get_shared_info_from_buff(xdp);
> + shinfo->nr_frags = 0;
> + shinfo->xdp_frags_size = 0;
> + }
> +
> + if ((*num_buf - 1) > MAX_SKB_FRAGS)
> + return -EINVAL;
> +
> + while ((--*num_buf) >= 1) {
> + buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
> + if (unlikely(!buf)) {
> + pr_debug("%s: rx error: %d buffers out of %d missing\n",
> + dev->name, *num_buf,
> + virtio16_to_cpu(vi->vdev, hdr->num_buffers));
> + dev->stats.rx_length_errors++;
> + return -EINVAL;
> + }
> +
> + if (!xdp_buff_has_frags(xdp))
> + xdp_buff_set_frags_flag(xdp);

Any reason to put this inside the loop?

> +
> + stats->bytes += len;
> + page = virt_to_head_page(buf);
> + offset = buf - page_address(page);
> + truesize = mergeable_ctx_to_truesize(ctx);
> + headroom = mergeable_ctx_to_headroom(ctx);
> +
> + cur_frag_size = truesize + (headroom ? (headroom + tailroom) : 0);
> + xdp_frags_truesz += cur_frag_size;

Not related to this patch, but it would easily confuse the future
readers that the we need another math for truesize. I think at least we
need some comments for this or

I guess the root cause is in get_mergeable_buf_len:

static unsigned int get_mergeable_buf_len(struct receive_queue *rq,
struct ewma_pkt_len *avg_pkt_len,
unsigned int room)
{
struct virtnet_info *vi = rq->vq->vdev->priv;
const size_t hdr_len = vi->hdr_len;
unsigned int len;

if (room)
return PAGE_SIZE - room;

And we do

len = get_mergeable_buf_len(rq, &rq->mrg_avg_pkt_len, room);

...

ctx = mergeable_len_to_ctx(len, headroom);

I wonder if it's better to pack the real truesize (PAGE_SIZE) here. This
may ease a lot of things.

Thanks

> + if (unlikely(len > truesize || cur_frag_size > PAGE_SIZE)) {
> + pr_debug("%s: rx error: len %u exceeds truesize %lu\n",
> + dev->name, len, (unsigned long)ctx);
> + dev->stats.rx_length_errors++;
> + return -EINVAL;
> + }
> +
> + frag = &shinfo->frags[shinfo->nr_frags++];
> + __skb_frag_set_page(frag, page);
> + skb_frag_off_set(frag, offset);
> + skb_frag_size_set(frag, len);
> + if (page_is_pfmemalloc(page))
> + xdp_buff_set_frag_pfmemalloc(xdp);
> +
> + shinfo->xdp_frags_size += len;
> + }
> +
> + *xdp_frags_truesize = xdp_frags_truesz;
> + return 0;
> +}
> +
> static struct sk_buff *receive_mergeable(struct net_device *dev,
> struct virtnet_info *vi,
> struct receive_queue *rq,