Re: [PATCH v2 5/9] virtio_net: construct multi-buffer xdp in mergeable

[Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>]

在 2022/12/27 下午3:01, Jason Wang 写道:
> 在 2022/12/20 22:14, Heng Qi 写道:
> > Build multi-buffer xdp using virtnet_build_xdp_buff_mrg().
> >
> > For the prefilled buffer before xdp is set, we will probably use
> > vq reset in the future. At the same time, virtio net currently
> > uses comp pages, and bpf_xdp_frags_increase_tail() needs to calculate
> > the tailroom of the last frag, which will involve the offset of the
> > corresponding page and cause a negative value, so we disable tail
> > increase by not setting xdp_rxq->frag_size.
> >
> > Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/net/virtio_net.c | 60 +++++++++++++++++++++++++++++-----------
> >   1 file changed, 44 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 8fc3b1841d92..40bc58fa57f5 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1018,6 +1018,7 @@ static struct sk_buff *receive_mergeable(struct 
> > net_device *dev,
> >                        unsigned int *xdp_xmit,
> >                        struct virtnet_rq_stats *stats)
> >   {
> > +    unsigned int tailroom = SKB_DATA_ALIGN(sizeof(struct 
> > skb_shared_info));
> >       struct virtio_net_hdr_mrg_rxbuf *hdr = buf;
> >       u16 num_buf = virtio16_to_cpu(vi->vdev, hdr->num_buffers);
> >       struct page *page = virt_to_head_page(buf);
> > @@ -1048,11 +1049,14 @@ static struct sk_buff 
> > *receive_mergeable(struct net_device *dev,
> >       rcu_read_lock();
> >       xdp_prog = rcu_dereference(rq->xdp_prog);
> >       if (xdp_prog) {
> > +        unsigned int xdp_frags_truesz = 0;
> > +        struct skb_shared_info *shinfo;
> >           struct xdp_frame *xdpf;
> >           struct page *xdp_page;
> >           struct xdp_buff xdp;
> >           void *data;
> >           u32 act;
> > +        int i;
> >             /* Transient failure which in theory could occur if
> >            * in-flight packets from before XDP was enabled reach
> > @@ -1061,19 +1065,23 @@ 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()
> > +        /* Now XDP core assumes frag size is PAGE_SIZE, but buffers
> > +         * with headroom may add hole in truesize, which
> > +         * make their length exceed PAGE_SIZE. So we disabled the
> > +         * hole mechanism for xdp. See add_recvbuf_mergeable().
> >            */
> >           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
> > -         * happen for the first several packets, so we don't
> > -         * care much about its performance.
> > +        /* This happens when headroom is not enough because
> > +         * of the buffer was prefilled before XDP is set.
> > +         * This should only happen for the first several packets.
> > +         * In fact, vq reset can be used here to help us clean up
> > +         * the prefilled buffers, but many existing devices do not
> > +         * support it, and we don't want to bother users who are
> > +         * using xdp normally.
> >            */
> > -        if (unlikely(num_buf > 1 ||
> > -                 headroom < virtnet_get_headroom(vi))) {
> > +        if (!xdp_prog->aux->xdp_has_frags &&
> > +            (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {
> >               /* linearize data for XDP */
> >               xdp_page = xdp_linearize_page(rq, &num_buf,
> >                                 page, offset,
> > @@ -1084,17 +1092,26 @@ static struct sk_buff 
> > *receive_mergeable(struct net_device *dev,
> >               if (!xdp_page)
> >                   goto err_xdp;
> >               offset = VIRTIO_XDP_HEADROOM;
> > +        } else if (unlikely(headroom < virtnet_get_headroom(vi))) {
>
>
> I believe we need to check xdp_prog->aux->xdp_has_frags at least since 
> this may not work if it needs more than one frags?

Sorry Jason, I didn't understand you, I'll try to answer. For 
multi-buffer xdp programs, if the first buffer is a pre-filled buffer 
(no headroom),
we need to copy it out and use the subsequent buffers of this packet as 
its frags (this is done in virtnet_build_xdp_buff_mrg()), therefore,
it seems that there is no need to check 'xdp_prog->aux->xdp_has_frags' 
to mark multi-buffer xdp (of course I can add it),

+ } else if (unlikely(headroom < virtnet_get_headroom(vi))) {

Because the linearization of single-buffer xdp has all been done before, 
the subsequent situation can only be applied to multi-buffer xdp:
+ if (!xdp_prog->aux->xdp_has_frags &&
+ (num_buf > 1 || headroom < virtnet_get_headroom(vi))) {

> Btw, I don't see a reason why we can't reuse xdp_linearize_page(), (we 
> probably don't need error is the buffer exceeds PAGE_SIZE).

For multi-buffer xdp, we only need to copy out the pre-filled first 
buffer, and use the remaining buffers of this packet as frags in 
virtnet_build_xdp_buff_mrg().

Thanks.

> Other looks good.
>
> Thanks
>
>
> > +            if ((VIRTIO_XDP_HEADROOM + len + tailroom) > PAGE_SIZE)
> > +                goto err_xdp;
> > +
> > +            xdp_page = alloc_page(GFP_ATOMIC);
> > +            if (!xdp_page)
> > +                goto err_xdp;
> > +
> > +            memcpy(page_address(xdp_page) + VIRTIO_XDP_HEADROOM,
> > +                   page_address(page) + offset, len);
> > +            frame_sz = PAGE_SIZE;
> > +            offset = VIRTIO_XDP_HEADROOM;
> >           } else {
> >               xdp_page = page;
> >           }
> > -
> > -        /* Allow consuming headroom but reserve enough space to push
> > -         * the descriptor on if we get an XDP_TX return code.
> > -         */
> >           data = page_address(xdp_page) + offset;
> > -        xdp_init_buff(&xdp, frame_sz - vi->hdr_len, &rq->xdp_rxq);
> > -        xdp_prepare_buff(&xdp, data - VIRTIO_XDP_HEADROOM + 
> > vi->hdr_len,
> > -                 VIRTIO_XDP_HEADROOM, len - vi->hdr_len, true);
> > +        err = virtnet_build_xdp_buff_mrg(dev, vi, rq, &xdp, data, 
> > len, frame_sz,
> > +                         &num_buf, &xdp_frags_truesz, stats);
> > +        if (unlikely(err))
> > +            goto err_xdp_frags;
> >             act = bpf_prog_run_xdp(xdp_prog, &xdp);
> >           stats->xdp_packets++;
> > @@ -1190,6 +1207,17 @@ static struct sk_buff 
> > *receive_mergeable(struct net_device *dev,
> >                   __free_pages(xdp_page, 0);
> >               goto err_xdp;
> >           }
> > +err_xdp_frags:
> > +        shinfo = xdp_get_shared_info_from_buff(&xdp);
> > +
> > +        if (unlikely(xdp_page != page))
> > +            __free_pages(xdp_page, 0);
> > +
> > +        for (i = 0; i < shinfo->nr_frags; i++) {
> > +            xdp_page = skb_frag_page(&shinfo->frags[i]);
> > +            put_page(xdp_page);
> > +        }
> > +        goto err_xdp;
> >       }
> >       rcu_read_unlock();