Re: [RFC PATCH 6/9] virtio_net: construct multi-buffer xdp in mergeable

Jason Wang
2022-12-16T03:47:20Z

On Wed, Dec 14, 2022 at 4:38 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
> On Tue, Dec 13, 2022 at 03:08:46PM +0800, Jason Wang wrote:
> > On Thu, Dec 8, 2022 at 4:30 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > >
> > >
> > >
> > > 在 2022/12/6 下午2:33, Jason Wang 写道:
> > > > On Tue, Nov 22, 2022 at 3:44 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > >> Build multi-buffer xdp using virtnet_build_xdp_buff() in mergeable.
> > > >>
> > > >> For the prefilled buffer before xdp is set, vq reset can be
> > > >> used to clear it, but most devices do not support it at present.
> > > >> In order not to bother users who are using xdp normally, we do
> > > >> not use vq reset for the time being.
> > > > I guess to tweak the part to say we will probably use vq reset in the future.
> > >
> > > OK, it works.
> > >
> > > >
> > > >> 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@linux.alibaba.com>
> > > >> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > >> ---
> > > >> drivers/net/virtio_net.c | 67 +++++++++++++++++++++++-----------------
> > > >> 1 file changed, 38 insertions(+), 29 deletions(-)
> > > >>
> > > >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > >> index 20784b1d8236..83e6933ae62b 100644
> > > >> --- a/drivers/net/virtio_net.c
> > > >> +++ b/drivers/net/virtio_net.c
> > > >> @@ -994,6 +994,7 @@ 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);
> > > >> @@ -1024,53 +1025,50 @@ 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
> > > >> - * the receive path after XDP is loaded.
> > > >> - */
> > > >> - if (unlikely(hdr->hdr.gso_type))
> > > >> - goto err_xdp;
> > > > Two questions:
> > > >
> > > > 1) should we keep this check for the XDP program that can't deal with XDP frags?
> > >
> > > Yes, the problem is the same as the xdp program without xdp.frags when
> > > GRO_HW, I will correct it.
> > >
> > > > 2) how could we guarantee that the vnet header (gso_type/csum_start
> > > > etc) is still valid after XDP (where XDP program can choose to
> > > > override the header)?
> > >
> > > We can save the vnet headr before the driver receives the packet and
> > > build xdp_buff, and then use
> > > the pre-saved value in the subsequent process.
> >
> > The problem is that XDP may modify the packet (header) so some fields
> > are not valid any more (e.g csum_start/offset ?).
> >
> > If I was not wrong, there's no way for the XDP program to access those
> > fields or does it support it right now?
> >
>
> When guest_csum feature is negotiated, xdp cannot be set, because the metadata
> of xdp_{buff, frame} may be adjusted by the bpf program, therefore,
> csum_{start, offset} itself is invalid. And at the same time,
> multi-buffer xdp programs should only Receive packets over larger MTU, so
> we don't need gso related information anymore and need to disable GRO_HW.

Ok, that's fine.

(But it requires a large pMTU).

Thanks

>
> Thanks.
>
> > >
> > > >> -
> > > >> - /* 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))) {
> > > >> - /* linearize data for XDP */
> > > >> - xdp_page = xdp_linearize_page(rq, &num_buf,
> > > >> - page, offset,
> > > >> - VIRTIO_XDP_HEADROOM,
> > > >> - &len);
> > > >> - frame_sz = PAGE_SIZE;
> > > >> + if (unlikely(headroom < virtnet_get_headroom(vi))) {
> > > >> + 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;
> > > > How can we know a single page is sufficient here? (before XDP is set,
> > > > we reserve neither headroom nor tailroom).
> > >
> > > This is only for the first buffer, refer to add_recvbuf_mergeable() and
> > > get_mergeable_buf_len() A buffer is always no larger than a page.
> >
> > Ok.
> >
> > Thanks
> >
> > >
> > > >
> > > >> offset = VIRTIO_XDP_HEADROOM;
> > > > I think we should still try to do linearization for the XDP program
> > > > that doesn't support XDP frags.
> > >
> > > Yes, you are right.
> > >
> > > Thanks.
> > >
> > > >
> > > > Thanks
> > > >
> > > >> } 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(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++;
> > > >> @@ -1164,6 +1162,17 @@ 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();
> > > >>
> > > >> --
> > > >> 2.19.1.6.gb485710b
> > > >>
> > >
>