Re: [PATCH net-next v6 08/10] virtio_net: xsk: rx: support recv small mode

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Fri, 28 Jun 2024 10:19:41 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Tue, Jun 18, 2024 at 3:57 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > In the process:
> > 1. We may need to copy data to create skb for XDP_PASS.
> > 2. We may need to call xsk_buff_free() to release the buffer.
> > 3. The handle for xdp_buff is difference from the buffer.
> >
> > If we pushed this logic into existing receive handle(merge and small),
> > we would have to maintain code scattered inside merge and small (and big).
> > So I think it is a good choice for us to put the xsk code into an
> > independent function.
>
> I think it's better to try to reuse the existing functions.
>
> More below:
>
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  drivers/net/virtio_net.c | 135 ++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 133 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 2ac5668a94ce..06608d696e2e 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -500,6 +500,10 @@ struct virtio_net_common_hdr {
> >  };
> >
> >  static void virtnet_sq_free_unused_buf(struct virtqueue *vq, void *buf);
> > +static int virtnet_xdp_handler(struct bpf_prog *xdp_prog, struct xdp_buff *xdp,
> > +                              struct net_device *dev,
> > +                              unsigned int *xdp_xmit,
> > +                              struct virtnet_rq_stats *stats);
> >
> >  static bool is_xdp_frame(void *ptr)
> >  {
> > @@ -1040,6 +1044,120 @@ static void sg_fill_dma(struct scatterlist *sg, dma_addr_t addr, u32 len)
> >         sg->length = len;
> >  }
> >
> > +static struct xdp_buff *buf_to_xdp(struct virtnet_info *vi,
> > +                                  struct receive_queue *rq, void *buf, u32 len)
> > +{
> > +       struct xdp_buff *xdp;
> > +       u32 bufsize;
> > +
> > +       xdp = (struct xdp_buff *)buf;
> > +
> > +       bufsize = xsk_pool_get_rx_frame_size(rq->xsk.pool) + vi->hdr_len;
> > +
> > +       if (unlikely(len > bufsize)) {
> > +               pr_debug("%s: rx error: len %u exceeds truesize %u\n",
> > +                        vi->dev->name, len, bufsize);
> > +               DEV_STATS_INC(vi->dev, rx_length_errors);
> > +               xsk_buff_free(xdp);
> > +               return NULL;
> > +       }
> > +
> > +       xsk_buff_set_size(xdp, len);
> > +       xsk_buff_dma_sync_for_cpu(xdp);
> > +
> > +       return xdp;
> > +}
> > +
> > +static struct sk_buff *xdp_construct_skb(struct receive_queue *rq,
> > +                                        struct xdp_buff *xdp)
> > +{
>
> So we have a similar caller which is receive_small_build_skb(). Any
> chance to reuse that?

receive_small_build_skb works with build_skb.

Here we need to copy the packet from the xsk buffer to the skb buffer.
So I do not think we can reuse it.


>
> > +       unsigned int metasize = xdp->data - xdp->data_meta;
> > +       struct sk_buff *skb;
> > +       unsigned int size;
> > +
> > +       size = xdp->data_end - xdp->data_hard_start;
> > +       skb = napi_alloc_skb(&rq->napi, size);
> > +       if (unlikely(!skb)) {
> > +               xsk_buff_free(xdp);
> > +               return NULL;
> > +       }
> > +
> > +       skb_reserve(skb, xdp->data_meta - xdp->data_hard_start);
> > +
> > +       size = xdp->data_end - xdp->data_meta;
> > +       memcpy(__skb_put(skb, size), xdp->data_meta, size);
> > +
> > +       if (metasize) {
> > +               __skb_pull(skb, metasize);
> > +               skb_metadata_set(skb, metasize);
> > +       }
> > +
> > +       xsk_buff_free(xdp);
> > +
> > +       return skb;
> > +}
> > +
> > +static struct sk_buff *virtnet_receive_xsk_small(struct net_device *dev, struct virtnet_info *vi,
> > +                                                struct receive_queue *rq, struct xdp_buff *xdp,
> > +                                                unsigned int *xdp_xmit,
> > +                                                struct virtnet_rq_stats *stats)
> > +{
> > +       struct bpf_prog *prog;
> > +       u32 ret;
> > +
> > +       ret = XDP_PASS;
> > +       rcu_read_lock();
> > +       prog = rcu_dereference(rq->xdp_prog);
> > +       if (prog)
> > +               ret = virtnet_xdp_handler(prog, xdp, dev, xdp_xmit, stats);
> > +       rcu_read_unlock();
> > +
> > +       switch (ret) {
> > +       case XDP_PASS:
> > +               return xdp_construct_skb(rq, xdp);
> > +
> > +       case XDP_TX:
> > +       case XDP_REDIRECT:
> > +               return NULL;
> > +
> > +       default:
> > +               /* drop packet */
> > +               xsk_buff_free(xdp);
> > +               u64_stats_inc(&stats->drops);
> > +               return NULL;
> > +       }
> > +}
> > +
> > +static struct sk_buff *virtnet_receive_xsk_buf(struct virtnet_info *vi, struct receive_queue *rq,
> > +                                              void *buf, u32 len,
> > +                                              unsigned int *xdp_xmit,
> > +                                              struct virtnet_rq_stats *stats)
> > +{
> > +       struct net_device *dev = vi->dev;
> > +       struct sk_buff *skb = NULL;
> > +       struct xdp_buff *xdp;
> > +
> > +       len -= vi->hdr_len;
> > +
> > +       u64_stats_add(&stats->bytes, len);
> > +
> > +       xdp = buf_to_xdp(vi, rq, buf, len);
> > +       if (!xdp)
> > +               return NULL;
> > +
> > +       if (unlikely(len < ETH_HLEN)) {
> > +               pr_debug("%s: short packet %i\n", dev->name, len);
> > +               DEV_STATS_INC(dev, rx_length_errors);
> > +               xsk_buff_free(xdp);
> > +               return NULL;
> > +       }
> > +
> > +       if (!vi->mergeable_rx_bufs)
> > +               skb = virtnet_receive_xsk_small(dev, vi, rq, xdp, xdp_xmit, stats);
> > +
> > +       return skb;
> > +}
> > +
> >  static int virtnet_add_recvbuf_xsk(struct virtnet_info *vi, struct receive_queue *rq,
> >                                    struct xsk_buff_pool *pool, gfp_t gfp)
> >  {
> > @@ -2363,9 +2481,22 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
> >         void *buf;
> >         int i;
> >
> > -       if (!vi->big_packets || vi->mergeable_rx_bufs) {
> > -               void *ctx;
> > +       if (rq->xsk.pool) {
> > +               struct sk_buff *skb;
> > +
> > +               while (packets < budget) {
> > +                       buf = virtqueue_get_buf(rq->vq, &len);
> > +                       if (!buf)
> > +                               break;
> >
> > +                       skb = virtnet_receive_xsk_buf(vi, rq, buf, len, xdp_xmit, &stats);
> > +                       if (skb)
> > +                               virtnet_receive_done(vi, rq, skb);
> > +
> > +                       packets++;
> > +               }
>
> If reusing turns out to be hard, I'd rather add new paths in receive_small().

The exist function is called after virtnet_rq_get_buf(), that will do dma unmap.
But for xsk, the dma unmap is not need. So xsk receive handle should use
virtqueue_get_buf directly.

Thanks.

>
> > +       } else if (!vi->big_packets || vi->mergeable_rx_bufs) {
> > +               void *ctx;
> >                 while (packets < budget &&
> >                        (buf = virtnet_rq_get_buf(rq, &len, &ctx))) {
> >                         receive_buf(vi, rq, buf, len, ctx, xdp_xmit, &stats);
> > --
> > 2.32.0.3.g01195cf9f
> >
>
> Thanks
>