Re: [PATCH v2 6/9] virtio_net: transmit the multi-buffer xdp

[Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>]

在 2022/12/27 下午3:12, Jason Wang 写道:
> 在 2022/12/20 22:14, Heng Qi 写道:
> > This serves as the basis for XDP_TX and XDP_REDIRECT
> > to send a multi-buffer xdp_frame.
> >
> > Signed-off-by: Heng Qi <hengqi@xxxxxxxxxxxxxxxxx>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@xxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/net/virtio_net.c | 27 ++++++++++++++++++++++-----
> >   1 file changed, 22 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 40bc58fa57f5..9f31bfa7f9a6 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -563,22 +563,39 @@ static int __virtnet_xdp_xmit_one(struct 
> > virtnet_info *vi,
> >                      struct xdp_frame *xdpf)
> >   {
> >       struct virtio_net_hdr_mrg_rxbuf *hdr;
> > -    int err;
> > +    struct skb_shared_info *shinfo;
> > +    u8 nr_frags = 0;
> > +    int err, i;
> >         if (unlikely(xdpf->headroom < vi->hdr_len))
> >           return -EOVERFLOW;
> >   -    /* Make room for virtqueue hdr (also change xdpf->headroom?) */
> > +    if (unlikely(xdp_frame_has_frags(xdpf))) {
> > +        shinfo = xdp_get_shared_info_from_frame(xdpf);
> > +        nr_frags = shinfo->nr_frags;
> > +    }
> > +
> > +    /* Need to adjust this to calculate the correct postion
> > +     * for shinfo of the xdpf.
> > +     */
> > +    xdpf->headroom -= vi->hdr_len;
>
>
> Any reason we need to do this here? (Or if it is, is it only needed 
> for multibuffer XDP?)

Going back to its wrapping function virtnet_xdp_xmit(), we need to free 
up the pending old buffers.
If the "is_xdp_frame(ptr)" condition is met, then we need to calculate 
the position of skb_shared_info
in xdp_get_frame_len() and xdp_return_frame(), which will involve to 
xdpf->data and xdpf->headroom.
Therefore, we need to update the value of headroom synchronously here.

Also, it's not necessary for single-buffer xdp, but we need to keep it 
because it's harmless and as it should be.

Thanks.

> Other looks good.
>
> Thanks
>
>
> >       xdpf->data -= vi->hdr_len;
> >       /* Zero header and leave csum up to XDP layers */
> >       hdr = xdpf->data;
> >       memset(hdr, 0, vi->hdr_len);
> >       xdpf->len   += vi->hdr_len;
> >   -    sg_init_one(sq->sg, xdpf->data, xdpf->len);
> > +    sg_init_table(sq->sg, nr_frags + 1);
> > +    sg_set_buf(sq->sg, xdpf->data, xdpf->len);
> > +    for (i = 0; i < nr_frags; i++) {
> > +        skb_frag_t *frag = &shinfo->frags[i];
> > +
> > +        sg_set_page(&sq->sg[i + 1], skb_frag_page(frag),
> > +                skb_frag_size(frag), skb_frag_off(frag));
> > +    }
> >   -    err = virtqueue_add_outbuf(sq->vq, sq->sg, 1, xdp_to_ptr(xdpf),
> > -                   GFP_ATOMIC);
> > +    err = virtqueue_add_outbuf(sq->vq, sq->sg, nr_frags + 1,
> > +                   xdp_to_ptr(xdpf), GFP_ATOMIC);
> >       if (unlikely(err))
> >           return -ENOSPC; /* Caller handle free/refcnt */