Re: [RFC PATCH 1/9] virtio_net: disable the hole mechanism for xdp

Jason Wang
2022-12-06T05:22:06Z

On Tue, Nov 22, 2022 at 3:44 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
>
> XDP core assumes that the frame_size of xdp_buff and the length of
> the frag are PAGE_SIZE. But before xdp is set, the length of the prefilled
> buffer may exceed PAGE_SIZE, which may cause the processing of xdp to fail,
> so we disable the hole mechanism when xdp is loaded.
>
> Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
> drivers/net/virtio_net.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9cce7dec7366..c5046d21b281 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1419,8 +1419,11 @@ static int add_recvbuf_mergeable(struct virtnet_info *vi,
> /* To avoid internal fragmentation, if there is very likely not
> * enough space for another buffer, add the remaining space to
> * the current buffer.
> + * XDP core assumes that frame_size of xdp_buff and the length
> + * of the frag are PAGE_SIZE, so we disable the hole mechanism.
> */
> - len += hole;
> + if (!vi->xdp_enabled)

How is this synchronized with virtnet_xdp_set()?

I think we need to use headroom here since it did:

static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
{
return vi->xdp_enabled ? VIRTIO_XDP_HEADROOM : 0;
}

Otherwise xdp_enabled could be re-read which may lead bugs.

Thanks

> + len += hole;
> alloc_frag->offset += hole;
> }
>
> --
> 2.19.1.6.gb485710b
>