On Tue, Nov 29, 2016 at 12:11:33PM -0800, John Fastabend wrote:
> virtio_net XDP support expects receive buffers to be contiguous.
> If this is not the case we enable a slowpath to allow connectivity
> to continue but at a significan performance overhead associated with
> linearizing data. To make it painfully aware to users that XDP is
> running in a degraded mode we throw an xdp buffer error.
>
> To linearize packets we allocate a page and copy the segments of
> the data, including the header, into it. After this the page can be
> handled by XDP code flow as normal.
>
> Then depending on the return code the page is either freed or sent
> to the XDP xmit path. There is no attempt to optimize this path.
>
> Signed-off-by: John Fastabend <john.r.fastab...@intel.com>
...
> +/* The conditions to enable XDP should preclude the underlying device from
> + * sending packets across multiple buffers (num_buf > 1). However per spec
> + * it does not appear to be illegal to do so but rather just against
> convention.
> + * So in order to avoid making a system unresponsive the packets are pushed
> + * into a page and the XDP program is run. This will be extremely slow and we
> + * push a warning to the user to fix this as soon as possible. Fixing this
> may
> + * require resolving the underlying hardware to determine why multiple
> buffers
> + * are being received or simply loading the XDP program in the ingress stack
> + * after the skb is built because there is no advantage to running it here
> + * anymore.
> + */
...
> if (num_buf > 1) {
> bpf_warn_invalid_xdp_buffer();
> - goto err_xdp;
> +
> + /* linearize data for XDP */
> + xdp_page = xdp_linearize_page(rq, num_buf,
> + page, offset, &len);
> + if (!xdp_page)
> + goto err_xdp;
in case when we're 'lucky' the performance will silently be bad.
Can we do warn_once here? so at least something in dmesg points out
that performance is not as expected. Am I reading it correctly that
you had to do a special kernel hack to trigger this situation and
in all normal cases it's not the case?
