On Tue, Dec 06, 2016 at 09:35:31AM -0800, John Fastabend wrote:
> On 16-12-03 07:17 PM, Martin KaFai Lau wrote:
> > This patch allows XDP prog to extend/remove the packet
> > data at the head (like adding or removing header). It is
> > done by adding a new XDP helper bpf_xdp_adjust_head().
> >
> > It also renames bpf_helper_changes_skb_data() to
> > bpf_helper_changes_pkt_data() to better reflect
> > that XDP prog does not work on skb.
> >
> > Acked-by: Alexei Starovoitov <a...@kernel.org>
> > Signed-off-by: Martin KaFai Lau <ka...@fb.com>
> > ---
>
>
> [...]
>
> >
> > -bool bpf_helper_changes_skb_data(void *func)
> > +BPF_CALL_2(bpf_xdp_adjust_head, struct xdp_buff *, xdp, int, offset)
> > +{
> > + /* Both mlx4 and mlx5 driver align each packet to PAGE_SIZE when
> > + * XDP prog is set.
> > + * If the above is not true for the other drivers to support
> > + * bpf_xdp_adjust_head, struct xdp_buff can be extended.
> > + */
> > + unsigned long addr = (unsigned long)xdp->data & PAGE_MASK;
> > + void *data_hard_start = (void *)addr;
> > + void *data = xdp->data + offset;
> > +
> > + if (unlikely(data < data_hard_start || data > xdp->data_end - ETH_HLEN))
> > + return -EINVAL;
> > +
> > + xdp->data = data;
> > +
> > + return 0;
> > +}
> > +
>
> Sorry for the delay but I don't like the assumptions being made here
> with regards to page alignment and free space.
>
> Instead of taking the offset from PAGE_MASK how about adding a pointer
> to xdp_buff so that we get,
>
> struct xdp_buff {
> void *data;
> void *data_end;
> void *data_start;
> };
>
> This gives the headroom explicitly in the data structure. This way we
> can handle non-paged aligned usages and also some of the drivers are
> putting metadata (descriptors) at the front of the page. With this
> patch we could stomp on that metadata with the above we avoid that
> problem altogether.
Extending the xdp_buff like this was my first local attempt. And
then I realized the assumption that mlx4/5 is making. Since
there is no immediate need for this patch series and the xdp_buff
can always be extended later if needed without breaking
the xdp prog, I dropped the xdp_buff addition for now in this patch.
Sure, it will be done at the next spin.
