Re: [PATCH v9 bpf-next 10/14] bpf: add multi-buffer support to xdp copy helpers

John Fastabend
2021-06-24T14:28:41Z

Eelco Chaudron wrote:
>
>
> On 23 Jun 2021, at 1:49, John Fastabend wrote:
>
> > Lorenzo Bianconi wrote:
> >> From: Eelco Chaudron <echaudro@redhat.com>
> >>
> >> This patch adds support for multi-buffer for the following helpers:
> >> - bpf_xdp_output()
> >> - bpf_perf_event_output()
> >>
> >> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
> >> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> >> ---
> >
> > Ah ok so at least xdp_output will work with all bytes. But this is
> > getting close to having access into the frags so I think doing
> > the last bit shouldn't be too hard?
>
>
> Guess you are talking about multi-buffer access in the XDP program?
>
> I did suggest an API a while back, https://lore.kernel.org/bpf/FD3E6E08-DE78-4FBA-96F6-646C93E88631@redhat.com/ but I had/have not time to work on it. Guess the difficult part is to convince the verifier to allow the data to be accessed.

Ah great I think we had the same idea I called it xdp_pull_data()
though.

Whats the complication though it looks like it can be done by simply
moving the data and data_end pointers around then marking them
invalidated. This way the verifier knows the program needs to
rewrite them. I can probably look more into next week.

From my first glance it looks relatively straight forward to do
now. I really would like to avoid yet another iteration of
programs features I have to discover and somehow work around
if we can get the helper into this series. If you really don't
have time I can probably take a look early next week on an
RFC for something like above helper.

.John