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

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Message-ID: <60d49690a87ae_2e84a2082c@john-XPS-13-9370.notmuch>
Date:   Thu, 24 Jun 2021 07:28:32 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Eelco Chaudron <echaudro@...hat.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, lorenzo.bianconi@...hat.com,
        davem@...emloft.net, kuba@...nel.org, ast@...nel.org,
        daniel@...earbox.net, shayagr@...zon.com, sameehj@...zon.com,
        dsahern@...nel.org, brouer@...hat.com, jasowang@...hat.com,
        alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v9 bpf-next 10/14] bpf: add multi-buffer support to xdp
 copy helpers

Eelco Chaudron wrote:
> 
> 
> On 23 Jun 2021, at 1:49, John Fastabend wrote:
> 
> > Lorenzo Bianconi wrote:
> >> From: Eelco Chaudron <echaudro@...hat.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@...hat.com>
> >> Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.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