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

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Message-ID: <YNsezApfos+47EZr@lore-desk>
Date:   Tue, 29 Jun 2021 15:23:24 +0200
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Eelco Chaudron <echaudro@...hat.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, 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

> > 
> > 
> > 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.

cool, thx :)
What about discussing APIs during the BPF mtg upstream on Thursday (probably
not next one since most of the people will be in PTO)? I will work on some docs.

Regards,
Lorenzo

> 
> 
> .John
> 

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