netdev - Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

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Message-ID: <CACAyw99S9v658UyiKz3ad4kja7rDNfYv+9VOXZHCUOtam_C8Wg@mail.gmail.com>
Date:   Wed, 29 Sep 2021 13:38:23 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        John Fastabend <john.fastabend@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        tirthendu.sarkar@...el.com
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

On Wed, 29 Sept 2021 at 13:10, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Lorenz Bauer <lmb@...udflare.com> writes:
>
> > On Thu, 16 Sept 2021 at 18:47, Jakub Kicinski <kuba@...nel.org> wrote:
> >>
> >> Won't applications end up building something like skb_header_pointer()
> >> based on bpf_xdp_adjust_data(), anyway? In which case why don't we
> >> provide them what they need?
> >>
> >> say:
> >>
> >> void *xdp_mb_pointer(struct xdp_buff *xdp_md, u32 flags,
> >>                      u32 offset, u32 len, void *stack_buf)
> >>
> >> flags and offset can be squashed into one u64 as needed. Helper returns
> >> pointer to packet data, either real one or stack_buf. Verifier has to
> >> be taught that the return value is NULL or a pointer which is safe with
> >> offsets up to @len.
> >>
> >> If the reason for access is write we'd also need:
> >>
> >> void *xdp_mb_pointer_flush(struct xdp_buff *xdp_md, u32 flags,
> >>                            u32 offset, u32 len, void *stack_buf)
> >
> > Yes! This would be so much better than bpf_skb_load/store_bytes(),
> > especially if we can use it for both XDP and skb contexts as stated
> > elsewhere in this thread.
>
> Alright. Let's see if we can go this route, then :)

Something I forgot to mention: you could infer that an XDP program is
mb-aware if it only does packet access via the helpers. Put another
way, it might be nice if ctx->data wasn't accessible in mb XDP. That
way I know that all packet access has to handle mb-aware ctx (think
pulling in functions via headers or even pre-compiled bpf libraries).

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com