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

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Message-ID: <CAADnVQ+XXGUxzqMdbPMYf+t_ViDkqvGDdogrmv-wH-dckzujLw@mail.gmail.com>
Date:   Wed, 29 Sep 2021 11:54:46 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Lorenz Bauer <lmb@...udflare.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        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, Sep 29, 2021 at 5:38 AM Lorenz Bauer <lmb@...udflare.com> wrote:
>
> 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)

I'm missing something. Why do we need a separate flush() helper?
Can't we do:
char buf[64], *p;
p = xdp_mb_pointer(ctx, flags, off, len, buf);
read/write p[]
if (p == buf)
    xdp_store_bytes(ctx, off, buf, len, flags);