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

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Date:   Wed, 29 Sep 2021 14:10:20 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Lorenz Bauer <lmb@...udflare.com>, Jakub Kicinski <kuba@...nel.org>
Cc:     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

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 :)

-Toke