netdev - Re: [PATCH net-next v2] samples/bpf: fixup some tools to be able to support xdp multibuffer

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Message-ID: <20230103172153.58f231ba@kernel.org>
Date:   Tue, 3 Jan 2023 17:21:53 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Tariq Toukan <ttoukan.linux@...il.com>,
        Andy Gospodarek <andrew.gospodarek@...adcom.com>,
        ast@...nel.org, daniel@...earbox.net, davem@...emloft.net,
        hawk@...nel.org, john.fastabend@...il.com, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        kpsingh@...nel.org, lorenzo.bianconi@...hat.com,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Andy Gospodarek <gospo@...adcom.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, gal@...dia.com,
        Saeed Mahameed <saeedm@...dia.com>, tariqt@...dia.com
Subject: Re: [PATCH net-next v2] samples/bpf: fixup some tools to be able to
 support xdp multibuffer

On Tue, 03 Jan 2023 16:19:49 +0100 Toke Høiland-Jørgensen wrote:
> Hmm, good question! I don't think we've ever explicitly documented any
> assumptions one way or the other. My own mental model has certainly
> always assumed the first frag would continue to be the same size as in
> non-multi-buf packets.

Interesting! :) My mental model was closer to GRO by frags 
so the linear part would have no data, just headers.

A random datapoint is that bpf_xdp_adjust_head() seems 
to enforce that there is at least ETH_HLEN.