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

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Date:   Thu, 5 Jan 2023 11:22:03 -0500
From:   Andy Gospodarek <andrew.gospodarek@...adcom.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        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>,
        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, Jan 03, 2023 at 05:21:53PM -0800, Jakub Kicinski wrote:
> 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.

As I mentioned in my mail just a few mins ago, I think this would be a good
model to consider.  All headers (including potentially tunnel headers) could be
in the linear area with the actual packet data in frags.

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