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

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Message-ID: <Y73Ry+nNqOkeZtaj@dragonfly.lan>
Date:   Tue, 10 Jan 2023 22:59:55 +0200
From:   Maxim Mikityanskiy <maxtram95@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>,
        Eelco Chaudron <echaudro@...hat.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tariq Toukan <tariqt@...dia.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.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>
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.
> 
> A random datapoint is that bpf_xdp_adjust_head() seems 
> to enforce that there is at least ETH_HLEN.

Also bpf_xdp_frags_increase_tail has the following check:

	if (!rxq->frag_size || rxq->frag_size > xdp->frame_sz)
		return -EOPNOTSUPP;

However, I can't seem to find where the `frag_size > frame_sz` part is
actually used. Maybe this condition can be dropped? Can someone shed
some light?

BTW, Tariq, we seem to have missed setting frag_size to a non-zero
value. Could you check that increasing the tail indeed doesn't work on
fragmented packets on mlx5e? I can send a oneliner to fix that.