netdev - Re: [PATCH v2 bpf 1/5] net: ethtool: add xdp properties flag set

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Message-ID: <eb305a4f-c189-6b32-f718-6e709ef0fa55@iogearbox.net>
Date:   Fri, 4 Dec 2020 23:19:55 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     alardam@...il.com, magnus.karlsson@...el.com,
        bjorn.topel@...el.com, andrii.nakryiko@...il.com, kuba@...nel.org,
        ast@...nel.org, netdev@...r.kernel.org, davem@...emloft.net,
        john.fastabend@...il.com, hawk@...nel.org,
        jonathan.lemon@...il.com, bpf@...r.kernel.org,
        jeffrey.t.kirsher@...el.com, maciejromanfijalkowski@...il.com,
        intel-wired-lan@...ts.osuosl.org,
        Marek Majtyka <marekx.majtyka@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v2 bpf 1/5] net: ethtool: add xdp properties flag set

On 12/4/20 6:20 PM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <daniel@...earbox.net> writes:
[...]
>> We tried to standardize on a minimum guaranteed amount, but unfortunately not
>> everyone seems to implement it, but I think it would be very useful to query
>> this from application side, for example, consider that an app inserts a BPF
>> prog at XDP doing custom encap shortly before XDP_TX so it would be useful to
>> know which of the different encaps it implements are realistically possible on
>> the underlying XDP supported dev.
> 
> How many distinct values are there in reality? Enough to express this in
> a few flags (XDP_HEADROOM_128, XDP_HEADROOM_192, etc?), or does it need
> an additional field to get the exact value? If we implement the latter
> we also run the risk of people actually implementing all sorts of weird
> values, whereas if we constrain it to a few distinct values it's easier
> to push back against adding new values (as it'll be obvious from the
> addition of new flags).

It's not everywhere straight forward to determine unfortunately, see also [0,1]
as some data points where Jesper looked into in the past, so in some cases it
might differ depending on the build/runtime config..

   [0] https://lore.kernel.org/bpf/158945314698.97035.5286827951225578467.stgit@firesoul/
   [1] https://lore.kernel.org/bpf/158945346494.97035.12809400414566061815.stgit@firesoul/