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

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Message-ID: <8735xwaxw0.fsf@toke.dk>
Date:   Tue, 16 Feb 2021 15:30:07 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Marek Majtyka <alardam@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Saeed Mahameed <saeed@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Maciej Fijalkowski <maciejromanfijalkowski@...il.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [PATCH v2 bpf 1/5] net: ethtool: add xdp properties flag set

Marek Majtyka <alardam@...il.com> writes:

> On Fri, Feb 12, 2021 at 3:05 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>>
>> On Thu, Feb 11, 2021 at 5:26 PM Jakub Kicinski <kuba@...nel.org> wrote:
>> >
>> > Perhaps I had seen one too many vendor incompatibility to trust that
>> > adding a driver API without a validation suite will result in something
>> > usable in production settings.
>>
>> I agree with Jakub. I don't see how extra ethtool reporting will help.
>> Anyone who wants to know whether eth0 supports XDP_REDIRECT can already do so:
>> ethtool -S eth0 | grep xdp_redirect
>
> Doing things right can never be treated as an addition. It is the
> other way around. Option -S is for statistics and additionally it can
> show something (AFAIR there wasn't such counter xdp_redirect, it must
> be something new, thanks for the info). But  nevertheless it cannot
> cover all needs IMO.
>
> Some questions worth to consider:
> Is this extra reporting function of statistics clearly documented in
> ethtool? Is this going to be clearly documented? Would it be easier
> for users/admins to find it?
> What about zero copy? Can it be available via statistics, too?
> What about drivers XDP transmit locking flag (latest request from Jesper)?


There is no way the statistics is enough. And saying "just grep for
xdp_redirect in ethtool -S" is bordering on active hostility towards
users.

We need drivers to export explicit features so we can do things like:

- Explicitly reject attaching a program that tries to do xdp_redirect on
  an interface that doesn't support it.

- Prevent devices that don't implement ndo_xdp_xmit() from being
  inserted into a devmap (oh, and this is on thing you can't know at all
  from the statistics, BTW).

- Expose the features in a machine-readable format (like the ethtool
  flags in your patch) so applications can discover in a reliable way
  what is available and do proper fallback if features are missing.

I can accept that we need some kind of conformance test to define what
each flag means (which would be kinda like a selftest for the feature
flags), but we definitely need the feature flags themselves!

-Toke