netdev - Re: Redux: Backwards compatibility for XDP multi-buff

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Message-ID: <20210921155443.507a8479@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 21 Sep 2021 15:54:43 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Lorenz Bauer <lmb@...udflare.com>,
        Lorenzo Bianconi <lbianconi@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: Redux: Backwards compatibility for XDP multi-buff

On Tue, 21 Sep 2021 18:06:35 +0200 Toke Høiland-Jørgensen wrote:
> 1. Do nothing. This would make it up to users / sysadmins to avoid
>    anything breaking by manually making sure to not enable multi-buffer
>    support while loading any XDP programs that will malfunction if
>    presented with an mb frame. This will probably break in interesting
>    ways, but it's nice and simple from an implementation PoV. With this
>    we don't need the declaration discussed above either.
> 
> 2. Add a check at runtime and drop the frames if they are mb-enabled and
>    the program doesn't understand it. This is relatively simple to
>    implement, but it also makes for difficult-to-understand issues (why
>    are my packets suddenly being dropped?), and it will incur runtime
>    overhead.
> 
> 3. Reject loading of programs that are not MB-aware when running in an
>    MB-enabled mode. This would make things break in more obvious ways,
>    and still allow a userspace loader to declare a program "MB-aware" to
>    force it to run if necessary. The problem then becomes at what level
>    to block this?
> 
>    Doing this at the driver level is not enough: while a particular
>    driver knows if it's running in multi-buff mode, we can't know for
>    sure if a particular XDP program is multi-buff aware at attach time:
>    it could be tail-calling other programs, or redirecting packets to
>    another interface where it will be processed by a non-MB aware
>    program.
> 
>    So another option is to make it a global toggle: e.g., create a new
>    sysctl to enable multi-buffer. If this is set, reject loading any XDP
>    program that doesn't support multi-buffer mode, and if it's unset,
>    disable multi-buffer mode in all drivers. This will make it explicit
>    when the multi-buffer mode is used, and prevent any accidental subtle
>    malfunction of existing XDP programs. The drawback is that it's a
>    mode switch, so more configuration complexity.

4. Add new program type, XDP_MB. Do not allow mixing of XDP vs XDP_MB
   thru tail calls.

IMHO that's very simple and covers majority of use cases.