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

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Date:   Wed, 22 Sep 2021 22:02:58 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
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

Jakub Kicinski <kuba@...nel.org> writes:

> 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.

Using the program type (or maybe the expected_attach_type) was how I was
imagining we'd encode the "I am MB aware" flag, yes. I hadn't actually
considered that this could be used to also restrict tail call/freplace
attachment, but that's a good point. So this leaves just the redirect
issue, then, see my other reply.

-Toke