netdev - Re: [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program based on kernel version

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Date:   Wed, 20 Jan 2021 15:56:57 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com,
        Marek Majtyka <alardam@...il.com>
Subject: Re: [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program
 based on kernel version

Björn Töpel <bjorn.topel@...el.com> writes:

> On 2021-01-20 13:52, Toke Høiland-Jørgensen wrote:
>> Björn Töpel <bjorn.topel@...il.com> writes:
>> 
>>> From: Björn Töpel <bjorn.topel@...el.com>
>>>
>>> Add detection for kernel version, and adapt the BPF program based on
>>> kernel support. This way, users will get the best possible performance
>>> from the BPF program.
>> 
>> Please do explicit feature detection instead of relying on the kernel
>> version number; some distro kernels are known to have a creative notion
>> of their own version, which is not really related to the features they
>> actually support (I'm sure you know which one I'm referring to ;)).
>>
>
> Right. For a *new* helper, like bpf_redirect_xsk, we rely on rejection
> from the verifier to detect support. What about "bpf_redirect_map() now
> supports passing return value as flags"? Any ideas how to do that in a
> robust, non-version number-based scheme?

Well, having a BPF program pass in a flag of '1' with an invalid lookup
and checking if it returns 1 or 0. But how to do that from libbpf, hmm,
good question. BPF_PROG_RUN()?

An alternative could be to default to a program that will handle both
cases in the BPF code, and make it opt-in to use the optimised versions
if the user knows their kernel supports it?

-Toke