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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 20 Jan 2021 15:56:57 +0100

Björn Töpel <[email-masked]> writes:

> On 2021-01-20 13:52, Toke Høiland-Jørgensen wrote:
>> Björn Töpel <[email-masked]> writes:
>> 
>>> From: Björn Töpel <[email-masked]>
>>>
>>> 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