netdev - Re: [PATCH bpf-next] libbpf: fix AF_XDP helper program to support kernels without the JMP32 eBPF instruction class

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Message-ID: <e8253d51-1821-af17-ce07-7c4cbc28f15e@iogearbox.net>
Date:   Fri, 20 Dec 2019 21:22:45 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Alex Forster <aforster@...udflare.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: fix AF_XDP helper program to support
 kernels without the JMP32 eBPF instruction class

On 12/20/19 8:20 AM, Björn Töpel wrote:
> On 2019-12-19 23:29, Alex Forster wrote:
>>> I though af_xdp landed after jmp32 ?
>>
>> They were indeed pretty close together, but AF_XDP became usable in
>> late 2018 with either 4.18 or 4.19. JMP32 landed in early 2019 with
>> 5.1.
> 
> Correct, but is anyone really using AF_XDP pre-5.1?
> 
> The rationale for doing JMP32:
> https://lore.kernel.org/bpf/87v9sip0i8.fsf@toke.dk/
> 
> I think going forward, a route where different AF_XDP programs is loaded
> based on what currently running kernel supports. "Every s^Hcycle is
> sacred", and we're actually paying for the extra checks.
> 
> Then again, failing to load is still pretty bad. :-) Thanks for fixing this.

Could we simply just test availability of JMP32 in underlying kernel and use
it if available, if not, fall back to regular JMP. libbpf already has infra
for this, so xsk code could make use of it.

Thanks,
Daniel