netdev - Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through chain calls

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Date:   Mon, 7 Oct 2019 20:21:22 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Lorenz Bauer <lmb@...udflare.com>
CC:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "John Fastabend" <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single
 interface through chain calls

On 07/10/2019 18:12, Lorenz Bauer wrote:
> Sure, but this is the simplest, not necessarily realistic use case. There
> is a reason that libbpf has the API it has. For example, we patch our
> eBPF before loading it. I'm sure there are other complications, which is
> why I prefer to keep loading my own programs.
Any reason why you can't have the client patch the eBPF (possibly with
 libbpf) before supplying the patched object file to the loaderiser?

>> No, I'm talking about doing a linker step (using the 'full-blown calls'
>>  _within_ an eBPF program that Alexei added a few months back) before the
>>  program is submitted to the kernel.  So the BPF_CALL|BPF_PSEUDO_CALL insn
>>  gets JITed to a direct call.
> Ah, I see. I'm not sure whether this restriction has been lifted, but those
> calls are incompatible with tail calls. So we wouldn't be able to use this.
Indeed, tail calls don't fit into my scheme, because being a tail-call from
 the subprogram doesn't make you a tail-call from the dispatcher program.
But AIUI tail calls are only in use today in various work-arounds for the
 lack of proper linking (including dynamic linking).  If we supported that,
 would you still need them?

>> OK, but in that case xdpd isn't evidence that the "loader" approach doesn't
>>  work, so I still think it should be tried before we go to the lengths of
>>  pushing something into the kernel (that we then have to maintain forever).
> Maybe this came across the wrong way, I never said it is.
No, you didn't (sorry).  Toke somewhat implied it, which is what I was
 responding to there.

-Ed