netdev - Re: [PATCH bpf-next v4 2/6] bpf: introduce BPF dispatcher

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Message-ID: <CAJ+HfNgg6dp3om=nfA_vJ+SbtstC=e9dVSm=z0yRJjc372L+=w@mail.gmail.com>
Date:   Fri, 13 Dec 2019 17:09:46 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Edward Cree <ecree@...arflare.com>,
        Toke Høiland-Jørgensen <thoiland@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH bpf-next v4 2/6] bpf: introduce BPF dispatcher

On Fri, 13 Dec 2019 at 17:03, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Dec 13, 2019 at 7:59 AM Björn Töpel <bjorn.topel@...il.com> wrote:
> >
> > On Fri, 13 Dec 2019 at 16:52, Alexei Starovoitov
> > <alexei.starovoitov@...il.com> wrote:
> > >
> > > On Fri, Dec 13, 2019 at 7:49 AM Björn Töpel <bjorn.topel@...il.com> wrote:
> > > >
> > > > On Fri, 13 Dec 2019 at 16:04, Alexei Starovoitov
> > > > <alexei.starovoitov@...il.com> wrote:
> > > > >
> > > > > On Fri, Dec 13, 2019 at 08:51:47AM +0100, Björn Töpel wrote:
> > > > > >
> > > > > > > I hope my guess that compiler didn't inline it is correct. Then extra noinline
> > > > > > > will not hurt and that's the only thing needed to avoid the issue.
> > > > > > >
> > > > > >
> > > > > > I'd say it's broken not marking it as noinline, and I was lucky. It
> > > > > > would break if other BPF entrypoints that are being called from
> > > > > > filter.o would appear. I'll wait for more comments, and respin a v5
> > > > > > after the weekend.
> > > > >
> > > > > Also noticed that EXPORT_SYMBOL for dispatch function is not necessary atm.
> > > > > Please drop it. It can be added later when need arises.
> > > > >
> > > >
> > > > It's needed for module builds, so I cannot drop it!
> > >
> > > Not following. Which module it's used out of?
> >
> > The trampoline is referenced from bpf_prog_run_xdp(), which is
> > inlined. Without EXPORT, the symbol is not visible for the module. So,
> > if, say i40e, is built as a module, you'll get a linker error.
>
> I'm still not following. i40e is not using this dispatch logic any more.

Hmm, *all* XDP users uses it, but indirectly via bpf_prog_run_xdp().
All drivers that execute an XDP program, does that via the
bpf_prog_run_xdp(), say, i40e_txrx.c and i40e_xsk.c.
bpf_prog_run_xdp() is inlined and expaned to __BPF_PROG_RUN(), which
calls into the dispatcher trampoline.

$ nm drivers/net/ethernet/intel/i40e/i40e_xsk.o|grep xdpfunc
                 U bpf_dispatcher_xdpfunc

Makes sense?