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

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: "Björn Töpel" <bjorn.topel@gmail.com>
Cc: Netdev <netdev@vger.kernel.org>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Björn Töpel" <bjorn.topel@intel.com>, bpf <bpf@vger.kernel.org>,
	"Magnus Karlsson" <magnus.karlsson@gmail.com>,
	"Karlsson, Magnus" <magnus.karlsson@intel.com>,
	"Jonathan Lemon" <jonathan.lemon@gmail.com>,
	"Edward Cree" <ecree@solarflare.com>,
	"Toke Høiland-Jørgensen" <thoiland@redhat.com>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Andrii Nakryiko" <andrii.nakryiko@gmail.com>
Subject: Re: [PATCH bpf-next v4 2/6] bpf: introduce BPF dispatcher
Date: Fri, 13 Dec 2019 09:18:43 -0800	[thread overview]
Message-ID: <20191213171841.kjrs3uhf6bgskgzq@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <CAJ+HfNgg6dp3om=nfA_vJ+SbtstC=e9dVSm=z0yRJjc372L+=w@mail.gmail.com>

On Fri, Dec 13, 2019 at 05:09:46PM +0100, Björn Töpel wrote:
> On Fri, 13 Dec 2019 at 17:03, Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Dec 13, 2019 at 7:59 AM Björn Töpel <bjorn.topel@gmail.com> wrote:
> > >
> > > On Fri, 13 Dec 2019 at 16:52, Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Fri, Dec 13, 2019 at 7:49 AM Björn Töpel <bjorn.topel@gmail.com> wrote:
> > > > >
> > > > > On Fri, 13 Dec 2019 at 16:04, Alexei Starovoitov
> > > > > <alexei.starovoitov@gmail.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?

ahh. got it. bpf_prog_run_xdp() is static inline in .h

Thank you for explaining!