netdev - Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from AF

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Message-ID: <CAJ+HfNgGTL-P-Qe5zOh=s0RBRMJGx0NXDLTj7DAunwk-HoVdxg@mail.gmail.com>
Date:   Tue, 22 Oct 2019 09:16:11 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Jiong Wang <jiong.wang@...ronome.com>
Subject: Re: [PATCH bpf-next v2] libbpf: use implicit XSKMAP lookup from
 AF_XDP XDP program

On Mon, 21 Oct 2019 at 17:43, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Björn Töpel <bjorn.topel@...il.com> writes:
>
> > On Mon, 21 Oct 2019 at 15:37, Björn Töpel <bjorn.topel@...il.com> wrote:
> >>
> >> On Mon, 21 Oct 2019 at 14:19, Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >> >
> >> > Björn Töpel <bjorn.topel@...il.com> writes:
> >> >
> >> [...]
> >> > >
> >> > > bpf_redirect_map() returns a 32-bit signed int, so the upper 32-bit
> >> > > will need to be cleared. Having an explicit AND is one instruction
> >> > > less than two shifts. So, it's an optimization (every instruction is
> >> > > sacred).
> >> >
> >> > OIC. Well, a comment explaining that might be nice (since you're doing
> >> > per-instruction comments anyway)? :)
> >> >
> >>
> >> Sure, I can do a v3 with a comment, unless someone has a better idea
> >> avoiding both shifts and AND.
> >>
> >> Thanks for taking a look!
> >>
> >
> > Now wait, there are the JMP32 instructions that Jiong added. So,
> > shifts/AND can be avoided. Now, regarding backward compat... JMP32 is
> > pretty new. I need to think a bit how to approach this. I mean, I'd
> > like to be able to use new BPF instructions.
>
> Well, they went into kernel 5.1 AFAICT; does AF_XDP even work properly
> in kernels older than that? For the xdp-tutorial we've just been telling
> people to upgrade their kernels to use it (see, e.g.,
> https://github.com/xdp-project/xdp-tutorial/issues/76).
>

Yeah, let's take that route, i.e. using JMP32 and one program. One
could argue that libbpf could do runtime checks and load the simpler
program w/o the fallback for post-5.3 only, and avoiding the branching
all together.


Björn


> -Toke
>