netdev - Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

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Date:   Fri, 15 Nov 2019 08:56:59 +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>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Edward Cree <ecree@...arflare.com>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On Fri, 15 Nov 2019 at 01:30, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
[...]
>
> Could you try optimizing emit_mov_imm64() to recognize s32 ?
> iirc there was a single x86 insns that could move and sign extend.
> That should cut down on bytecode size and probably make things a bit faster?
> Another alternative is compare lower 32-bit only, since on x86-64 upper 32
> should be ~0 anyway for bpf prog pointers.

Good ideas, thanks! I'll do the optimization, extend it to >4 entries
(as Toke suggested), and do a non-RFC respin.

> Looking at bookkeeping code, I think I should be able to generalize bpf
> trampoline a bit and share the code for bpf dispatch.

Ok, good!

> Could you also try aligning jmp target a bit by inserting nops?
> Some x86 cpus are sensitive to jmp target alignment. Even without considering
> JCC bug it could be helpful. Especially since we're talking about XDP/AF_XDP
> here that will be pushing millions of calls through bpf dispatch.
>

Yeah, I need to address the Jcc bug anyway, so that makes sense.

Another thought; I'm using the fentry nop as patch point, so it wont
play nice with other users of fentry atm -- but the plan is to move to
Steve's *_ftrace_direct work at some point, correct?


Björn