linux-kernel - Re: [RFC PATCH bpf-next] RV32G eBPF JIT

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Date:   Tue, 25 Jun 2019 13:26:21 -0700
From:   Luke Nelson <lukenels@...washington.edu>
To:     Jiong Wang <jiong.wang@...ronome.com>
Cc:     Luke Nelson <luke.r.nels@...il.com>, Xi Wang <xi.wang@...il.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Björn Töpel <bjorn.topel@...il.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next] RV32G eBPF JIT

On Mon, Jun 24, 2019 at 9:45 AM Jiong Wang <jiong.wang@...ronome.com> wrote:
>
> Looks to me 32-bit optimization is not enabled.
>
> If you define bpf_jit_needs_zext to return true
>
>   bool bpf_jit_needs_zext(void)
>   {
>         return true;
>   }
>
> Then you don't need to zero high 32-bit when writing 32-bit sub-register
> and you just need to implement the explicit zero extension insn which is a
> special variant of BPF_MOV. This can save quite a few instructions. RV64
> and arches like arm has implemented this, please search
> "aux->verifier_zext".
>
> And there is a doc for this optimization:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/Documentation/bpf/bpf_design_QA.rst#n168

Thanks for the pointer. I'll add this optimization before sending out
an updated version of the patch.

– Luke