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

[Luke Nelson]

On Mon, Jun 24, 2019 at 9:45 AM Jiong Wang <jiong.wang@xxxxxxxxxxxxx> 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.

â Luke