Re: [RFC PATCH 0/3] RV64G eBPF JIT

[Palmer Dabbelt <palmer@sifive.com>]

On Tue, 15 Jan 2019 00:35:15 PST (-0800), bjorn.topel@gmail.com wrote:
> Hi!
>
> I've been hacking on a RV64G eBPF JIT compiler, and would like some
> feedback.
>
> Codewise, it needs some refactoring. Currently there's a bit too much
> copy-and-paste going on, and I know some places where I could optimize
> the code generation a bit (mostly BPF_K type of instructions, dealing
> with immediates).
>
> From a features perspective, two things are missing:
>
> * tail calls
> * "far-branches", i.e. conditional branches that reach beyond 13b.
>
> The test_bpf.ko (only tested on 4.20!) passes all tests.
>
> I've done all the tests on QEMU (version 3.1.50), so no real hardware.
>
> Some questions/observations:
>
> * I've added "HAVE_EFFICIENT_UNALIGNED_ACCESS" to
>   arch/riscv/Kconfig. Is this assumption correct?
>
> * emit_imm() just relies on lui, adds and shifts. No fancy xori cost
>   optimizations like GCC does.
>
> * Suggestions on how to implement the tail call, given that the
>   prologue/epilogue has variable size. I will dig into the details of
>   mips/arm64/x86. :-)
>
> Next steps (prior patch proper) is cleaning up the code, add tail
> calls, and making sure that bpftool disassembly works correctly.
>
> All input are welcome. This is my first RISC-V hack, so I sure there
> are a lot things to improve!
>
>
> Thanks,
> Björn
>
>
> Björn Töpel (3):
>   riscv: set HAVE_EFFICIENT_UNALIGNED_ACCESS
>   riscv: add build infra for JIT compiler
>   bpf, riscv: added eBPF JIT for RV64G
>
>  arch/riscv/Kconfig            |    2 +
>  arch/riscv/Makefile           |    4 +
>  arch/riscv/net/Makefile       |    5 +
>  arch/riscv/net/bpf_jit_comp.c | 1612 +++++++++++++++++++++++++++++++++
>  4 files changed, 1623 insertions(+)
>  create mode 100644 arch/riscv/net/Makefile
>  create mode 100644 arch/riscv/net/bpf_jit_comp.c

Thanks for doing this.  I saw a few reviews go by and I'm behind on email, so 
I'm going to drop this until a v2.