linux-kernel - Re: [PATCH v3 bpf-next] RV32G eBPF JIT

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Date:   Fri, 28 Feb 2020 14:20:11 -0800
From:   Luke Nelson <lukenels@...washington.edu>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Jiong Wang <jiong.wang@...ronome.com>,
        Xi Wang <xi.wang@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.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>,
        Andrii Nakryiko <andriin@...com>,
        linux-riscv@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next] RV32G eBPF JIT

On Fri, Feb 28, 2020 at 6:25 AM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> Luke/Xi, apologies for the slow reponse. (All my RV work is done on
> non-payed time, so that's that. :-)) Very nice that you're still
> working on it!

No worries, thanks for reviewing!

> >
> > - ALU64 DIV/MOD: Requires loops to implement on 32-bit hardware.
>
> Even though it requires loops, JIT support would be nice. OTOH, arm
> doesn't support that either...

We could probably implement this by calling into a helper function
for BPF_ALU64 | BPF_DIV/BPF_MOD, but none of the other JITs (e.g.,
arm,x86) for 32-bit architectures do this. We could add support to
rv32 and other architectures in a future patch.

> > - BPF_XADD | BPF_DW: Requires either an 8-byte atomic instruction
> >   in the target (which doesn't exist in RV32), or acqusition of
> >   locks in generated code.
> >
>
> Any ideas how this could be addressed for RV32G?

I don't believe there is a simple way to correctly implement BPF_XADD
| BPF_DW without hardware support for 64-bit atomic operations,
like for other 32-bit JITs.

> In general I agree with Song; It would be good if the 64/32 bit
> variants would share more code. RISC-V 64/32 *are* very similar, and
> we should be able to benefit from that codewise.
>
> Pull out all functions are that common -- most of the emit_*, the
> parts of the registers, the branch relaxation, and context
> structs. Hopefully, the acutal RV32/64 specfic parts will be pretty
> small.

Thanks for the suggestion. I'll factor out the common functionality
into a header for the next revision.

> Finally; There are some checkpatch issues: run 'checkpatch.pl --strict'.
> [...]

I'll fix the rest of the issues you found in the next revision.

> Thanks for the hard work! I'll take it for a spin, with help from the
> guide above, this weekend!

Thanks again!
- Luke