netdev - Re: [PATCH bpf-next v4 1/4] riscv, bpf: move common riscv JIT code to header

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Date:   Tue, 3 Mar 2020 18:31:11 -0800
From:   Luke Nelson <lukenels@...washington.edu>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Luke Nelson <luke.r.nels@...il.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>, Xi Wang <xi.wang@...il.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Rob Herring <robh@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH bpf-next v4 1/4] riscv, bpf: move common riscv JIT code to header

Hi Björn,

Thanks for the comments! Inlined responses below:

On Mon, Mar 2, 2020 at 11:50 PM Björn Töpel <bjorn.topel@...il.com> wrote:
>
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Common functionality for RV32 and RV64 BPF JIT compilers
> > + *
> > + * Copyright (c) 2019 Björn Töpel <bjorn.topel@...il.com>
> > + * Copyright (c) 2020 Luke Nelson <luke.r.nels@...il.com>
> > + * Copyright (c) 2020 Xi Wang <xi.wang@...il.com>
>
> I'm no lawyer, so this is more of a question; You've pulled out code
> into a header, and renamed two functions. Does that warrant copyright
> line additions? Should my line be removed?

This header also includes new code for emitting instructions required
for the RV32 JIT (e.g., sltu) and some additional pseudoinstructions
(e.g., bgtu and similar). I'm also no lawyer, so I don't know either
if this rises to the level of adding copyright lines. I'm happy to
do the following in v5 if it looks better:

+ * Copyright (c) 2019 Björn Töpel <bjorn.topel@...il.com>
+ *
+ * Modified by ...

> > +#if __riscv_xlen == 64
>
> Please remove this. If the inlined functions are not used, they're not
> part of the binary. This adds complexity to the code, and without it
> we can catch build errors early on!

I agree in general we should avoid #if. The reason for using it
here is to cause build errors if the RV32 JIT ever tries to emit
an RV64-only instruction by mistake. Otherwise, what is now a build
error would be delayed to an illegal instruction trap when the JITed
code is executed, which is much harder to find and diagnose.

We could use separate files, bpf_jit_32.h and bpf_jit_64.h (the
latter will include the former), if we want to avoid #if. Though
this adds another form of complexity.

So the options here are 1) using no #if, with the risk of hiding
subtle bugs in the RV32 JIT; 2) using #if as is; and 3) using
separate headers. What do you think?

Thanks!

Luke