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

From: "Björn Töpel" <bjorn.topel@gmail.com>
To: Luke Nelson <lukenels@cs.washington.edu>
Cc: bpf <bpf@vger.kernel.org>, Luke Nelson <luke.r.nels@gmail.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, Xi Wang <xi.wang@gmail.com>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Stephen Hemminger <stephen@networkplumber.org>,
	Rob Herring <robh@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
	Netdev <netdev@vger.kernel.org>,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH bpf-next v4 1/4] riscv, bpf: move common riscv JIT code to header
Date: Wed, 4 Mar 2020 06:44:24 +0100	[thread overview]
Message-ID: <CAJ+HfNgHyX_zMh7Wm00twwY75YLftZ8GFMw3rx5k+yiLH8p0eg@mail.gmail.com> (raw)
In-Reply-To: <CADasFoC5EEXdq43waj9pQDb9HtpG2bWE2yMVySBZ4rpopYbROQ@mail.gmail.com>

On Wed, 4 Mar 2020 at 03:31, Luke Nelson <lukenels@cs.washington.edu> wrote:
>
> Hi Björn,
>
> Thanks for the comments! Inlined responses below:
>
> On Mon, Mar 2, 2020 at 11:50 PM Björn Töpel <bjorn.topel@gmail.com> wrote:
> >
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +/*
> > > + * Common functionality for RV32 and RV64 BPF JIT compilers
> > > + *
> > > + * Copyright (c) 2019 Björn Töpel <bjorn.topel@gmail.com>
> > > + * Copyright (c) 2020 Luke Nelson <luke.r.nels@gmail.com>
> > > + * Copyright (c) 2020 Xi Wang <xi.wang@gmail.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@gmail.com>
> + *
> + * Modified by ...
>

Ah, my mistake! Feel free to keep the Copyright. I was honestly just
curious what the correct way (if any) was. So; Keep your copyright!
Sorry for the noise!

> > > +#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?
>

Ok, that is a valid concern. We could go the route of compile-time checking:

if (__riscv_xlen != 64)
    bad_usage();

That's overkill in this case. Keep the #if.

Cheers,
Björn

> Thanks!
>
> Luke