linux-kernel - Re: [PATCH v4 16/20] Kbuild: add Rust support

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Date:   Sat, 12 Feb 2022 16:47:33 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux <rust-for-linux@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Finn Behrens <me@...enk.de>,
        Adam Bratschi-Kaye <ark.email@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sven Van Asbroeck <thesven73@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Boris-Chengbiao Zhou <bobo1239@....de>,
        Boqun Feng <boqun.feng@...il.com>,
        Douglas Su <d0u9.su@...look.com>,
        Dariusz Sosnowski <dsosnowski@...snowski.pl>,
        Antonio Terceiro <antonio.terceiro@...aro.org>,
        Daniel Xu <dxu@...uu.xyz>, Miguel Cano <macanroj@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v4 16/20] Kbuild: add Rust support

Hi Russell,

On Sat, Feb 12, 2022 at 3:17 PM Russell King (Oracle)
<linux@...linux.org.uk> wrote:
>
> Please don't use CPU_32v6* here.
>
> It probably makes more sense to add a symbol "HAVE_RUST" and have the
> appropriate architecture Kconfig files select HAVE_RUST.

We can do it whatever way arch maintainers prefer, of course. Why
would you prefer one over the other?

> Does Rust support Thumb on ARMv6 and ARMv7 architectures?

Yes, the main backend is LLVM. Some built-in targets and their support
level are listed here, if you want to take a look:

    https://doc.rust-lang.org/nightly/rustc/platform-support.html

There is also a GCC backend in `rustc` [1], which is making good
progress and may eventually give us access to architectures supported
by GCC. Furthermore, a from-scratch Rust frontend for GCC [2] is being
worked on, though this effort will likely take more time to reach a
point where it may be used for the kernel.

[1] https://github.com/rust-lang/rust/tree/master/compiler/rustc_codegen_gcc
[2] https://github.com/Rust-GCC/gccrs

> Please remove every utterance of "default n" from your patch; n is the
> default default which default defaults to, so you don't need to specify
> default n to make the option default to n. It will default to n purely
> because n is the default when no default is specified.

Certainly. I am curious, though: is there a reason for most of the
other 500+ instances in the kernel tree?

> As Rust doesn't support all the architectures that the kernel supports,
> Rust must not be used for core infrastructure.

Yeah, although I am not sure I understand what you are getting at here.

Cheers,
Miguel