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

From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Miguel Ojeda <ojeda@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	rust-for-linux <rust-for-linux@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Alex Gaynor <alex.gaynor@gmail.com>, Finn Behrens <me@kloenk.de>,
	Adam Bratschi-Kaye <ark.email@gmail.com>,
	Wedson Almeida Filho <wedsonaf@google.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Sven Van Asbroeck <thesven73@gmail.com>,
	Gary Guo <gary@garyguo.net>,
	Boris-Chengbiao Zhou <bobo1239@web.de>,
	Boqun Feng <boqun.feng@gmail.com>,
	Douglas Su <d0u9.su@outlook.com>,
	Dariusz Sosnowski <dsosnowski@dsosnowski.pl>,
	Antonio Terceiro <antonio.terceiro@linaro.org>,
	Daniel Xu <dxu@dxuuu.xyz>, Miguel Cano <macanroj@gmail.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	linux-riscv <linux-riscv@lists.infradead.org>
Subject: Re: [PATCH v4 16/20] Kbuild: add Rust support
Date: Sat, 12 Feb 2022 16:47:33 +0100	[thread overview]
Message-ID: <CANiq72nMhUH1s0HMeLb+hfp5=u3h20ryC4uqAgB1Znuq52e=Pw@mail.gmail.com> (raw)
In-Reply-To: <YgfBUhYdLXA46kOX@shell.armlinux.org.uk>

Hi Russell,

On Sat, Feb 12, 2022 at 3:17 PM Russell King (Oracle)
<linux@armlinux.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