linux-kernel - Re: [PATCH] RISC-V: Fix building rust when using GCC toolchain

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Message-ID: <CANiq72=ShT8O0GcN8G-YRE1CB8Z9Ztr-ZNcQ6cphHYvDGanTKg@mail.gmail.com>
Date: Thu, 26 Sep 2024 18:11:17 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Conor Dooley <conor@...nel.org>
Cc: Gary Guo <gary@...yguo.net>, Jason Montleon <jmontleo@...hat.com>, ojeda@...nel.org, 
	alex.gaynor@...il.com, boqun.feng@...il.com, bjorn3_gh@...tonmail.com, 
	benno.lossin@...ton.me, a.hindborg@...nel.org, aliceryhl@...gle.com, 
	paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu, 
	nathan@...nel.org, ndesaulniers@...gle.com, morbo@...gle.com, 
	justinstitt@...gle.com, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org, 
	llvm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] RISC-V: Fix building rust when using GCC toolchain

On Thu, Sep 26, 2024 at 5:56 PM Conor Dooley <conor@...nel.org> wrote:
>
> Mixed builds are allowed on the c side, since we can figure out what the

I am not sure what you mean by allowed on the C side. For out-of-tree
modules you mean?

> versions of each tool are. If there's a way to detect the version of
> libclang in use by the rust side, then I would be okay with mixed gcc +
> rustc builds.

If you mean the libclang used by bindgen, yes, we have such a check
(warning) in scripts/rust_is_available.sh. We can also have it as a
Kconfig symbol if needed.

Regarding rustc's LLVM version, I wanted to have a similar check in
scripts/rust_is_available.sh (also a warning), but it would have been
quite noisy, and if LTO is not enabled it should generally be OK. So
we are adding instead a Kconfig symbol for that, which will be used
for a couple things. Gary has a WIP patch for this one.

> Yes, I would rather this was not applied at all. My plan was to send a
> patch making HAVE_RUST depend on CC_IS_CLANG, but just ain't got around
> to it yet, partly cos I was kinda hoping to mention this to you guys at
> LPC last week, but I never got the chance to talk to any rust people (or
> go to any rust talks either!).

To be clear, that `depends on` would need to be only for RISC-V, i.e.
other architectures are "OK" with those. It is really, really
experimental, as we have always warned, but some people is using it
successfully, apparently.

> Sure, I can add a comment there.

Thanks!

> In sorta related news, is there a plan for config "options" that will
> allow us to detect gcc-rs or the gcc rust backend?

gccrs is way too early to even think about that. rustc_codegen_gcc,
yeah, if needed, we can add a symbol for that when we start supporting
the backend.

Cheers,
Miguel