Message-ID: <20250901-lasso-kabob-de32b8fcede8@spud>
Date: Mon, 1 Sep 2025 15:08:31 +0100
From: Conor Dooley <conor@...nel.org>
To: Asuna Yang <spriteovo@...il.com>
Cc: Jason Montleon <jmontleo@...hat.com>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <lossin@...nel.org>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Danilo Krummrich <dakr@...nel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: RISC-V: Re-enable GCC+Rust builds

On Sat, Aug 30, 2025 at 07:17:48PM +0100, Conor Dooley wrote:
> On Sat, Aug 30, 2025 at 01:00:56PM +0800, Asuna Yang wrote:
> > I noticed that GCC+Rust builds for RISC-V were disabled about a year ago, as
> > discussed in
> > https://lore.kernel.org/all/20240917000848.720765-1-jmontleo@redhat.com/
> > 
> > I'm a bit lost here. What are the main obstacles to re-enabling GCC builds
> > now?
> > 
> > Conor said:
> > > Okay. Short term then is deny gcc + rust, longer term is allow it with the
> > same caveats as the aforementioned mixed stuff.
> > "the same caveats" means detecting what specifically?
> 
> There's "code" in the riscv Kconfig/Makefile that makes sure that the
> assembler has the same understanding of what extensions are enabled as
> the compiler. This is done by detecting which version of the tools are
> in use, and adjusting march etc as a result. See
> TOOLCHAIN_NEEDS_EXPLICIT_ZICSR_ZIFENCEI for an example. When I wrote the
> comment you're citing, there was no "off the shelf" way to figure out
> the version of libclang in use to ensure that it has the same
> understanding of -march as the version of gcc being used on the c side
> does. For clang build, it's not a concern since it's almost certainly
> the exact same as the compiler building the c side.
> 
> > We have a RISC-V PWM driver being written in Rust. Currently, GCC being
> > disabled for building the kernel with Rust for RISC-V is the primary blocker
> > for including these drivers in RISC-V distros. Therefore, I'd like to push
> > forward and contribute to the re-enabling of GCC builds. Is there a more
> > detailed direction on what I can do here?
> 
> Add the version of libclang as a Kconfig symbol, so that the kernel's
> build system can ensure that both sides are built using the same
> configuration. Off the top of my head, using a pre-17 libclang with a
> new gcc would require having zicsr in -march for the c side and it
> removed for rust. It's been a while (1 year+) since I fiddled with this
> though, so my recollection there could well be inaccurate.

Hmm, while I think of it, there's some other things that are problematic
that are not currently checked but would have to be. For example,
there's a check in the riscv Kconfig menu to see if
stack-protector-guard=tls can be used via a cc-option check. If that
check passes with gcc as the compiler that option will be passed to the
rust side of the build, where llvm might not support it.
Similarly, turning on an extension like Zacas via a cc-option check could
pass for gcc but not be usable when passed to the rust side, causing
errors. These sorts of things should be prevented via Kconfig, not show
up as confusing build errors.


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