RISC-V: Re-enable GCC+Rust builds

[Conor Dooley]

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.

Cheers,
Conor.
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