[PATCH v1] RISC-V: disallow gcc + rust builds

[Nathan Chancellor]

On Tue, Oct 01, 2024 at 12:28:13PM +0100, Conor Dooley wrote:
> From: Conor Dooley <conor.dooley at microchip.com>
> 
> During the discussion before supporting rust on riscv, it was decided
> not to support gcc yet, due to differences in extension handling
> compared to llvm (only the version of libclang matching the c compiler
> is supported). Recently Jason Montleon reported [1] that building with
> gcc caused build issues, due to unsupported arguments being passed to
> libclang. After some discussion between myself and Miguel, it is better
> to disable gcc + rust builds to match the original intent, and
> subsequently support it when an appropriate set of extensions can be
> deduced from the version of libclang.
> 
> Closes: https://lore.kernel.org/all/20240917000848.720765-2-jmontleo@redhat.com/ [1]
> Link: https://lore.kernel.org/all/20240926-battering-revolt-6c6a7827413e@spud/ [2]
> Fixes: 70a57b247251a ("RISC-V: enable building 64-bit kernels with rust support")
> Reported-by: Jason Montleon <jmontleo at redhat.com>
> Signed-off-by: Conor Dooley <conor.dooley at microchip.com>

Seems like a reasonable restriction to me.

Reviewed-by: Nathan Chancellor <nathan at kernel.org>

I assume the difficulty is not in getting the libclang version itself
but actually mapping extensions to the versions and then deciding how to
make that available to the build system? Or is it more that a solution
is somewhat known but someone has to actually implement it? Sorry if
this has been talked about before.

> ---
> 
> Palmer, this is yours to take.
> 
> CC: jmontleo at redhat.com
> CC: ojeda at kernel.org
> CC: alex.gaynor at gmail.com
> CC: boqun.feng at gmail.com
> CC: gary at garyguo.net
> CC: bjorn3_gh at protonmail.com
> CC: benno.lossin at proton.me
> CC: a.hindborg at kernel.org
> CC: aliceryhl at google.com
> CC: paul.walmsley at sifive.com
> CC: palmer at dabbelt.com
> CC: nathan at kernel.org
> CC: ndesaulniers at google.com
> CC: morbo at google.com
> CC: justinstitt at google.com
> CC: rust-for-linux at vger.kernel.org
> CC: linux-riscv at lists.infradead.org
> CC: llvm at lists.linux.dev
> ---
>  Documentation/rust/arch-support.rst | 2 +-
>  arch/riscv/Kconfig                  | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/rust/arch-support.rst b/Documentation/rust/arch-support.rst
> index 750ff371570a0..54be7ddf3e57a 100644
> --- a/Documentation/rust/arch-support.rst
> +++ b/Documentation/rust/arch-support.rst
> @@ -17,7 +17,7 @@ Architecture   Level of support  Constraints
>  =============  ================  ==============================================
>  ``arm64``      Maintained        Little Endian only.
>  ``loongarch``  Maintained        \-
> -``riscv``      Maintained        ``riscv64`` only.
> +``riscv``      Maintained        ``riscv64`` and LLVM/Clang only.
>  ``um``         Maintained        \-
>  ``x86``        Maintained        ``x86_64`` only.
>  =============  ================  ==============================================
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 22dc5ea4196ce..76843584960c2 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -177,7 +177,7 @@ config RISCV
>  	select HAVE_REGS_AND_STACK_ACCESS_API
>  	select HAVE_RETHOOK if !XIP_KERNEL
>  	select HAVE_RSEQ
> -	select HAVE_RUST if RUSTC_SUPPORTS_RISCV
> +	select HAVE_RUST if RUSTC_SUPPORTS_RISCV && CC_IS_CLANG
>  	select HAVE_SAMPLE_FTRACE_DIRECT
>  	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
>  	select HAVE_STACKPROTECTOR
> -- 
> 2.45.2
>