Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Nathan Chancellor <nathan@kernel.org>]

On Sun, Mar 22, 2026 at 08:21:59PM +0100, Miguel Ojeda wrote:
>   - Clang passes `-Werror=unused-command-line-argument`, which means
>     under arm (i.e. 32-bit) we get:
> 
>       clang: error: argument unused during compilation: '-U arm' [-Werror,-Wunused-command-line-argument]
> 
>     And under UML I see:
> 
>       clang: error: argument unused during compilation: '-I ./arch/um/include/shared' [-Werror,-Wunused-command-line-argument]
>       clang: error: argument unused during compilation: '-I ./arch/x86/um/shared' [-Werror,-Wunused-command-line-argument]
>       clang: error: argument unused during compilation: '-I ./arch/um/include/shared/skas' [-Werror,-Wunused-command-line-argument]
> 
>     So we would need e.g. `-Wno-unused-command-line-argument` there
>     close to the `-Wno-override-module` one, unless Kbuild or
>     ClangBuiltLinux thinks it is important to keep it for this case.

No, I don't think it is worth trying to make -Wunused-command-line-argument
work for this command. Just disable it. This mirrors what is being done
for cmd_cc_o_bc in the distributed ThinLTO:

  https://lore.kernel.org/20260316212930.120438-3-xur@google.com/

>     On the other hand, regardless of whether we fix this (and another
>     issue in a separate email found thanks to the UML build), we could
>     instead add `depends on` listing explicitly the architectures where
>     this is going to be actually tested. That way maintainers can decide
>     whether they want to support it when they are ready. Thoughts?
> 
>     Cc'ing Nathan, Nicolas, Nick, Bill, Justin, David, UML, ARM.

I do agree with some of the concerns that adding an architecure
dimension to this is a little complicated. I would rather try to flush
out those build problems with patches and keep it enabled for all
architectures. At the same time though, I understand that enabling it
for the "tier 1" architectures is a low barrier of entry for getting the
feature upstream, validated, and distributed to the majority of people
that would actually use and depend on it, so I ultimately leave that
call up to you.

>   - If we use the `.bc` extension, we need to add a `.gitignore` for
>     `.bc` files, and an exception for `kernel/time/timeconst.bc`.
> 
>     I guess we will not have too many `bc` scripts in the future for
>     that to be a problem. On the other hand, we have the chance to use
>     another extension (either for LLVM bitcode or for `bc` scripts).
> 
>     But please let me know on e.g. the Kbuild side if someone has
>     concerns...

No real concern on that front but .gitignore has a command to run when
modifying it, which will require a !timeconst.bc in a
kernel/time/.gitignore file.

Cheers,
Nathan