Re: [PATCH] kbuild: rust: add AutoFDO support

[Rong Xu]

I have reviewed this patch and it looks good to me. I suggest updating
autofdo.rst to explicitly mention support for Rust.

Regarding the naming convention, I understand Gary's point that Clang
and Rust should both be viewed as LLVM frontends. I am open to
renaming *FLAGS_AUTOFDO_CLANG to *FLAGS_AUTOFDO_LLVM. However, I would
prefer not to use *FLAGS_AUTOFDO, as GCC also supports AutoFDO, but it
hasn't been integrated into kernel builds yet.

These naming updates can be handled in a separate patch.

Reviewed-by: Rong Xu <xur@xxxxxxxxxx>

-Rong

On Fri, Mar 20, 2026 at 9:48 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
>
> On Thu Mar 19, 2026 at 11:54 PM GMT, Nathan Chancellor wrote:
> > On Thu, Mar 19, 2026 at 07:09:22PM +0000, Gary Guo wrote:
> >> On Thu Mar 19, 2026 at 4:33 PM GMT, Alice Ryhl wrote:
> >> > On Thu, Mar 19, 2026 at 12:47 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
> >> >> On Thu Mar 19, 2026 at 11:44 AM GMT, Alice Ryhl wrote:
> >> >> > Do you think it's worth having Rust not match the CFLAGS here?
> >> >> >
> >> >>
> >> >> I think the C flags should probably be renamed AUTOFDO_LLVM too. After call, all
> >> >> the perf tools involved here are called llvm-foobar as well.
> >> >
> >> > But isn't it just called this because the config option is
> >> > CONFIG_AUTOFDO_CLANG? So it's the CFLAGS or RUSTFLAGS related to that
> >> > particular config option. It may make sense to rename
> >> > CONFIG_AUTOFDO_CLANG to CONFIG_AUTOFDO, but that's out-of-scope for
> >> > this change, in my opinion.
> >>
> >> I get that it's an existing config name, but previously it is a correct
> >> description, while after this change it is no longer accurate.
> >>
> >> I don't know if there's an established practice on renaming CONFIG options when
> >> this sort thing happens though. Perhaps one way is to have add
> >> CONFIG_AUTOFDO_CLANG that selects CONFIG_AUTOFDO.
> >
> > As Miguel mentioned, there is the transitional keyword for Kconfig to
> > handle renames now but I would really like there to be a compelling
> > reason for the rename other than LLVM is a little more accurate than
> > CLANG. The kernel has never really done a great job of referring to
> > Clang when it means the C compiler vs. the whole LLVM toolchain (like
> > LTO_CLANG and all its derivatives should be LTO_LLVM since it requires
> > ld.lld, llvm-nm, and llvm-ar in addition to clang). Additionally, aside
> > from the configuration, there is the user facing Make variable
> > CLANG_AUTOFDO_PROFILE that would presumably need to be changed for
> > consistency.
>
> If you think the config name should be kept as CONFIG_AUTOFDO_CLANG, then I
> think there is no real benefit in changing the variable name.
>
> Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>
>
> Best,
> Gary