[PATCH] powerpc/ftrace: Disable ftrace on ppc32 if using clang

[Nick Desaulniers]

On Thu, Jun 8, 2023 at 8:47 PM Naveen N Rao <naveen at kernel.org> wrote:
>
> Ftrace on ppc32 expects a three instruction sequence at the beginning of
> each function when specifying -pg:
>         mflr    r0
>         stw     r0,4(r1)
>         bl      _mcount
>
> This is the case with all supported versions of gcc. Clang however emits
> a branch to _mcount after the function prologue, similar to the pre
> -mprofile-kernel ABI on ppc64. This is not supported.
>
> Disable ftrace on ppc32 if using clang for now. This can be re-enabled
> later if clang picks up support for -fpatchable-function-entry on ppc32.
>
> Signed-off-by: Naveen N Rao <naveen at kernel.org>

Thanks for the patch! I've filed the below bug, a link to whom I'd
like to see retained in the commit message. In the future, please file
bugs against the compiler vendors first, then include the relevant
link.

Link: https://github.com/llvm/llvm-project/issues/63220
Acked-by: Nick Desaulniers <ndesaulniers at google.com>

> ---
>  arch/powerpc/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index bff5820b7cda14..d85e3cf4016d90 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -236,7 +236,7 @@ config PPC
>         select HAVE_FUNCTION_DESCRIPTORS        if PPC64_ELF_ABI_V1
>         select HAVE_FUNCTION_ERROR_INJECTION
>         select HAVE_FUNCTION_GRAPH_TRACER
> -       select HAVE_FUNCTION_TRACER
> +       select HAVE_FUNCTION_TRACER             if PPC64 || (PPC32 && CC_IS_GCC)
>         select HAVE_GCC_PLUGINS                 if GCC_VERSION >= 50200   # plugin support on gcc <= 5.1 is buggy on PPC
>         select HAVE_GENERIC_VDSO
>         select HAVE_HARDLOCKUP_DETECTOR_ARCH    if PPC_BOOK3S_64 && SMP
>
> base-commit: bd517a8442b6c6646a136421cd4c1b95bf4ce32b
> --
> 2.40.1
>


-- 
Thanks,
~Nick Desaulniers