Re: [PATCH 2/2] Kconfig: CC_HAS_NO_PROFILE_FN_ATTR, depend on for GCOV and PGO

Marco Elver
2021-06-19T06:23:52Z

On Sat, 19 Jun 2021 at 01:30, Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> We don't want compiler instrumentation to touch noinstr functions, which
> are annotated with the no_profile function attribute. Add a Kconfig test
> for this and make PGO and GCOV depend on it.
>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
> Link: https://lore.kernel.org/lkml/YMTn9yjuemKFLbws@hirez.programming.kicks-ass.net/
> Link: https://lore.kernel.org/lkml/YMcssV%2Fn5IBGv4f0@hirez.programming.kicks-ass.net/
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> init/Kconfig | 3 +++
> kernel/gcov/Kconfig | 1 +
> kernel/pgo/Kconfig | 3 ++-
> 3 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 1ea12c64e4c9..540f862b40c6 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -83,6 +83,9 @@ config TOOLS_SUPPORT_RELR
> config CC_HAS_ASM_INLINE
> def_bool $(success,echo 'void foo(void) { asm inline (""); }' | $(CC) -x c - -c -o /dev/null)
>
> +config CC_HAS_NO_PROFILE_FN_ATTR
> + def_bool $(success,echo '__attribute__((no_profile)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
> +
> config CONSTRUCTORS
> bool
>
> diff --git a/kernel/gcov/Kconfig b/kernel/gcov/Kconfig
> index 58f87a3092f3..19facd4289cd 100644
> --- a/kernel/gcov/Kconfig
> +++ b/kernel/gcov/Kconfig
> @@ -5,6 +5,7 @@ config GCOV_KERNEL
> bool "Enable gcov-based kernel profiling"
> depends on DEBUG_FS
> depends on !CC_IS_CLANG || CLANG_VERSION >= 110000
> + depends on !X86 || (X86 && CC_HAS_NO_PROFILE_FN_ATTR)

[+Cc Mark]

arm64 is also starting to rely on noinstr working properly.

This should probably be a 'select ARCH_HAS_GCOV_PROFILE_ALL if
CC_HAS_NO_PROFILE_FN_ATTR' in the relevant arch/../Kconfig.

Alternatively, using:
https://lkml.kernel.org/r/YMcssV/n5IBGv4f0@hirez.programming.kicks-ass.net

But I'd probably not overcomplicate things at this point and just use
ARCH_HAS_GCOV_PROFILE_ALL, because GCOV seems to be a) rarely used,
and b) if someone decides to selectively instrument stuff like entry
code, we can just say it's user error.

> select CONSTRUCTORS
> default n
> help
> diff --git a/kernel/pgo/Kconfig b/kernel/pgo/Kconfig
> index d2053df1111c..26f75ac4c6c1 100644
> --- a/kernel/pgo/Kconfig
> +++ b/kernel/pgo/Kconfig
> @@ -8,7 +8,8 @@ config PGO_CLANG
> bool "Enable clang's PGO-based kernel profiling"
> depends on DEBUG_FS
> depends on ARCH_SUPPORTS_PGO_CLANG
> - depends on CC_IS_CLANG && CLANG_VERSION >= 120000
> + depends on CC_IS_CLANG
> + depends on CC_HAS_NO_PROFILE_FN_ATTR
> help
> This option enables clang's PGO (Profile Guided Optimization) based
> code profiling to better optimize the kernel.
> --
> 2.32.0.288.g62a8d224e6-goog
>