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

[Nick Desaulniers <ndesaulniers@google.com>]

On Fri, Jun 18, 2021 at 11:23 PM Marco Elver <elver@google.com> wrote:
>
> 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.

Sure,
Will, Catalin, other arm64 folks:
Any thoughts on requiring GCC 7.1+/Clang 13.0+ for GCOV support?  That
way we can better guarantee that GCOV (and eventually, PGO) don't
touch noinstr functions?

If that's ok, I'll add modify the above like:

+ depends on !ARM64 || (ARM64 && CC_HAS_NO_PROFILE_FN_ATTR)

to the above hunk in v2.  Oh, looks like arch/s390 also uses noinstr.
Same question applies then:

+ depends on !S390 || (S390 && CC_HAS_NO_PROFILE_FN_ATTR)

Or, we could just do

+ depends on CC_HAS_NO_PROFILE_FN_ATTR

Though that will penalize architectures not using noinstr, that still
would like to use GCOV with versions of GCC older than 7.1.  Perhaps
there are no such such users, or they should consider upgrading their
tools to we can stick with the simpler Kconfig? Thoughts?

>
> 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
> >



-- 
Thanks,
~Nick Desaulniers