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

On Mon, Jun 21, 2021 at 01:43:54PM -0700, Nick Desaulniers wrote:

On Mon, Jun 21, 2021 at 11:50 AM Bill Wendling [off-list ref] wrote:

quoted

On Mon, Jun 21, 2021 at 11:22 AM Nick Desaulniers
[off-list ref] wrote:

quoted

On Fri, Jun 18, 2021 at 11:23 PM Marco Elver [off-list ref] wrote:

quoted

On Sat, 19 Jun 2021 at 01:30, Nick Desaulniers [off-list ref] wrote:

quoted

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/ (local)
Link: https://lore.kernel.org/lkml/YMcssV%2Fn5IBGv4f0@hirez.programming.kicks-ass.net/ (local)
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)

Wouldn't "!ARM64 || CC_HAS_NO_PROFILE_FN_ATTR" be more succinct?

We need to be able to express via Kconfig "GCOV should not be enabled
for architectures that use noinstr when the toolchain does not support
__attribute__((no_profile_instrument_function))."

Where "architectures that use noinstr" are currently arm64, s390, and
x86.  So I guess we could do:

+ depends on !ARM64 || !S390 || !X86 || CC_HAS_NO_PROFILE_FN_ATTR

I think you want:

  depends on !(ARM64 || S390 || X86) || CC_HAS_NO_PROFILE_FN_ATTR

(We could add a Kconfig for ARCH_WANTS_NO_INSTR, which might be more
informative than listed out architectures which might be non-obvious
to passers-by).

That would probably look better.

-- 
Catalin