Re: [PATCH v2 3/3] Kconfig: add ARCH_WANTS_NO_INSTR+CC_HAS_NO_PROFILE_FN_ATTR, depend on for GCOV and PGO

On Tue, 22 Jun 2021 at 09:26, Peter Zijlstra [off-list ref] wrote:

On Mon, Jun 21, 2021 at 04:18:22PM -0700, Nick Desaulniers wrote:

quoted

We don't want compiler instrumentation to touch noinstr functions, which
are annotated with the no_profile_instrument_function function
attribute. Add a Kconfig test for this and make PGO and GCOV depend on
it.

If an architecture is using noinstr, it should denote that via this
Kconfig value. That makes Kconfigs that depend on noinstr able to
express dependencies in an architecturally agnostic way.

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: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
Changes V1 -> V2:
* Add ARCH_WANTS_NO_INSTR
* Change depdendencies to be !ARCH_WANTS_NO_INSTR || CC_HAS_NO_PROFILE_FN_ATTR
  rather than list architectures explicitly, as per Nathan.
* s/no_profile/no_profile_instrument_function/

 arch/Kconfig        | 7 +++++++
 arch/arm64/Kconfig  | 1 +
 arch/s390/Kconfig   | 1 +
 arch/x86/Kconfig    | 1 +
 init/Kconfig        | 3 +++
 kernel/gcov/Kconfig | 1 +
 kernel/pgo/Kconfig  | 3 ++-
 7 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 2b4109b0edee..2113c6b3b801 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -285,6 +285,13 @@ config ARCH_THREAD_STACK_ALLOCATOR
 config ARCH_WANTS_DYNAMIC_TASK_STRUCT
      bool

+config ARCH_WANTS_NO_INSTR
+     bool
+     help
+       An architecure should select this if the noinstr macro is being used on
+       functions to denote that the toolchain should avoid instrumenting such
+       functions and is required for correctness.
+
 config ARCH_32BIT_OFF_T
      bool
      depends on !64BIT

There's also CC_HAS_WORKING_NOSANITIZE_ADDRESS in lib/Kconfig.kasan that
might want to be hooked into this, but that can be done separately I
suppose.

KASAN already depends on this for all compiler instrumentation modes,
not just for 'noinstr' but also to avoid false positives. So it's not
just for noinstr's benefit, and we should not weaken the requirement
there.

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