Re: [PATCH kernel v3] powerpc/makefile: Do not redefine $(CPP) for preprocessor

On 14/05/2021 04:59, Nathan Chancellor wrote:

On 5/13/2021 4:59 AM, Alexey Kardashevskiy wrote:

quoted

The $(CPP) (do only preprocessing) macro is already defined in Makefile.
However POWERPC redefines it and adds $(KBUILD_CFLAGS) which results
in flags duplication. Which is not a big deal by itself except for
the flags which depend on other flags and the compiler checks them
as it parses the command line.

Specifically, scripts/Makefile.build:304 generates ksyms for .S files.
If clang+llvm+sanitizer are enabled, this results in

-emit-llvm-bc -fno-lto -flto -fvisibility=hidden \
  -fsanitize=cfi-mfcall -fno-lto  ...

in the clang command line and triggers error:

clang-13: error: invalid argument '-fsanitize=cfi-mfcall' only allowed 
with '-flto'

This removes unnecessary CPP redefinition. Which works fine as in most
place KBUILD_CFLAGS is passed to $CPP except
arch/powerpc/kernel/vdso64/vdso(32|64).lds. To fix vdso, this does:
1. add -m(big|little)-endian to $CPP
2. add target to $KBUILD_CPPFLAGS as otherwise clang ignores 
-m(big|little)-endian if
the building platform does not support big endian (such as x86).

Signed-off-by: Alexey Kardashevskiy <redacted>
---
Changes:
v3:
* moved vdso cleanup in a separate patch
* only add target to KBUILD_CPPFLAGS for CLANG

v2:
* fix KBUILD_CPPFLAGS
* add CLANG_FLAGS to CPPFLAGS
---
  Makefile              | 1 +
  arch/powerpc/Makefile | 3 ++-
  2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 15b6476d0f89..5b545bef7653 100644
--- a/Makefile
+++ b/Makefile

@@ -576,6 +576,7 @@ CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) 
--version 2>/dev/null | head -

  ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
  ifneq ($(CROSS_COMPILE),)
  CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
+KBUILD_CPPFLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))

You can avoid the duplication here by just doing:

KBUILD_CPPFLAGS    += $(CLANG_FLAGS)

This has potential of duplicating even more flags which is exactly what 
I am trying to avoid here.

I am still not super happy about the flag duplication but I am not sure 
I can think of a better solution. If KBUILD_CPPFLAGS are always included 
when building .o files,

My understanding is that KBUILD_CPPFLAGS should not be added for .o. Who 
does know or decide for sure about what CPPFLAGS are for? :)

maybe we should just add $(CLANG_FLAGS) to 
KBUILD_CPPFLAGS instead of KBUILD_CFLAGS?

quoted

  endif
  ifeq ($(LLVM_IAS),1)
  CLANG_FLAGS    += -integrated-as

diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index 3212d076ac6a..306bfd2797ad 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile

@@ -76,6 +76,7 @@ endif

  ifdef CONFIG_CPU_LITTLE_ENDIAN
  KBUILD_CFLAGS    += -mlittle-endian
+KBUILD_CPPFLAGS    += -mlittle-endian
  KBUILD_LDFLAGS    += -EL
  LDEMULATION    := lppc
  GNUTARGET    := powerpcle

@@ -83,6 +84,7 @@ MULTIPLEWORD    := -mno-multiple

  KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-save-toc-indirect)
  else
  KBUILD_CFLAGS += $(call cc-option,-mbig-endian)
+KBUILD_CPPFLAGS += $(call cc-option,-mbig-endian)
  KBUILD_LDFLAGS    += -EB
  LDEMULATION    := ppc
  GNUTARGET    := powerpc

@@ -208,7 +210,6 @@ KBUILD_CPPFLAGS    += -I $(srctree)/arch/$(ARCH) 

$(asinstr)
  KBUILD_AFLAGS    += $(AFLAGS-y)
  KBUILD_CFLAGS    += $(call cc-option,-msoft-float)
  KBUILD_CFLAGS    += -pipe $(CFLAGS-y)
-CPP        = $(CC) -E $(KBUILD_CFLAGS)
  CHECKFLAGS    += -m$(BITS) -D__powerpc__ -D__powerpc$(BITS)__
  ifdef CONFIG_CPU_BIG_ENDIAN

-- 
Alexey