need -Qunused-arguments with -fuse-lld for cc-option tests

[nickdesaulniers]

When trying LLD with x86, I was seeing:

$ make CC=clang HOSTCC=clang LD=ld.lld -j46
scripts/kconfig/conf  --syncconfig Kconfig
You are building kernel with non-retpoline compiler.
Please update your compiler.
arch/x86/Makefile:298: recipe for target 'checkbin' failed

Looks like __cc-option adds -Werror which breaks the cc-option for the RETPOLINE_CFLAGS. ex:

$ clang -fuse-ld=ld.lld -x c -c /dev/null
clang-9: warning: argument unused during compilation: '-fuse-ld=lld' [-Wunused-command-line-argument]
$ clang -fuse-ld=ld.lld -x c -c /dev/null -Werror
clang-9: error: argument unused during compilation: '-fuse-ld=lld' [-Werror,-Wunused-command-line-argument]
$ clang -fuse-ld=ld.lld -x c -c /dev/null -Werror -Qunused-arguments
$

diff --git a/Makefile b/Makefile
index 8011555745aa..84c2335e8e4a 100644
--- a/Makefile
+++ b/Makefile
@@ -501,7 +501,7 @@ CLANG_FLAGS += --gcc-toolchain=$(GCC_TOOLCHAIN)
 endif
 CLANG_FLAGS    += -no-integrated-as
 ifneq ($(shell $(LD) --version 2>&1 | head -n 1 | grep LLD),)
-CLANG_FLAGS    += -fuse-ld=lld
+CLANG_FLAGS    += -fuse-ld=lld -Qunused-arguments
 endif
 KBUILD_CFLAGS  += $(CLANG_FLAGS)
 KBUILD_AFLAGS  += $(CLANG_FLAGS)

[nathanchance]

Turns out this fixes #357.

I think that the -Qunused-arguments line can just be moved up because KBUILD_CPPFLAGS is supplied to __cc-option via cc-option: https://github.com/torvalds/linux/blob/v5.0-rc6/scripts/Kbuild.include#L119-L120

diff --git a/Makefile b/Makefile
index 28728a394362..cfaef110bb33 100644
--- a/Makefile
+++ b/Makefile
@@ -509,6 +509,7 @@ endif
 KBUILD_CFLAGS  += $(CLANG_FLAGS)
 KBUILD_AFLAGS  += $(CLANG_FLAGS)
 export CLANG_FLAGS
+KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 endif
 
 RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
@@ -703,7 +704,6 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 KBUILD_CFLAGS += $(stackp-flags-y)
 
 ifdef CONFIG_CC_IS_CLANG
-KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
 KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
 KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
 KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)

[nickdesaulniers]

Does -Qunused-arguments work as a CPPFLAG? (I had it as a CFLAG). If so, we don't need to wrap it in a cc-option call there, since I'm pretty sure that block is clang only.

[nathanchance]

Does -Qunused-arguments work as a CPPFLAG?

It should, as I believe it has been there since Clang support was first added.

If so, we don't need to wrap it in a cc-option call there, since I'm pretty sure that block is clang only.

Yeah, I was thinking the same thing. I assume there was good reason for wrapping it initially (maybe an older version of Clang doesn't support it?) but given that I wouldn't recommend anything less than clang 7 right now, I think we'd be fine to unwrap it.

[nickdesaulniers]

Oh, I remembered now why I didn't move -Qunused-arguments. When not using LLD, we still want that flag; your approach of using it with clang regardless of linker is better than my initial suggestion.

[nickdesaulniers]

Just hit this again internally when trying to build KASAN builds.

[dileks]

Isn't this Clang specific, so do it unconditionally and pass to both KBUILD_CFLAGS and KBUILD_CPPFLAGS (by not knowing if you can pass -no-integrated-as to KBUILD_CPPFLAGS)?

CLANG_FLAGS	+= -no-integrated-as
CLANG_FLAGS	+= -Qunused-arguments
KBUILD_CFLAGS	+= $(CLANG_FLAGS)
KBUILD_AFLAGS	+= $(CLANG_FLAGS)
KBUILD_CPPFLAGS += $(CLANG_FLAGS)
export CLANG_FLAGS

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Makefile#n502

[dileks]

Or like this:

CLANG_FLAGS	+= -no-integrated-as
KBUILD_CFLAGS	+= $(CLANG_FLAGS)
KBUILD_AFLAGS	+= $(CLANG_FLAGS)
export CLANG_FLAGS
KBUILD_CFLAGS   += -Qunused-arguments
KBUILD_CPPFLAGS += -Qunused-arguments

[nathanchance]

I agree that we can probably drop cc-option but I think that Qunused-arguments shouldn't be added to KBUILD_CFLAGS otherwise we won't catch issues like #307.

[dileks]

Agreed.
Will this be part of the lld patchset (was sent by @nickdesaulniers to linux-kbuild ML)?

[dileks]

See this one how import KBUILD_CPPFLAGS are:

"ftrace: Build with CPPFLAGS to get -Qunused-arguments"

[1] https://git.kernel.org/linus/5a4630aadb9a9525474e9ac92965829f990cb5c4

[nickdesaulniers]

Will this be part of the lld patchset

I will include it in the next revision I send. Sorry for a quiet week; performance review season at work right now. ☹️ 🔫

[dileks]

I was able to use LLD as linker:

KConfig (will try with enabled later):

scripts/config -d CRYPTO_SHA512_SSSE3

Patchset:

$ git log --oneline --no-merges origin/HEAD..
8ad71b67b524 (for-5.0/lld-nickd-v2) Makefile: lld: set -O2 linker flag when linking with LLD
074326c6337c Makefile: lld: tell clang to use lld
b25bec66b4fd Makefile: clang: choose GCC_TOOLCHAIN_DIR not on LD
0112a3cb6e75 init/Kconfig: add config support for detecting linker
da5f007eb18e (for-5.0/lld-dileks) kbuild: clang: lld: Move -Qunused-arguments CPPFLAGS
ceb8873a6687 (for-5.1/scsi-fcoe) scsi: fcoe: make use of fip_mode enum complete
aadf16734696 (for-5.0/kvm-x86-cbl-asmgoto) KVM: x86: asm-goto: Workaround CBL issue #315 and #316
0670525ded48 (for-5.0/x86-cbl-asmgoto) WIP: asm goto: x86: convert some __always_inline functions to macros

Plus this diff:

$ git diff
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 0d618ee634ac..ee3b5c7d662e 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -401,7 +401,7 @@ SECTIONS
  * Per-cpu symbols which need to be offset from __per_cpu_load
  * for the boot processor.
  */
-#define INIT_PER_CPU(x) init_per_cpu__##x = x + __per_cpu_load
+#define INIT_PER_CPU(x) init_per_cpu__##x = ABSOLUTE(x) + __per_cpu_load
 INIT_PER_CPU(gdt_page);
 INIT_PER_CPU(irq_stack_union);
 
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 3d7a6a9c2370..f687c834810c 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -733,7 +733,7 @@
                KEEP(*(.orc_unwind_ip))                                 \
                __stop_orc_unwind_ip = .;                               \
        }                                                               \
-       . = ALIGN(6);                                                   \
+       . = ALIGN(8);                                                   \
        .orc_unwind : AT(ADDR(.orc_unwind) - LOAD_OFFSET) {             \
                __start_orc_unwind = .;                                 \
                KEEP(*(.orc_unwind))                                    \

Attached patch inspired by #366.

kbuild-clang-lld-Move-Qunused-arguments-CPPFLAGS-patch.txt

[nickdesaulniers]

submitted: https://lore.kernel.org/r/20190402070858.49597-1-ndesaulniers@google.com/

[nickdesaulniers]

Will follow up with #342