On Mon, Dec 17, 2018 at 01:23:52PM -0500, Nicolas Pitre wrote:
> On Sat, 15 Dec 2018, Nathan Chancellor wrote:
>
> > While building arm32 allyesconfig, I ran into the following errors:
> >
> > arch/arm/lib/xor-neon.c:17:2: error: You should compile this file with
> > '-mfloat-abi=softfp -mfpu=neon'
> >
> > In file included from lib/raid6/neon1.c:27:
> > /home/nathan/cbl/prebuilt/lib/clang/8.0.0/include/arm_neon.h:28:2:
> > error: "NEON support not enabled"
> >
> > Building V=1 showed NEON_FLAGS getting passed along to Clang but
> > __ARM_NEON__ was not getting defined. Ultimately, it boils down to Clang
> > only defining __ARM_NEON__ when targeting armv7, rather than armv6k,
> > which is the '-march' value for allyesconfig.
> >
> > From lib/Basic/Targets/ARM.cpp in the Clang source:
> >
> > // This only gets set when Neon instructions are actually available, unlike
> > // the VFP define, hence the soft float and arch check. This is subtly
> > // different from gcc, we follow the intent which was that it should be set
> > // when Neon instructions are actually available.
> > if ((FPU & NeonFPU) && !SoftFloat && ArchVersion >= 7) {
> > Builder.defineMacro("__ARM_NEON", "1");
> > Builder.defineMacro("__ARM_NEON__");
> > // current AArch32 NEON implementations do not support double-precision
> > // floating-point even when it is present in VFP.
> > Builder.defineMacro("__ARM_NEON_FP",
> > "0x" + Twine::utohexstr(HW_FP & ~HW_FP_DP));
> > }
> >
> > Ard Biesheuvel recommended explicitly adding '-march=armv7-a' at the
> > beginning of the NEON_FLAGS definitions so that __ARM_NEON__ always gets
> > definined by Clang. This doesn't functionally change anything because
> > that code will only run where NEON is supported, which is implicitly
> > armv7.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/287
> > Suggested-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
>
> Did you test that this doesn't create issues with gcc e.g. complaints
> from the linker that objects have incompatible architecture
> specifications or similar annoyance? This already happened in the past
> but I forget the exact scenario. If you already did, or after you do
> validate with gcc as well, then you may add:
>
> Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
>
>
Hi Nicolas,
I was 99% sure that I checked GCC before sending this but I just did
another run to confirm and everything links successfully. We still use
binutils for assembling/linking the kernel so I assume that I would have
seen a warning from ld.bfd even with Clang.
Thank you for the review!
Nathan
>
> > ---
> > Documentation/arm/kernel_mode_neon.txt | 4 ++--
> > arch/arm/lib/Makefile | 2 +-
> > arch/arm/lib/xor-neon.c | 2 +-
> > lib/raid6/Makefile | 2 +-
> > 4 files changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/Documentation/arm/kernel_mode_neon.txt b/Documentation/arm/kernel_mode_neon.txt
> > index 525452726d31..b9e060c5b61e 100644
> > --- a/Documentation/arm/kernel_mode_neon.txt
> > +++ b/Documentation/arm/kernel_mode_neon.txt
> > @@ -6,7 +6,7 @@ TL;DR summary
> > * Use only NEON instructions, or VFP instructions that don't rely on support
> > code
> > * Isolate your NEON code in a separate compilation unit, and compile it with
> > - '-mfpu=neon -mfloat-abi=softfp'
> > + '-march=armv7-a -mfpu=neon -mfloat-abi=softfp'
> > * Put kernel_neon_begin() and kernel_neon_end() calls around the calls into your
> > NEON code
> > * Don't sleep in your NEON code, and be aware that it will be executed with
> > @@ -87,7 +87,7 @@ instructions appearing in unexpected places if no special care is taken.
> > Therefore, the recommended and only supported way of using NEON/VFP in the
> > kernel is by adhering to the following rules:
> > * isolate the NEON code in a separate compilation unit and compile it with
> > - '-mfpu=neon -mfloat-abi=softfp';
> > + '-march=armv7-a -mfpu=neon -mfloat-abi=softfp';
> > * issue the calls to kernel_neon_begin(), kernel_neon_end() as well as the calls
> > into the unit containing the NEON code from a compilation unit which is *not*
> > built with the GCC flag '-mfpu=neon' set.
> > diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
> > index ad25fd1872c7..0bff0176db2c 100644
> > --- a/arch/arm/lib/Makefile
> > +++ b/arch/arm/lib/Makefile
> > @@ -39,7 +39,7 @@ $(obj)/csumpartialcopy.o: $(obj)/csumpartialcopygeneric.S
> > $(obj)/csumpartialcopyuser.o: $(obj)/csumpartialcopygeneric.S
> >
> > ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
> > - NEON_FLAGS := -mfloat-abi=softfp -mfpu=neon
> > + NEON_FLAGS := -march=armv7-a -mfloat-abi=softfp -mfpu=neon
> > CFLAGS_xor-neon.o += $(NEON_FLAGS)
> > obj-$(CONFIG_XOR_BLOCKS) += xor-neon.o
> > endif
> > diff --git a/arch/arm/lib/xor-neon.c b/arch/arm/lib/xor-neon.c
> > index a6741a895189..4600b62d845f 100644
> > --- a/arch/arm/lib/xor-neon.c
> > +++ b/arch/arm/lib/xor-neon.c
> > @@ -14,7 +14,7 @@
> > MODULE_LICENSE("GPL");
> >
> > #ifndef __ARM_NEON__
> > -#error You should compile this file with '-mfloat-abi=softfp -mfpu=neon'
> > +#error You should compile this file with '-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
> > #endif
> >
> > /*
> > diff --git a/lib/raid6/Makefile b/lib/raid6/Makefile
> > index 2f8b61dfd9b0..bfec7c87c61e 100644
> > --- a/lib/raid6/Makefile
> > +++ b/lib/raid6/Makefile
> > @@ -25,7 +25,7 @@ endif
> > ifeq ($(CONFIG_KERNEL_MODE_NEON),y)
> > NEON_FLAGS := -ffreestanding
> > ifeq ($(ARCH),arm)
> > -NEON_FLAGS += -mfloat-abi=softfp -mfpu=neon
> > +NEON_FLAGS += -march=armv7-a -mfloat-abi=softfp -mfpu=neon
> > endif
> > CFLAGS_recov_neon_inner.o += $(NEON_FLAGS)
> > ifeq ($(ARCH),arm64)
> > --
> > 2.20.1
> >
> >
