linux-kernel - Re: [PATCH] selftests: default to host arch for LLVM builds

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Message-ID: <20240428120806.19275-1-kernel@valentinobst.de>
Date: Sun, 28 Apr 2024 14:08:06 +0200
From: Valentin Obst <kernel@...entinobst.de>
To: jhubbard@...dia.com
Cc: anders.roxell@...aro.org,
	bpoirier@...dia.com,
	broonie@...nel.org,
	guillaume.tucker@...labora.com,
	kernel@...entinobst.de,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	mpdesouza@...e.com,
	nathan@...nel.org,
	sashal@...nel.org,
	shuah@...nel.org
Subject: Re: [PATCH] selftests: default to host arch for LLVM builds

> > Align the behavior for gcc and clang builds by interpreting unset
> > `ARCH` and `CROSS_COMPILE` variables in `LLVM` builds as a sign that the
> > user wants to build for the host architecture.
> >
> > This patch preserves the properties that setting the `ARCH` variable to an
> > unknown value will trigger an error that complains about insufficient
> > information, and that a set `CROSS_COMPILE` variable will override the
> > target triple that is determined based on presence/absence of `ARCH`.
> >
> > When compiling with clang, i.e., `LLVM` is set, an unset `ARCH` variable in
> > combination with an unset `CROSS_COMPILE` variable, i.e., compiling for
> > the host architecture, leads to compilation failures since `lib.mk` can
> > not determine the clang target triple. In this case, the following error
> > message is displayed for each subsystem that does not set `ARCH` in its
> > own Makefile before including `lib.mk` (lines wrapped at 75 chrs):
> >
> >    make[1]: Entering directory '/mnt/build/linux/tools/testing/selftests/
> >     sysctl'
> >    ../lib.mk:33: *** Specify CROSS_COMPILE or add '--target=' option to
> >     lib.mk.  Stop.
> >    make[1]: Leaving directory '/mnt/build/linux/tools/testing/selftests/
> >     sysctl'
>
> Thanks for fixing this.
>
> And yes, the selftests "normal" (non-cross-compile) build is *broken*
> right now, for clang. I didn't realize from the patch title that this is
> actually a significant fix. Maybe we should change the subject line (patch
> title) to something like:
>
>     [PATCH] selftests: fix the clang build: default to host arch for LLVM builds

Yes, I agree that the title should contain the word 'fix' somewhere. For
me its okay if maintainers reword the title when applying the patch,
alternatively I can send a v2. (Is it still a v2 if I change the title, or
rather a new patch?).

Any thoughts on whether this also needs a 'Cc stable'? Its not quite
clear to me if this fix meets the requirements. As above, no objections if
maintainers should decide to add it.

>
> ?
>
> Just a thought. The "Fixes:" tag covers it already, I realize.
>
> Anyway, this looks correct, and fixes that aspect of the build for me, so
> either way, please feel free to add:
>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>

Thanks!

	- Best Valentin
>
>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > In the same scenario a gcc build would default to the host architecture,
> > i.e., it would use plain `gcc`.
> >
> > Fixes: 795285ef2425 ("selftests: Fix clang cross compilation")
> > Reviewed-by: Mark Brown <broonie@...nel.org>
> > Signed-off-by: Valentin Obst <kernel@...entinobst.de>
> > ---
> > I am not entirely sure whether this behavior is in fact known and intended
> > and whether the way to obtain the host target triple is sufficiently
> > general. The flag was introduced in llvm-8 with [1], it will be an error in
> > older clang versions.
> >
> > The target triple you get with `-print-target-triple` may not be the
> > same that you would get when explicitly setting ARCH to you host
> > architecture. For example on my x86_64 system it get
> > `x86_64-pc-linux-gnu` instead of `x86_64-linux-gnu`, similar deviations
> > were observed when testing other clang binaries on compiler-explorer,
> > e.g., [2].
> >
> > An alternative could be to simply do:
> >
> >        ARCH ?= $(shell uname -m)
> >
> > before using it to select the target. Possibly with some post processing,
> > but at that point we would likely be replicating `scripts/subarch.include`.
> > This is what some subsystem Makefiles do before including `lib.mk`. This
> > change might make it possible to remove the explicit setting of `ARCH` from
> > the few subsystem Makefiles that do it.
> >
> > [1]: https://reviews.llvm.org/D50755
> > [2]: https://godbolt.org/z/r7Gn9bvv1
> >
> > Changes in v1:
> > - Shortened commit message.
> > - Link to RFC: https://lore.kernel.org/r/20240303-selftests-libmk-llvm-rfc-v1-1-9ab53e365e31@valentinobst.de
> > ---
> >   tools/testing/selftests/lib.mk | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
> > index da2cade3bab0..8ae203d8ed7f 100644
> > --- a/tools/testing/selftests/lib.mk
> > +++ b/tools/testing/selftests/lib.mk
> > @@ -7,6 +7,8 @@ else ifneq ($(filter -%,$(LLVM)),)
> >   LLVM_SUFFIX := $(LLVM)
> >   endif
> >
> > +CLANG := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
> > +
> >   CLANG_TARGET_FLAGS_arm          := arm-linux-gnueabi
> >   CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu
> >   CLANG_TARGET_FLAGS_hexagon      := hexagon-linux-musl
> > @@ -18,7 +20,13 @@ CLANG_TARGET_FLAGS_riscv        := riscv64-linux-gnu
> >   CLANG_TARGET_FLAGS_s390         := s390x-linux-gnu
> >   CLANG_TARGET_FLAGS_x86          := x86_64-linux-gnu
> >   CLANG_TARGET_FLAGS_x86_64       := x86_64-linux-gnu
> > -CLANG_TARGET_FLAGS              := $(CLANG_TARGET_FLAGS_$(ARCH))
> > +
> > +# Default to host architecture if ARCH is not explicitly given.
> > +ifeq ($(ARCH),)
> > +CLANG_TARGET_FLAGS := $(shell $(CLANG) -print-target-triple)
> > +else
> > +CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(ARCH))
> > +endif
> >
> >   ifeq ($(CROSS_COMPILE),)
> >   ifeq ($(CLANG_TARGET_FLAGS),)
> > @@ -30,7 +38,7 @@ else
> >   CLANG_FLAGS     += --target=$(notdir $(CROSS_COMPILE:%-=%))
> >   endif # CROSS_COMPILE
> >
> > -CC := $(LLVM_PREFIX)clang$(LLVM_SUFFIX) $(CLANG_FLAGS) -fintegrated-as
> > +CC := $(CLANG) $(CLANG_FLAGS) -fintegrated-as
> >   else
> >   CC := $(CROSS_COMPILE)gcc
> >   endif # LLVM
> >
> > ---
> > base-commit: 4cece764965020c22cff7665b18a012006359095
> > change-id: 20240303-selftests-libmk-llvm-rfc-5fe3cfa9f094
> >
> > Best regards,
> > --
> > Valentin Obst <kernel@...entinobst.de>