Re: clang-nightly: vdso/compat_gettimeofday.h:152:15: error: instruction variant requires ARMv6 or later

[Sylvestre Ledru <sylvestre@debian.org>]

Hello,


Le 04/12/2023 à 23:33, Nathan Chancellor a écrit :
> On Mon, Dec 04, 2023 at 11:13:04AM -0700, Nathan Chancellor wrote:
>> Hi Naresh,
>>
>> On Mon, Dec 04, 2023 at 05:33:26PM +0530, Naresh Kamboju wrote:
>>> Following build errors noticed on Linux next-20231204 tag with clang-nightly
>>> for arm and arm64.
>>>
>>> ## Test Regressions (compared to next-20231201)
>>> * arm64, build
>>>    - clang-nightly-defconfig
>>>    - clang-nightly-defconfig-40bc7ee5
>>>    - clang-nightly-lkftconfig
>>>    - clang-nightly-lkftconfig-kselftest
>>>
>>> * arm, build
>>>    - clang-nightly-allnoconfig
>>>    - clang-nightly-axm55xx_defconfig
>>>    - clang-nightly-bcm2835_defconfig
>>>    - clang-nightly-clps711x_defconfig
>>>    - clang-nightly-defconfig
>>>    - clang-nightly-exynos_defconfig
>>>    - clang-nightly-imx_v6_v7_defconfig
>>>    - clang-nightly-keystone_defconfig
>>>    - clang-nightly-lkftconfig
>>>    - clang-nightly-lkftconfig-kselftest
>>>    - clang-nightly-omap2plus_defconfig
>>>    - clang-nightly-pxa910_defconfig
>>>    - clang-nightly-s3c6400_defconfig
>>>    - clang-nightly-s5pv210_defconfig
>>>    - clang-nightly-sama5_defconfig
>>>    - clang-nightly-shmobile_defconfig
>>>    - clang-nightly-tinyconfig
>>>    - clang-nightly-u8500_defconfig
>>>    - clang-nightly-vexpress_defconfig
>>>
>>>
>>> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
>>>
>>>
>>> Build log on arm64:
>>> ---------
>>> In file included from lib/vdso/gettimeofday.c:5:
>>> In file included from include/vdso/datapage.h:135:
>>> arch/arm64/include/asm/vdso/compat_gettimeofday.h:152:15: error:
>>> instruction variant requires ARMv6 or later
>>>    152 |         asm volatile("mov %0, %1" : "=r"(ret) : "r"(_vdso_data));
>>>        |                      ^
>>> <inline asm>:1:2: note: instantiated into assembly here
>>>      1 |         mov r4, r1
>>>        |         ^
>>> In file included from <built-in>:3:
>>> lib/vdso/gettimeofday.c:139:3: error: invalid instruction
>>>    139 |                 smp_rmb();
>>>        |                 ^
>>>
>>> Build log on arm:
>>> ---------
>>> In file included from arch/arm/vfp/vfpmodule.c:23:
>>> arch/arm/include/asm/cp15.h:101:2: error: instruction requires: data-barriers
>>>    101 |         isb();
>>>        |         ^
>> This is caused by a change to Debian's LLVM that changes the internal
>> defaults of the arm-linux-gnueabi and arm-linux-gnueabihf tuples:
>>
>> https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/commit/907baf024b9a5a1626893d9e731b6c79ccf45c87
>>
>> We use arm-linux-gnueabi for the kernel (see scripts/Makefile.clang) so
>> now we have a hardcoded armv5te CPU, even if we are building for armv7
>> or such.
>>
>> I am still investigating into what (if anything) can be done to resolve
>> this on the kernel side. We could potentially revert commit
>> ddc72c9659b5 ("kbuild: clang: do not use CROSS_COMPILE for target
>> triple") but I am not sure that will save us from that change, as
>> tuxmake's CROSS_COMPILE=arm-linux-gnueabihf will cause us to have an
>> armv7 CPU even though we may not be building for armv7.
> Okay, this is a pretty awful situation the more I look into it :(
>
> The arm64 compat vDSO build is easy enough to fix because we require use
> of the integrated assembler, which means we can add '-mcpu=generic' (the
> default in LLVM for those files based on my debugging) to those files
> and be done with it:
>
> diff --git a/arch/arm64/kernel/vdso32/Makefile b/arch/arm64/kernel/vdso32/Makefile
> index 1f911a76c5af..5f5cb722cfc2 100644
> --- a/arch/arm64/kernel/vdso32/Makefile
> +++ b/arch/arm64/kernel/vdso32/Makefile
> @@ -9,6 +9,10 @@ include $(srctree)/lib/vdso/Makefile
>   ifeq ($(CONFIG_CC_IS_CLANG), y)
>   CC_COMPAT ?= $(CC)
>   CC_COMPAT += --target=arm-linux-gnueabi
> +# Some distributions (such as Debian) change the default CPU for the
> +# arm-linux-gnueabi target triple, which can break the build. Explicitly set
> +# the CPU to generic, which is the default for Linux in LLVM.
> +CC_COMPAT += -mcpu=generic
>   else
>   CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
>   endif
>
> The failures for all the ARCH=arm configurations appear to be much more
> difficult to fix because the default CPU value changes based on the
> '-march' value, which basically means that we would have to hardcode
> LLVM's default CPU logic into the kernel's Makefile, which is just not
> maintainable in my opinion. Just doing a multi_v7_defconfig build of
> arch/arm/ shows the value returned from ARM::getARMCPUForArch() in
> llvm/lib/TargetParser/ARMTargetParser.cpp can vary between "arm7tdmi" or
> "generic". Supplying '-mcpu=generic' explicitly won't work with
> LLVM_IAS=0 because GNU as does not support it and clang just happily
> passes it along, even though it does not do that in the implicit default
> case.
>
> Sylvestre, I strongly believe you should consider reverting that change
> or give us some compiler flag that allows us to fallback to upstream
> LLVM's default CPU selection logic. I think that hardcoding Debian's
> architecture defintions based on the target triple into the compiler
> could cause issues for other projects as well. For example,
> '--target=arm-linux-gnueabi -march=armv7-a' won't actually target ARMv7:
>
>    $ echo 'int main(void) { asm("dsb"); return 0; }' | \
>          clang --target=arm-linux-gnueabi -march=armv7-a \
>          -x c -c -o /dev/null -v -
>    ...
>     "/usr/bin/clang-17" -cc1 -triple armv7-unknown-linux-gnueabi ...
>    ...
>
> vs.
>
>    $ echo 'int main(void) { asm("dsb"); return 0; }' | \
>          clang --target=arm-linux-gnueabi -march=armv7-a \
>          -x c -c -o /dev/null -v -
>    ...
>    "<prefix>/bin/clang-18" -cc1 -triple armv5e-unknown-linux-gnueabi ...
>    ...
>
I guess it is this patch, right?

https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/commit/97633b6d51ebc8579c5dbecd12a02fb933620620

if so, do you want me to revert it?

Thanks
S


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