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

[Nathan Chancellor]

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@xxxxxxxxxx>
> 
> 
> 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.

Cheers,
Nathan