BUILD_BUG_ON(!__builtin_constant_p(feature)) breaks bcc trace tool

[Naveen N. Rao]

On 2017/01/24 04:36PM, Michael Ellerman wrote:
> Anton Blanchard <anton at samba.org> writes:
> > We added:
> >
> > BUILD_BUG_ON(!__builtin_constant_p(feature)) 
> >
> > to cpu_has_feature() and mmu_has_feature() in order to catch usage
> > issues (such as cpu_has_feature(cpu_has_feature(X)). Unfortunately LLVM
> > isn't smart enough to resolve this, and it errors out.
> >
> > I work around it in my clang/LLVM builds of the kernel, but I have just
> > discovered that it causes a lot of issues for the bcc (eBPF) trace tool
> > (which uses LLVM).
> 
> I didn't understand that part, but Aneesh explained to me that it's
> because bcc pulls in the kernel-internal headers.
> 
> I guess as a quick fix we just have to #ifdef it, can you confirm this
> works?

This works and solves the issue for me. In the absence of a better 
approach, can you please push this for v4.10?

Tested-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>

Thanks,
Naveen

> 
> 
> diff --git a/arch/powerpc/include/asm/cpu_has_feature.h b/arch/powerpc/include/asm/cpu_has_feature.h
> index b312b152461b..6e834caa3720 100644
> --- a/arch/powerpc/include/asm/cpu_has_feature.h
> +++ b/arch/powerpc/include/asm/cpu_has_feature.h
> @@ -23,7 +23,9 @@ static __always_inline bool cpu_has_feature(unsigned long feature)
>  {
>  	int i;
> 
> +#ifndef __clang__ /* clang can't cope with this */
>  	BUILD_BUG_ON(!__builtin_constant_p(feature));
> +#endif
> 
>  #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
>  	if (!static_key_initialized) {
> diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
> index a34c764ca8dd..233a7e8cc8e3 100644
> --- a/arch/powerpc/include/asm/mmu.h
> +++ b/arch/powerpc/include/asm/mmu.h
> @@ -160,7 +160,9 @@ static __always_inline bool mmu_has_feature(unsigned long feature)
>  {
>  	int i;
> 
> +#ifndef __clang__ /* clang can't cope with this */
>  	BUILD_BUG_ON(!__builtin_constant_p(feature));
> +#endif
> 
>  #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
>  	if (!static_key_initialized) {
> 
> 
> cheers
>