Re: Question about arch/x86/xen/mmu.c

["Jürgen Groß" <jgross@suse.com>]

On 12/04/2014 10:30 PM, Jan-Simon Moeller wrote:
> Hi !
>
> My name is Jan-Simon Moeller and I'm looking into compiling the kernel with
> LLVM/Clang (see llvm.linuxfoundation.org) .
>
> Right now we face this issue when compiling with clang:
>
>   CC      arch/x86/xen/mmu.o
> arch/x86/xen/mmu.c:1343:18: error: fields must have a constant size:
>       'variable length array in structure' extension will never be
>       supported
>                 DECLARE_BITMAP(mask, num_processors);
>                                ^
> include/linux/types.h:10:16: note: expanded from macro 'DECLARE_BITMAP'
>         unsigned long name[BITS_TO_LONGS(bits)]
>                       ^
> 1 error generated.
>
>
> Question to the experts: why can't we just use NR_CPUS and be done with it ?
> NR_CPUS will be setup by CONFIG_NR_CPUS and thus static.
> ( e.g. arch/x86/configs/x86_64_defconfig:CONFIG_NR_CPUS=64 )

This would expand the structure on kernels configured for many cpus
(e.g. 4096) but running on a smaller machine dramatically.


Juergen