Revert NR_CPUS=1 fix from 4.15 (was: Re: [PATCH] fix for_each_cpu() again for NR_CPUS=1)

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 31, 2021 at 04:52:47PM +0200, Jan Beulich wrote:
> Unfortunately aa50f45332f1 ("xen: fix for_each_cpu when NR_CPUS=1") has
> caused quite a bit of fallout with gcc10, e.g. (there are at least two
> more similar ones, and I didn't bother trying to find them all):
> 
> In file included from .../xen/include/xen/config.h:13,
>                  from <command-line>:
> core_parking.c: In function ‘core_parking_power’:
> .../xen/include/asm/percpu.h:12:51: error: array subscript 1 is above array 
> bounds of ‘long unsigned int[1]’ [-Werror=array-bounds]
>    12 |     (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
> .../xen/include/xen/compiler.h:141:29: note: in definition of macro 
> ‘RELOC_HIDE’
>   141 |     (typeof(ptr)) (__ptr + (off)); })
>       |                             ^~~
> core_parking.c:133:39: note: in expansion of macro ‘per_cpu’
>   133 |             core_tmp = cpumask_weight(per_cpu(cpu_core_mask, cpu));
>       |                                       ^~~~~~~
> In file included from .../xen/include/xen/percpu.h:4,
>                  from .../xen/include/asm/msr.h:7,
>                  from .../xen/include/asm/time.h:5,
>                  from .../xen/include/xen/time.h:76,
>                  from .../xen/include/xen/spinlock.h:4,
>                  from .../xen/include/xen/cpu.h:5,
>                  from core_parking.c:19:
> .../xen/include/asm/percpu.h:6:22: note: while referencing ‘__per_cpu_offset’
>     6 | extern unsigned long __per_cpu_offset[NR_CPUS];
>       |                      ^~~~~~~~~~~~~~~~

At this point, should be consider reverting the original fix from the
4.15 branch, so that we don't release something that's build broken
with gcc 10?

Roger.