[xen stable-4.15] fix for_each_cpu() again for NR_CPUS=1

[patchbot@xxxxxxx]

commit 5788a7e61145d0ad4f93d3ff22f7d6b5dfef7478
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Apr 20 11:56:11 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 20 11:56:11 2021 +0200

    fix for_each_cpu() again for NR_CPUS=1
    
    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 â??core_parking_powerâ??:
    .../xen/include/asm/percpu.h:12:51: error: array subscript 1 is above array 
bounds of â??long unsigned int[1]â?? [-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 
â??RELOC_HIDEâ??
      141 |     (typeof(ptr)) (__ptr + (off)); })
          |                             ^~~
    core_parking.c:133:39: note: in expansion of macro â??per_cpuâ??
      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 
â??__per_cpu_offsetâ??
        6 | extern unsigned long __per_cpu_offset[NR_CPUS];
          |                      ^~~~~~~~~~~~~~~~
    
    One of the further errors even went as far as claiming that an array
    index (range) of [0, 0] was outside the bounds of a [1] array, so
    something fishy is pretty clearly going on there.
    
    The compiler apparently wants to be able to see that the loop isn't
    really a loop in order to avoid triggering such warnings, yet what
    exactly makes it consider the loop exit condition constant and within
    the [0, 1] range isn't obvious - using ((mask)->bits[0] & 1) instead of
    cpumask_test_cpu() for example did _not_ help.
    
    Re-instate a special form of for_each_cpu(), experimentally "proven" to
    avoid the diagnostics.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Dario Faggioli <dfaggioli@xxxxxxxx>
    master commit: 625faf9f002bd6ff4b6457a016b8ff338223b659
    master date: 2021-04-07 12:24:45 +0200
---
 xen/include/xen/cpumask.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/include/xen/cpumask.h b/xen/include/xen/cpumask.h
index e69589fc08..9826707909 100644
--- a/xen/include/xen/cpumask.h
+++ b/xen/include/xen/cpumask.h
@@ -368,10 +368,15 @@ static inline void free_cpumask_var(cpumask_var_t mask)
 #define FREE_CPUMASK_VAR(m) free_cpumask_var(m)
 #endif
 
+#if NR_CPUS > 1
 #define for_each_cpu(cpu, mask)                        \
        for ((cpu) = cpumask_first(mask);       \
             (cpu) < nr_cpu_ids;                \
             (cpu) = cpumask_next(cpu, mask))
+#else /* NR_CPUS == 1 */
+#define for_each_cpu(cpu, mask)                        \
+       for ((cpu) = 0; (cpu) < cpumask_test_cpu(0, mask); ++(cpu))
+#endif /* NR_CPUS */
 
 /*
  * The following particular system cpumasks and operations manage
--
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