[PATCH] xen: fix for_each_cpu when NR_CPUS=1

[Dario Faggioli <dfaggioli@xxxxxxxx>]

When running an hypervisor build with NR_CPUS=1 for_each_cpu does not
take into account whether the bit of the CPU is set or not in the
provided mask.

This means that whatever we have in the bodies of these loops is always
done once, even if the mask was empty and it should never be done. This
is clearly a bug and was in fact causing an assert to trigger in credit2
code.

Removing the special casing of NR_CPUS == 1 makes things work again.

Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
Cc: Ian Jackson <iwj@xxxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Julien Grall <julien@xxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Wei Liu <wl@xxxxxxx>
---
I'm not really sure whether this should be 4.15 material.

It's definitely a bug, IMO. The risk is also pretty low, considering
that no one should really run Xen in this configuration (NR_CPUS=1, I
mean). Which is also the reason why it's probably not really important
that we fix it at this point of the release cycle.
---
 xen/include/xen/cpumask.h |    5 -----
 1 file changed, 5 deletions(-)

diff --git a/xen/include/xen/cpumask.h b/xen/include/xen/cpumask.h
index 256b60b106..e69589fc08 100644
--- a/xen/include/xen/cpumask.h
+++ b/xen/include/xen/cpumask.h
@@ -368,15 +368,10 @@ 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) < 1; (cpu)++, (void)(mask))
-#endif /* NR_CPUS */
 
 /*
  * The following particular system cpumasks and operations manage