[PATCH] x86: avoid wrong use of all-but-self IPI shorthand

[Jan Beulich <jbeulich@xxxxxxxx>]

With "nosmp" I did observe a flood of "APIC error on CPU0: 04(04), Send
accept error" log messages on an AMD system. And rightly so - nothing
excludes the use of the shorthand in send_IPI_mask() in this case. Set
"unaccounted_cpus" to "true" also when command line restrictions are the
cause.

Note that PV-shim mode is unaffected by this change, first and foremost
because "nosmp" and "maxcpus=" are ignored in this case.

Fixes: 5500d265a2a8 ("x86/smp: use APIC ALLBUT destination shorthand when 
possible")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While in "nosmp" mode it's probably benign that we switch to the bigsmp
APIC driver simply because there are more than 8 physical CPUs, I
suppose that's inefficient when "maxcpus=" with a value between 2 and 8
(inclusive) is in use. Question is whether that's worthwhile to find a
solution for.

--- a/xen/arch/x86/mpparse.c
+++ b/xen/arch/x86/mpparse.c
@@ -84,9 +84,14 @@ void __init set_nr_cpu_ids(unsigned int
        if (!park_offline_cpus)
                tot_cpus = max_cpus;
        nr_cpu_ids = min(tot_cpus, NR_CPUS + 0u);
-       if (park_offline_cpus && nr_cpu_ids < num_processors)
-               printk(XENLOG_WARNING "SMP: Cannot bring up %u further CPUs\n",
-                      num_processors - nr_cpu_ids);
+       if (nr_cpu_ids < num_processors)
+       {
+               unaccounted_cpus = true;
+               if (park_offline_cpus)
+                       printk(XENLOG_WARNING
+                              "SMP: Cannot bring up %u further CPUs\n",
+                              num_processors - nr_cpu_ids);
+       }
 
 #ifndef nr_cpumask_bits
        nr_cpumask_bits = ROUNDUP(nr_cpu_ids, BITS_PER_LONG);