xen: sched: simplify ACPI S3 resume path.

In fact, when domains are being unpaused:
 - it's not necessary to put the vcpu to sleep, as
   they are all already paused;
 - it is not necessary to call vcpu_migrate(), as
   the vcpus are still paused, and therefore won't
   wakeup anyway.

Basically, the only important thing is to call
pick_cpu, to let the scheduler run and figure out
what would be the best initial placement (i.e., the
value stored in v->processor), for the vcpus, as
they come back up, one after another.

Note that this is consistent with what was happening
before this change, as vcpu_migrate() calls pick_cpu.
But much simpler and quicker.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>

xen/common/schedule.c

@@ -635,7 +635,11 @@ void restore_vcpu_affinity(struct domain *d)
 
     for_each_vcpu ( d, v )
     {
-        spinlock_t *lock = vcpu_schedule_lock_irq(v);
+        spinlock_t *lock;
+
+        ASSERT(!vcpu_runnable(v));
+
+        lock = vcpu_schedule_lock_irq(v);
 
         if ( v->affinity_broken )
         {
@@ -659,17 +663,11 @@ void restore_vcpu_affinity(struct domain *d)
                     cpupool_domain_cpumask(v->domain));
         v->processor = cpumask_any(cpumask_scratch_cpu(cpu));
 
-        if ( v->processor == cpu )
-        {
-            set_bit(_VPF_migrating, &v->pause_flags);
-            spin_unlock_irq(lock);;
-            vcpu_sleep_nosync(v);
-            vcpu_migrate(v);
-        }
-        else
-        {
-            spin_unlock_irq(lock);
-        }
+        spin_unlock_irq(lock);;
+
+        lock = vcpu_schedule_lock_irq(v);
+        v->processor = SCHED_OP(VCPU2OP(v), pick_cpu, v);
+        spin_unlock_irq(lock);
     }
 
     domain_update_node_affinity(d);