cpu: Introduce sanity checks when switching between CPUs

This patch introduces the following sanity checks when switching
between CPUs:

 * Check that the set of new and old CPUs do not overlap. Having an
   overlap between the set of new CPUs and the set of old CPUs is
   currently not supported. Doing such a switch used to result in the
   following assertion error:
     BaseCPU::takeOverFrom(BaseCPU*): \
       Assertion `!new_itb_port->isConnected()' failed.

 * Check that all new CPUs are in the switched out state.

 * Check that all old CPUs are in the switched in state.

src/python/m5/simulate.py

@@ -228,11 +228,21 @@ def switchCpus(cpuList):
         if not isinstance(item, tuple) or len(item) != 2:
             raise RuntimeError, "List must have tuples of (oldCPU,newCPU)"
 
+    old_cpu_set = set([old_cpu for old_cpu, new_cpu in cpuList])
     for old_cpu, new_cpu in cpuList:
         if not isinstance(old_cpu, objects.BaseCPU):
             raise TypeError, "%s is not of type BaseCPU" % old_cpu
         if not isinstance(new_cpu, objects.BaseCPU):
             raise TypeError, "%s is not of type BaseCPU" % new_cpu
+        if new_cpu in old_cpu_set:
+            raise RuntimeError, \
+                "New CPU (%s) is in the list of old CPUs." % (old_cpu,)
+        if not new_cpu.switchedOut():
+            raise RuntimeError, \
+                "New CPU (%s) is already active." % (new_cpu,)
+        if old_cpu.switchedOut():
+            raise RuntimeError, \
+                "Old CPU (%s) is inactive." % (new_cpu,)
 
     # Now all of the CPUs are ready to be switched out
     for old_cpu, new_cpu in cpuList: