sim: Move the draining interface into a separate base class

This patch moves the draining interface from SimObject to a separate
class that can be used by any object needing draining. However,
objects not visible to the Python code (i.e., objects not deriving
from SimObject) still depend on their parents informing them when to
drain. This patch also gets rid of the CountedDrainEvent (which isn't
really an event) and replaces it with a DrainManager.

1 files changed, 9 insertions, 102 deletions

diff --git a/src/sim/sim_object.hh b/src/sim/sim_object.hh
index c1238e23f..f04289e2f 100644
--- a/src/sim/sim_object.hh
+++ b/src/sim/sim_object.hh
@@ -45,6 +45,7 @@
 
 #include "enums/MemoryMode.hh"
 #include "params/SimObject.hh"
+#include "sim/drain.hh"
 #include "sim/eventq.hh"
 #include "sim/serialize.hh"
 
@@ -72,40 +73,7 @@ class Event;
  *     </ul>
  * <li>SimObject::resetStats()
  * <li>SimObject::startup()
- * <li>SimObject::resume() if resuming from a checkpoint.
- * </ol>
- *
- * An object's internal state needs to be drained when creating a
- * checkpoint, switching between CPU models, or switching between
- * timing models. Once the internal state has been drained from
- * <i>all</i> objects in the system, the objects are serialized to
- * disc or the configuration change takes place. The process works as
- * follows (see simulate.py for details):
- *
- * <ol>
- * <li>An instance of a CountedDrainEvent is created to keep track of
- *     how many objects need to be drained. The object maintains an
- *     internal counter that is decreased every time its
- *     CountedDrainEvent::process() method is called. When the counter
- *     reaches zero, the simulation is stopped.
- *
- * <li>Call SimObject::drain() for every object in the
- *     system. Draining has completed if all of them return
- *     zero. Otherwise, the sum of the return values is loaded into
- *     the counter of the CountedDrainEvent. A pointer of the drain
- *     event is passed as an argument to the drain() method.
- *
- * <li>Continue simulation. When an object has finished draining its
- *     internal state, it calls CountedDrainEvent::process() on the
- *     CountedDrainEvent. When counter in the CountedDrainEvent reaches
- *     zero, the simulation stops.
- *
- * <li>Check if any object still needs draining, if so repeat the
- *     process above.
- *
- * <li>Serialize objects, switch CPU model, or change timing model.
- *
- * <li>Call SimObject::resume() and continue the simulation.
+ * <li>Drainable::drainResume() if resuming from a checkpoint.
  * </ol>
  *
  * @note Whenever a method is called on all objects in the simulator's
@@ -114,42 +82,8 @@ class Event;
  * SimObject.py). This has the effect of calling the method on the
  * parent node <i>before</i> its children.
  */
-class SimObject : public EventManager, public Serializable
+class SimObject : public EventManager, public Serializable, public Drainable
 {
-  public:
-    /**
-     * Object drain/handover states
-     *
-     * An object starts out in the Running state. When the simulator
-     * prepares to take a snapshot or prepares a CPU for handover, it
-     * calls the drain() method to transfer the object into the
-     * Draining or Drained state. If any object enters the Draining
-     * state (drain() returning >0), simulation continues until it all
-     * objects have entered the Drained.
-     *
-     * The before resuming simulation, the simulator calls resume() to
-     * transfer the object to the Running state.
-     *
-     * \note Even though the state of an object (visible to the rest
-     * of the world through getState()) could be used to determine if
-     * all objects have entered the Drained state, the protocol is
-     * actually a bit more elaborate. See drain() for details.
-     */
-    enum State {
-        Running,  /** Running normally */
-        Draining, /** Draining buffers pending serialization/handover */
-        Drained   /** Buffers drained, ready for serialization/handover */
-    };
-
-  private:
-    State state;
-
-  protected:
-    void changeState(State new_state) { state = new_state; }
-
-  public:
-    State getState() { return state; }
-
   private:
     typedef std::vector<SimObject *> SimObjectList;
 
@@ -217,43 +151,16 @@ class SimObject : public EventManager, public Serializable
     virtual void startup();
 
     /**
-     * Serialize all SimObjects in the system.
+     * Provide a default implementation of the drain interface that
+     * simply returns 0 (draining completed) and sets the drain state
+     * to Drained.
      */
-    static void serializeAll(std::ostream &os);
+    unsigned int drain(DrainManager *drainManger);
 
     /**
-     * Determine if an object needs draining and register a drain
-     * event.
-     *
-     * When draining the state of an object, the simulator calls drain
-     * with a pointer to a drain event. If the object does not need
-     * further simulation to drain internal buffers, it switched to
-     * the Drained state and returns 0, otherwise it switches to the
-     * Draining state and returns the number of times that it will
-     * call Event::process() on the drain event. Most objects are
-     * expected to return either 0 or 1.
-     *
-     * The default implementation simply switches to the Drained state
-     * and returns 0.
-     *
-     * @note An object that has entered the Drained state can be
-     * disturbed by other objects in the system and consequently be
-     * forced to enter the Draining state again. The simulator
-     * therefore repeats the draining process until all objects return
-     * 0 on the first call to drain().
-     *
-     * @param drain_event Event to use to inform the simulator when
-     * the draining has completed.
-     *
-     * @return 0 if the object is ready for serialization now, >0 if
-     * it needs further simulation.
-     */
-    virtual unsigned int drain(Event *drain_event);
-
-    /**
-     * Switch an object in the Drained stated into the Running state.
+     * Serialize all SimObjects in the system.
      */
-    virtual void resume();
+    static void serializeAll(std::ostream &os);
 
 #ifdef DEBUG
   public: