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, 216 insertions, 0 deletions

diff --git a/src/sim/drain.hh b/src/sim/drain.hh
new file mode 100644
index 000000000..252022bc7
--- /dev/null
+++ b/src/sim/drain.hh
@@ -0,0 +1,216 @@
+/*
+ * Copyright (c) 2012 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Andreas Sandberg
+ */
+
+#ifndef __SIM_DRAIN_HH__
+#define __SIM_DRAIN_HH__
+
+#include <cassert>
+#include <vector>
+
+#include "base/flags.hh"
+
+class Event;
+
+/**
+ * This class coordinates draining of a System.
+ *
+ * When draining a System, we need to make sure that all SimObjects in
+ * that system have drained their state before declaring the operation
+ * to be successful. This class keeps track of how many objects are
+ * still in the process of draining their state. Once it determines
+ * that all objects have drained their state, it exits the simulation
+ * loop.
+ *
+ * @note A System might not be completely drained even though the
+ * DrainManager has caused the simulation loop to exit. Draining needs
+ * to be restarted until all Drainable objects declare that they don't
+ * need further simulation to be completely drained. See Drainable for
+ * more information.
+ */
+class DrainManager
+{
+  public:
+    DrainManager();
+    virtual ~DrainManager();
+
+    /**
+     * Get the number of objects registered with this DrainManager
+     * that are currently draining their state.
+     *
+     * @return Number of objects currently draining.
+     */
+    unsigned int getCount() const { return _count; }
+
+    void setCount(int count) { _count = count; }
+
+    /**
+     * Notify the DrainManager that a Drainable object has finished
+     * draining.
+     */
+    void signalDrainDone() {
+        assert(_count > 0);
+        if (--_count == 0)
+            drainCycleDone();
+    }
+
+  protected:
+    /**
+     * Callback when all registered Drainable objects have completed a
+     * drain cycle.
+     */
+    virtual void drainCycleDone();
+
+    /** Number of objects still draining. */
+    unsigned int _count;
+};
+
+/**
+ * Interface for objects that might require draining before
+ * checkpointing.
+ *
+ * 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 DrainManager 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::signalDrainDone() method is called. When the
+ *     counter reaches zero, the simulation is stopped.
+ *
+ * <li>Call Drainable::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 DrainManager. A pointer to the drain
+ *     manager is passed as an argument to the drain() method.
+ *
+ * <li>Continue simulation. When an object has finished draining its
+ *     internal state, it calls CountedDrainEvent::signalDrainDone()
+ *     on the manager. When the counter in the manager 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 Drainable::drainResume() and continue the simulation.
+ * </ol>
+ *
+ */
+class 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 state.
+     *
+     * 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 */
+    };
+
+    Drainable();
+    virtual ~Drainable();
+
+    /**
+     * Determine if an object needs draining and register a
+     * DrainManager.
+     *
+     * When draining the state of an object, the simulator calls drain
+     * with a pointer to a drain manager. 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.
+     *
+     * @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 drainManager DrainManager to use to inform the simulator
+     * when draining has completed.
+     *
+     * @return 0 if the object is ready for serialization now, >0 if
+     * it needs further simulation.
+     */
+    virtual unsigned int drain(DrainManager *drainManager) = 0;
+
+    /**
+     * Resume execution after a successful drain.
+     *
+     * @note This method is normally only called from the simulation
+     * scripts.
+     */
+    virtual void drainResume();
+
+    State getDrainState() const { return _drainState; }
+
+  protected:
+    void setDrainState(State new_state) { _drainState = new_state; }
+
+
+  private:
+    State _drainState;
+
+};
+
+#endif