config: Fix to SystemC example's event handling

This patch fixes checkpoint restore in the SystemC hosting example by handling
early PollEvent events correctly before any EventQueue events are posted.

The SystemC event queue handler (SCEventQueue) reports an error if the event
loop is entered with no Events posted.  It is possible for this to happen
after instantiate due to PollEvent events.  This patch separates out
`external' events into a different handler in sc_module.cc to prevent the
error from occurring.

This fix also improves the event handling of asynchronous events by:

    1) Making asynchronous events 'catch up' gem5 time to SystemC
        time to avoid the appearance that events have been lost
        while servicing an asynchronous event that schedules an
        event loop exit event

    2) Add an in_simulate data member to Module to allow the event
        loop to check whether events should be processed or deferred
        until the next time Module::simulate is entered

    3) Cancel pending events around the entry/exit of the event loop
        in Module::simulate

    4) Moving the state initialisation of the example entirely into
        run to correct a problem with early events in checkpoint
        restore.

It is still possible to schedule asynchronous events (and talk PollQueue
actions) while simulate is not running.  This behaviour may stil cause
some problems.

1 files changed, 54 insertions, 51 deletions

diff --git a/util/systemc/main.cc b/util/systemc/main.cc
index 6ffaeb6da..2ca6bbe65 100644
--- a/util/systemc/main.cc
+++ b/util/systemc/main.cc
@@ -120,8 +120,6 @@ class SimControl : public Gem5SystemC::Module
 
     SimControl(sc_core::sc_module_name name, int argc_, char **argv_);
 
-    void before_end_of_elaboration();
-
     void run();
 };
 
@@ -141,14 +139,26 @@ SimControl::SimControl(sc_core::sc_module_name name,
 
     cxxConfigInit();
 
+    /* Pass DPRINTF messages to SystemC */
     Trace::setDebugLogger(&logger);
 
+    /* @todo need this as an option */
     Gem5SystemC::setTickFrequency();
-    sc_core::sc_set_time_resolution(1, sc_core::SC_PS);
 
+    /* Make a SystemC-synchronising event queue and install it as the
+     *  sole top level gem5 EventQueue */
     Gem5SystemC::Module::setupEventQueues(*this);
+
+    if (sc_core::sc_get_time_resolution() !=
+        sc_core::sc_time(1, sc_core::SC_PS))
+    {
+        fatal("Time resolution must be set to 1 ps for gem5 to work");
+    }
+
+    /* Enable keyboard interrupt, async I/O etc. */
     initSignals();
 
+    /* Enable stats */
     Stats::initSimStats();
     Stats::registerHandlers(CxxConfig::statsReset, CxxConfig::statsDump);
 
@@ -168,10 +178,9 @@ SimControl::SimControl(sc_core::sc_module_name name,
 
     CxxConfigFileBase *conf = new CxxIniFile();
 
-    if (!conf->load(config_file.c_str())) {
-        std::cerr << "Can't open config file: " << config_file << '\n';
-        std::exit(EXIT_FAILURE);
-    }
+    if (!conf->load(config_file.c_str()))
+        fatal("Can't open config file: %s", config_file);
+
     arg_ptr++;
 
     config_manager = new CxxConfigManager(*conf);
@@ -231,8 +240,7 @@ SimControl::SimControl(sc_core::sc_module_name name,
             }
         }
     } catch (CxxConfigManager::Exception &e) {
-        std::cerr << e.name << ": " << e.message << "\n";
-        std::exit(EXIT_FAILURE);
+        fatal("Config problem in sim object %s: %s", e.name, e.message);
     }
 
     CxxConfig::statsEnable();
@@ -241,31 +249,51 @@ SimControl::SimControl(sc_core::sc_module_name name,
     try {
         config_manager->instantiate();
     } catch (CxxConfigManager::Exception &e) {
-        std::cerr << "Config problem in sim object " << e.name
-            << ": " << e.message << "\n";
-
-        std::exit(EXIT_FAILURE);
+        fatal("Config problem in sim object %s: %s", e.name, e.message);
     }
 }
 
-void SimControl::before_end_of_elaboration()
+void SimControl::run()
 {
-    if (!checkpoint_restore) {
-        try {
+    EventQueue *eventq = getEventQueue(0);
+    GlobalSimLoopExitEvent *exit_event = NULL;
+
+    /* There *must* be no scheduled events yet */
+    fatal_if(!eventq->empty(), "There must be no posted events"
+        " before SimControl::run");
+
+    try {
+        if (checkpoint_restore) {
+            std::cerr << "Restoring checkpoint\n";
+
+            Checkpoint *checkpoint = new Checkpoint(checkpoint_dir,
+                config_manager->getSimObjectResolver());
+
+            /* Catch SystemC up with gem5 after checkpoint restore.
+             *  Note that gem5 leading SystemC is always a violation of the
+             *  required relationship between the two, hence this careful
+             *  catchup */
+            Tick systemc_time = sc_core::sc_time_stamp().value();
+            if (curTick() > systemc_time) {
+                Tick wait_period = curTick() - systemc_time;
+
+                std::cerr << "Waiting for " << wait_period << "ps for"
+                    " SystemC to catch up to gem5\n";
+                wait(sc_core::sc_time(wait_period, sc_core::SC_PS));
+            }
+
+            config_manager->loadState(checkpoint);
+            config_manager->startup();
+            config_manager->drainResume();
+        } else {
             config_manager->initState();
             config_manager->startup();
-        } catch (CxxConfigManager::Exception &e) {
-            std::cerr << "Config problem in sim object " << e.name
-                << ": " << e.message << "\n";
-
-            std::exit(EXIT_FAILURE);
         }
+    } catch (CxxConfigManager::Exception &e) {
+        fatal("Config problem in sim object %s: %s", e.name, e.message);
     }
-}
 
-void SimControl::run()
-{
-    GlobalSimLoopExitEvent *exit_event = NULL;
+    fatal_if(eventq->empty(), "No events to process after system startup");
 
     if (checkpoint_save) {
         exit_event = simulate(pre_run_time);
@@ -299,32 +327,6 @@ void SimControl::run()
         config_manager->drainResume();
     }
 
-    if (checkpoint_restore) {
-        std::cerr << "Restoring checkpoint\n";
-
-        Checkpoint *checkpoint = new Checkpoint(checkpoint_dir,
-            config_manager->getSimObjectResolver());
-
-        Serializable::unserializeGlobals(checkpoint);
-
-        /* gem5 time can have changed, so lets wait until SystemC
-         *  catches up */
-        Tick systemc_time = sc_core::sc_time_stamp().value();
-        if (curTick() > systemc_time) {
-            Tick wait_period = curTick() - systemc_time;
-
-            std::cerr << "Waiting for " << wait_period << "ps for"
-                " SystemC to catch up to gem5\n";
-            wait(sc_core::sc_time(wait_period, sc_core::SC_PS));
-        }
-
-        config_manager->loadState(checkpoint);
-
-        config_manager->drainResume();
-
-        std::cerr << "Restored from checkpoint\n";
-    }
-
     if (switch_cpus) {
         exit_event = simulate(pre_switch_time);
 
@@ -350,6 +352,7 @@ void SimControl::run()
 
         old_cpu.switchOut();
         system.setMemoryMode(Enums::timing);
+
         new_cpu.takeOverFrom(&old_cpu);
         config_manager->drainResume();