sim: EventQueue wakeup on events scheduled outside the event loop

This patch adds a 'wakeup' member function to EventQueue which should be
called on an event queue whenever an event is scheduled on the event queue
from outside code within the call tree of the gem5 event loop.

This clearly isn't necessary for normal gem5 EventQueue operation but
becomes the minimum necessary interface to allow hosting gem5's event loop
onto other schedulers where there may be calls into gem5 from external
code which schedules events onto an EventQueue between the current time and
the time of the next scheduled event.

The use case I have in mind is a SystemC hosting where the event loop is:

    while (more events) {
        wait(time_to_next_event or wakeup)
        setCurTick
        service events at this time
    }

where the 'wait' needs to be woken up if time_to_next_event becomes shorter
due to a scheduled event from SystemC arriving in a gem5 object.

Requiring 'wakeup' to be called is a more efficient interface than
requiring all gem5 event scheduling actions to affect the host scheduler.

This interface could be located elsewhere, say on another global object,
or by being passed by the host scheduler to objects which will schedule
such events, but it seems cleanest to put it on EventQueue as it is
actually a signal to the queue.

EventQueue::wakeup is called for async_event events on event queue 0 as
it's only important that *some* queue be triggered for such events.

1 files changed, 22 insertions, 0 deletions

diff --git a/src/sim/eventq.hh b/src/sim/eventq.hh
index c390d2155..394fd4d8d 100644
--- a/src/sim/eventq.hh
+++ b/src/sim/eventq.hh
@@ -606,6 +606,21 @@ class EventQueue : public Serializable
     void handleAsyncInsertions();
 
     /**
+     *  Function to signal that the event loop should be woken up because
+     *  an event has been scheduled by an agent outside the gem5 event
+     *  loop(s) whose event insertion may not have been noticed by gem5.
+     *  This function isn't needed by the usual gem5 event loop but may
+     *  be necessary in derived EventQueues which host gem5 onto other
+     *  schedulers.
+     *
+     *  @param when Time of a delayed wakeup (if known). This parameter
+     *  can be used by an implementation to schedule a wakeup in the
+     *  future if it is sure it will remain active until then.
+     *  Or it can be ignored and the event queue can be woken up now.
+     */
+    virtual void wakeup(Tick when = (Tick)-1) { }
+
+    /**
      *  function for replacing the head of the event queue, so that a
      *  different set of events can run without disturbing events that have
      *  already been scheduled. Already scheduled events can be processed
@@ -635,6 +650,8 @@ class EventQueue : public Serializable
     virtual void serialize(std::ostream &os);
     virtual void unserialize(Checkpoint *cp, const std::string &section);
 #endif
+
+    virtual ~EventQueue() { }
 };
 
 void dumpMainQueue();
@@ -693,6 +710,11 @@ class EventManager
         eventq->reschedule(event, when, always);
     }
 
+    void wakeupEventQueue(Tick when = (Tick)-1)
+    {
+        eventq->wakeup(when);
+    }
+
     void setCurTick(Tick newVal) { eventq->setCurTick(newVal); }
 };