stdlib: This commit added warning for exit default behavior

This commit imported the warn from m5.util library and added a function
 named "defaultBehaviorWarning" to the exit_event_generators.py file
under src/python/gem5/simulator.

This function takes two string variable and output a warning that
contains a warning about the default behavior, the behavior type,
 and an detail explaination about it.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-1235

Change-Id: I54500425eaa1a556769aa1f8ea6b32852694c94d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61189
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/python/gem5/simulate/exit_event_generators.py

@@ -27,17 +27,25 @@
 import m5.stats
 from ..components.processors.abstract_processor import AbstractProcessor
 from ..components.processors.switchable_processor import SwitchableProcessor
+from m5.util import warn
 
 """
 In this package we store generators for simulation exit events.
 """
-
+def defaultBehaviorWarning(type, effect):
+    warn(
+        "As no behavior was set by the user, default behavior is being carried"
+        f" out.\n Type: {type} \n Detail: {effect} \n")
 
 def default_exit_generator():
     """
     A default generator for an exit event. It will return True, indicating that
     the Simulator run loop should exit.
     """
+    defaultBehaviorWarning(
+        "default_exit_generator",
+        "A default generator for an exit event. It will return True, "
+        "indicating that the Simulator run loop should exit.")
     while True:
         yield True
 
@@ -48,6 +56,11 @@ def default_switch_generator(processor: AbstractProcessor):
     SwitchableProcessor, this generator will switch it. Otherwise nothing will
     happen.
     """
+    defaultBehaviorWarning(
+        "default_switch_generator",
+        "A default generator for a switch exit event.If the processor is a "
+        "SwitchableProcessor, this generator will switch it. Otherwise nothing"
+        " will happen.")
     is_switchable = isinstance(processor, SwitchableProcessor)
     while True:
         if is_switchable:
@@ -61,6 +74,10 @@ def default_workbegin_generator():
     A default generator for a workbegin exit event. It will reset the
     simulation statistics.
     """
+    defaultBehaviorWarning(
+        "default_workbegin_generator",
+        "A default generator for a workbegin exit event. It will reset the "
+        "simulation statistics.")
     while True:
         m5.stats.reset()
         yield False
@@ -71,6 +88,10 @@ def default_workend_generator():
     A default generator for a workend exit event. It will dump the simulation
     statistics.
     """
+    defaultBehaviorWarning(
+        "default_workend_generator",
+        "A default generator for a workend exit event. It will dump the "
+        "simulation statistics.")
     while True:
         m5.stats.dump()
         yield False