stdlib: Move 'sim_quantum' set from Simulator to Processor

The setting of the `sim_quantum` parameter makes considerably more sense
to occur in the Processor. Through the `_pre_instnatiate` functions this
is now possible.

src/python/gem5/components/boards/abstract_board.py

@@ -406,11 +406,11 @@ class AbstractBoard:
                             occurs whether a simulation is to be run in FS or
                             SE mode will be determined by the board set."""
 
-        # Connect the memory, processor, and cache hierarchy.
+        # 1. Connect the memory, processor, and cache hierarchy.
         self._connect_things()
 
-        # Return the Root object.
-        return Root(
+        # 2. Create the root object
+        root = Root(
             full_system=(
                 full_system
                 if full_system is not None
@@ -419,6 +419,13 @@ class AbstractBoard:
             board=self,
         )
 
+        # 3. Call any of the components' `_pre_instantiate` functions.
+        # Right now, only the processor requires this.
+        self.get_processor()._pre_instantiate(root)
+
+        # 4. Return the root object.
+        return root
+
     def _connect_things_check(self):
         """
         Here we check that connect things has been called and throw an

src/python/gem5/components/processors/abstract_processor.py

@@ -33,7 +33,10 @@ from typing import (
     Optional,
 )
 
-from m5.objects import SubSystem
+from m5.objects import (
+    Root,
+    SubSystem,
+)
 
 from ...isas import ISA
 from ...utils.requires import requires
@@ -83,3 +86,12 @@ class AbstractProcessor(SubSystem):
     def _post_instantiate(self) -> None:
         """Called to set up anything needed after ``m5.instantiate``."""
         pass
+
+    def _pre_instantiate(self, root: Root) -> None:
+        """Called in the `AbstractBoard`'s `_pre_instantiate` method. This is
+        called after `connect_things`, after the creation of the root object
+        (which is passed in as an argument), but before `m5.instantiate`).
+
+        Subclasses should override this method to set up any connections.
+        """
+        pass

src/python/gem5/components/processors/base_cpu_processor.py

@@ -27,12 +27,14 @@
 
 from typing import List
 
+import m5
 from m5.objects import (
     BaseAtomicSimpleCPU,
     BaseMinorCPU,
     BaseNonCachingSimpleCPU,
     BaseO3CPU,
     BaseTimingSimpleCPU,
+    Root,
 )
 from m5.util import warn
 
@@ -99,3 +101,9 @@ class BaseCPUProcessor(AbstractProcessor):
                 board.set_mem_mode(MemMode.ATOMIC)
         else:
             raise NotImplementedError
+
+    def _pre_instantiate(self, root: Root) -> None:
+        super()._pre_instantiate(root)
+        if any(core.is_kvm_core() for core in self.get_cores()):
+            m5.ticks.fixGlobalFrequency()
+            root.sim_quantum = m5.ticks.fromSeconds(0.001)

src/python/gem5/components/processors/switchable_processor.py

@@ -31,6 +31,7 @@ from typing import (
 )
 
 import m5
+from m5.objects import Root
 
 from ...utils.override import *
 from ..boards.abstract_board import AbstractBoard
@@ -155,3 +156,24 @@ class SwitchableProcessor(AbstractProcessor):
 
         # Ensure the current processor is updated.
         self._current_cores = to_switch
+
+    def _pre_instantiate(self, root: Root) -> None:
+        super()._pre_instantiate(root)
+        # The following is a bit of a hack. If a simulation is to use a KVM
+        # core then the `sim_quantum` value must be set. However, in the
+        # case of using a SwitchableProcessor the KVM cores may be
+        # switched out and therefore not accessible via `get_cores()`.
+        # This is the reason for the `isinstance` check.
+        #
+        # We cannot set the `sim_quantum` value in every simulation as
+        # setting it causes the scheduling of exits to be off by the
+        # `sim_quantum` value (something necessary if we are using KVM
+        # cores). Ergo we only set the value of KVM cores are present.
+        #
+        # There is still a bug here in that if the user is switching to and
+        # from KVM and non-KVM cores via the SwitchableProcessor then the
+        # scheduling of exits for the non-KVM cores will be incorrect. This
+        # will be fixed at a later date.
+        if self._prepare_kvm:
+            m5.ticks.fixGlobalFrequency()
+            root.sim_quantum = m5.ticks.fromSeconds(0.001)

src/python/gem5/simulate/simulator.py

@@ -670,29 +670,6 @@ class Simulator:
                 full_system=self._full_system
             )
 
-            # The following is a bit of a hack. If a simulation is to use a KVM
-            # core then the `sim_quantum` value must be set. However, in the
-            # case of using a SwitchableProcessor the KVM cores may be
-            # switched out and therefore not accessible via `get_cores()`.
-            # This is the reason for the `isinstance` check.
-            #
-            # We cannot set the `sim_quantum` value in every simulation as
-            # setting it causes the scheduling of exits to be off by the
-            # `sim_quantum` value (something necessary if we are using KVM
-            # cores). Ergo we only set the value of KVM cores are present.
-            #
-            # There is still a bug here in that if the user is switching to and
-            # from KVM and non-KVM cores via the SwitchableProcessor then the
-            # scheduling of exits for the non-KVM cores will be incorrect. This
-            # will be fixed at a later date.
-            processor = self._board.processor
-            if any(core.is_kvm_core() for core in processor.get_cores()) or (
-                isinstance(processor, SwitchableProcessor)
-                and any(core.is_kvm_core() for core in processor._all_cores())
-            ):
-                m5.ticks.fixGlobalFrequency()
-                root.sim_quantum = m5.ticks.fromSeconds(0.001)
-
             # m5.instantiate() takes a parameter specifying the path to the
             # checkpoint directory. If the parameter is None, no checkpoint
             # will be restored.