stdlib: Only set 'sim_quantum' value of KVM cores included

This commit:
https://gem5-review.googlesource.com/c/public/gem5/+/62471
set `sim_quantum` for any simulation done via the Simulator module.
However, this causes issues when setting exit events at a particular
tick. It resulted in the exit being off by the `sim_quantum` value. This
is required for KVM setups but is undesirable for non-KVM setups. Ergo,
this commit ensures the `sim_quantum` is only set in cases where KVM
cores are included in a simulation.

There are two items of note here:

1. When using the SwitchableProcessor the KVM cores may be switched out
   and therefore not accessable via the `get_cores` method. To get round
   this we check if the processor is a SwitchableProcessor and run an
   additionial check that _any_ of the cores in the SwitchableProcessor
   are KVM. This is a big hacky; the Processor API should be changed to
   make this easier.
2. This only partially fixes the problem of exit events being off given
   a specified tick. This will still occur in the case a
   SwitchableProcessor is used containing KVM cores. E.g., non-KVM cores
   will still be "off" when KVM cores are switched out. This issue will
   be addressed in a later commit.

Change-Id: Id966d76cd1630b6c41c5972fa9423c9e48eafaf6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/63051
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/gem5/simulate/simulator.py

@@ -43,6 +43,7 @@ from .exit_event_generators import (
 )
 from .exit_event import ExitEvent
 from ..components.boards.abstract_board import AbstractBoard
+from ..components.processors.switchable_processor import SwitchableProcessor
 from ..components.processors.cpu_types import CPUTypes
 
 
@@ -303,8 +304,28 @@ class Simulator:
             # (for example, in `get_stats()`).
             self._root = root
 
-            m5.ticks.fixGlobalFrequency()
-            root.sim_quantum = m5.ticks.fromSeconds(0.001)
+            # 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