sim: Allow passing a user-defined L2XBar to addTwoLevelCacheHierarchy().

Before this CL, the addTwoLevelCacheHierarchy() function uses the
default L2XBar class as the interconnect between CPU L1 caches and
L2. This CL allows passing a user-defined bus to overwrite the
default L2XBar by adding an optional argument to the function.

Change-Id: I917657272fd4924ee0bed882a226851afba26847
Reviewed-on: https://gem5-review.googlesource.com/7364
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

src/cpu/BaseCPU.py

@@ -299,9 +299,10 @@ class BaseCPU(MemObject):
                 self._cached_ports += ["checker.itb.walker.port", \
                                        "checker.dtb.walker.port"]
 
-    def addTwoLevelCacheHierarchy(self, ic, dc, l2c, iwc = None, dwc = None):
+    def addTwoLevelCacheHierarchy(self, ic, dc, l2c, iwc=None, dwc=None,
+                                  xbar=None):
         self.addPrivateSplitL1Caches(ic, dc, iwc, dwc)
-        self.toL2Bus = L2XBar()
+        self.toL2Bus = xbar if xbar else L2XBar()
         self.connectCachedPorts(self.toL2Bus)
         self.l2cache = l2c
         self.toL2Bus.master = self.l2cache.cpu_side