mem, config: Selective use of snoop filter

Disable the default snoop filter in the SystemXBar so that the
typical membus does not have a snoop filter by default. Instead,
add the snoop filter only when there are caches added to the system
(with the caches / l2cache options).

The underlying problem is that the snoop filter grows without
bounds (for now) if there are no caches to tell it that lines have
been evicted. This causes slow regression runs for all the atomic
regressions. This patch fixes this behaviour.

--HG--
extra : source : f97c20511828209757440839ed48d741d02d428f

configs/common/CacheConfig.py

@@ -153,6 +153,11 @@ def config_cache(options, system):
         else:
             system.cpu[i].connectAllPorts(system.membus)
 
+        # Add a snoop filter to the membus if there are caches above it
+        if (options.l2cache or options.caches) and \
+        (system.membus.snoop_filter == NULL):
+            system.membus.snoop_filter = SnoopFilter()
+
     return system
 
 # ExternalSlave provides a "port", but when that port connects to a cache,

tests/configs/base_config.py

@@ -126,6 +126,9 @@ class BaseSystem(object):
             cpu.createInterruptController()
             cpu.connectAllPorts(sha_bus if sha_bus != None else system.membus,
                                 system.membus)
+            # System has caches before the membus -> add snoop filter
+            if sha_bus and system.membus.snoop_filter == NULL:
+                system.membus.snoop_filter = SnoopFilter()
 
     def init_kvm(self, system):
         """Do KVM-specific system initialization.
@@ -149,6 +152,9 @@ class BaseSystem(object):
             self.init_kvm(system)
 
         sha_bus = self.create_caches_shared(system)
+        # System has caches before the membus -> add snoop filter
+        if sha_bus and system.membus.snoop_filter == NULL:
+            system.membus.snoop_filter = SnoopFilter()
         for cpu in system.cpu:
             self.init_cpu(system, cpu, sha_bus)