mem: Add cache clusivity

This patch adds a parameter to control the cache clusivity, that is if
the cache is mostly inclusive or exclusive. At the moment there is no
intention to support strict policies, and thus the options are: 1)
mostly inclusive, or 2) mostly exclusive.

The choice of policy guides the behaviuor on a cache fill, and a new
helper function, allocOnFill, is created to encapsulate the decision
making process. For the timing mode, the decision is annotated on the
MSHR on sending out the downstream packet, and in atomic we directly
pass the decision to handleFill. We (ab)use the tempBlock in cases
where we are not allocating on fill, leaving the rest of the cache
unaffected. Simple and effective.

This patch also makes it more explicit that multiple caches are
allowed to consider a block writable (this is the case
also before this patch). That is, for a mostly inclusive cache,
multiple caches upstream may also consider the block exclusive. The
caches considering the block writable/exclusive all appear along the
same path to memory, and from a coherency protocol point of view it
works due to the fact that we always snoop upwards in zero time before
querying any downstream cache.

Note that this patch does not introduce clean writebacks. Thus, for
clean lines we are essentially removing a cache level if it is made
mostly exclusive. For example, lines from the read-only L1 instruction
cache or table-walker cache are always clean, and simply get dropped
rather than being passed to the L2. If the L2 is mostly exclusive and
does not allocate on fill it will thus never hold the line. A follow
on patch adds the clean writebacks.

The patch changes the L2 of the O3_ARM_v7a CPU configuration to be
mostly exclusive (and stats are affected accordingly).

1 files changed, 16 insertions, 0 deletions

diff --git a/src/mem/cache/Cache.py b/src/mem/cache/Cache.py
index 8ad1177e7..48e52a8d5 100644
--- a/src/mem/cache/Cache.py
+++ b/src/mem/cache/Cache.py
@@ -84,6 +84,22 @@ class BaseCache(MemObject):
 
     system = Param.System(Parent.any, "System we belong to")
 
+# Enum for cache clusivity, currently mostly inclusive or mostly
+# exclusive.
+class Clusivity(Enum): vals = ['mostly_incl', 'mostly_excl']
+
 class Cache(BaseCache):
     type = 'Cache'
     cxx_header = 'mem/cache/cache.hh'
+
+    # Control whether this cache should be mostly inclusive or mostly
+    # exclusive with respect to upstream caches. The behaviour on a
+    # fill is determined accordingly. For a mostly inclusive cache,
+    # blocks are allocated on all fill operations. Thus, L1 caches
+    # should be set as mostly inclusive even if they have no upstream
+    # caches. In the case of a mostly exclusive cache, fills are not
+    # allocating unless they came directly from a non-caching source,
+    # e.g. a table walker. Additionally, on a hit from an upstream
+    # cache a line is dropped for a mostly exclusive cache.
+    clusivity = Param.Clusivity('mostly_incl',
+                                "Clusivity with upstream cache")