ruby: Remove the RubyCache/CacheMemory latency

The RubyCache (CacheMemory) latency parameter is only used for top-level caches
instantiated for Ruby coherence protocols. However, the top-level cache hit
latency is assessed by the Sequencer as accesses flow through to the cache
hierarchy. Further, protocol state machines should be enforcing these cache hit
latencies, but RubyCaches do not expose their latency to any existng state
machines through the SLICC/C++ interface. Thus, the RubyCache latency parameter
is superfluous for all caches. This is confusing for users.

As a step toward pushing L0/L1 cache hit latency into the top-level cache
controllers, move their latencies out of the RubyCache declarations and over to
their Sequencers. Eventually, these Sequencer parameters should be exposed as
parameters to the top-level cache controllers, which should assess the latency.
NOTE: Assessing these latencies in the cache controllers will require modifying
each to eliminate instantaneous Ruby hit callbacks in transitions that finish
accesses, which is likely a large undertaking.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/mem/ruby/system/Sequencer.py b/src/mem/ruby/system/Sequencer.py
index e545000cf..7494986e9 100644
--- a/src/mem/ruby/system/Sequencer.py
+++ b/src/mem/ruby/system/Sequencer.py
@@ -61,6 +61,12 @@ class RubySequencer(RubyPort):
 
     icache = Param.RubyCache("")
     dcache = Param.RubyCache("")
+    # Cache latencies currently assessed at the beginning of each access
+    # NOTE: Setting these values to a value greater than one will result in
+    # O3 CPU pipeline bubbles and negatively impact performance
+    # TODO: Latencies should be migrated into each top-level cache controller
+    icache_hit_latency = Param.Cycles(1, "Inst cache hit latency")
+    dcache_hit_latency = Param.Cycles(1, "Data cache hit latency")
     max_outstanding_requests = Param.Int(16,
         "max requests (incl. prefetches) outstanding")
     deadlock_threshold = Param.Cycles(500000,