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.

3 files changed, 26 insertions, 4 deletions

diff --git a/src/mem/ruby/system/Sequencer.cc b/src/mem/ruby/system/Sequencer.cc
index 01b868017..36bd9cd62 100644
--- a/src/mem/ruby/system/Sequencer.cc
+++ b/src/mem/ruby/system/Sequencer.cc
@@ -58,6 +58,8 @@ Sequencer::Sequencer(const Params *p)
 
     m_instCache_ptr = p->icache;
     m_dataCache_ptr = p->dcache;
+    m_data_cache_hit_latency = p->dcache_hit_latency;
+    m_inst_cache_hit_latency = p->icache_hit_latency;
     m_max_outstanding_requests = p->max_outstanding_requests;
     m_deadlock_threshold = p->deadlock_threshold;
 
@@ -65,6 +67,8 @@ Sequencer::Sequencer(const Params *p)
     assert(m_deadlock_threshold > 0);
     assert(m_instCache_ptr != NULL);
     assert(m_dataCache_ptr != NULL);
+    assert(m_data_cache_hit_latency > 0);
+    assert(m_inst_cache_hit_latency > 0);
 
     m_usingNetworkTester = p->using_network_tester;
 }
@@ -691,12 +695,17 @@ Sequencer::issueRequest(PacketPtr pkt, RubyRequestType secondary_type)
             msg->getPhysicalAddress(),
             RubyRequestType_to_string(secondary_type));
 
-    Cycles latency(0);  // initialzed to an null value
-
+    // The Sequencer currently assesses instruction and data cache hit latency
+    // for the top-level caches at the beginning of a memory access.
+    // TODO: Eventually, this latency should be moved to represent the actual
+    // cache access latency portion of the memory access. This will require
+    // changing cache controller protocol files to assess the latency on the
+    // access response path.
+    Cycles latency(0);  // Initialize to zero to catch misconfigured latency
     if (secondary_type == RubyRequestType_IFETCH)
-        latency = m_instCache_ptr->getLatency();
+        latency = m_inst_cache_hit_latency;
     else
-        latency = m_dataCache_ptr->getLatency();
+        latency = m_data_cache_hit_latency;
 
     // Send the message to the cache controller
     assert(latency > 0);
diff --git a/src/mem/ruby/system/Sequencer.hh b/src/mem/ruby/system/Sequencer.hh
index d5cd17f5f..505b3f3bc 100644
--- a/src/mem/ruby/system/Sequencer.hh
+++ b/src/mem/ruby/system/Sequencer.hh
@@ -180,6 +180,13 @@ class Sequencer : public RubyPort
     CacheMemory* m_dataCache_ptr;
     CacheMemory* m_instCache_ptr;
 
+    // The cache access latency for top-level caches (L0/L1). These are
+    // currently assessed at the beginning of each memory access through the
+    // sequencer.
+    // TODO: Migrate these latencies into top-level cache controllers.
+    Cycles m_data_cache_hit_latency;
+    Cycles m_inst_cache_hit_latency;
+
     typedef m5::hash_map<Address, SequencerRequest*> RequestTable;
     RequestTable m_writeRequestTable;
     RequestTable m_readRequestTable;
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,