cpu: Clarify meaning of cachePorts variable in lsq_unit.hh of O3

cachePorts currently constrains the number of store packets written to the
D-Cache each cycle), but loads currently affect this variable. This leads
to unexpected congestion (e.g., setting cachePorts to a realistic 1 will
in fact allow a store to WB only if no loads have accessed the D-Cache
this cycle). In the absence of arbitration, this patch decouples how many
loads can be done per cycle from how many stores can be done per cycle.

Signed-off-by: Jason Lowe-Power <jason@lowepower.com>

src/cpu/o3/O3CPU.py

@@ -52,7 +52,8 @@ class DerivO3CPU(BaseCPU):
 
     activity = Param.Unsigned(0, "Initial count")
 
-    cachePorts = Param.Unsigned(200, "Cache Ports")
+    cacheStorePorts = Param.Unsigned(200, "Cache Ports. "
+          "Constrains stores only. Loads are constrained by load FUs.")
 
     decodeToFetchDelay = Param.Cycles(1, "Decode to fetch delay")
     renameToFetchDelay = Param.Cycles(1 ,"Rename to fetch delay")

src/cpu/o3/lsq_unit.hh

@@ -113,7 +113,7 @@ class LSQUnit {
      * @todo: Move the number of used ports up to the LSQ level so it can
      * be shared by all LSQ units.
      */
-    void tick() { usedPorts = 0; }
+    void tick() { usedStorePorts = 0; }
 
     /** Inserts an instruction. */
     void insert(DynInstPtr &inst);
@@ -429,11 +429,11 @@ class LSQUnit {
     int storeTail;
 
     /// @todo Consider moving to a more advanced model with write vs read ports
-    /** The number of cache ports available each cycle. */
-    int cachePorts;
+    /** The number of cache ports available each cycle (stores only). */
+    int cacheStorePorts;
 
-    /** The number of used cache ports in this cycle. */
-    int usedPorts;
+    /** The number of used cache ports in this cycle by stores. */
+    int usedStorePorts;
 
     //list<InstSeqNum> mshrSeqNums;
 
@@ -765,8 +765,6 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
         load_inst->memData = new uint8_t[req->getSize()];
     }
 
-    ++usedPorts;
-
     // if we the cache is not blocked, do cache access
     bool completedFirst = false;
     PacketPtr data_pkt = Packet::createRead(req);
@@ -800,6 +798,11 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
         state->mainPkt = data_pkt;
     }
 
+    // For now, load throughput is constrained by the number of
+    // load FUs only, and loads do not consume a cache port (only
+    // stores do).
+    // @todo We should account for cache port contention
+    // and arbitrate between loads and stores.
     bool successful_load = true;
     if (!dcachePort->sendTimingReq(fst_data_pkt)) {
         successful_load = false;
@@ -811,7 +814,8 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
         // load will be squashed, so indicate this to the state object.
         // The first packet will return in completeDataAccess and be
         // handled there.
-        ++usedPorts;
+        // @todo We should also account for cache port contention
+        // here.
         if (!dcachePort->sendTimingReq(snd_data_pkt)) {
             // The main packet will be deleted in completeDataAccess.
             state->complete();

src/cpu/o3/lsq_unit_impl.hh

@@ -176,7 +176,7 @@ LSQUnit<Impl>::init(O3CPU *cpu_ptr, IEW *iew_ptr, DerivO3CPUParams *params,
 
     depCheckShift = params->LSQDepCheckShift;
     checkLoads = params->LSQCheckLoads;
-    cachePorts = params->cachePorts;
+    cacheStorePorts = params->cacheStorePorts;
     needsTSO = params->needsTSO;
 
     resetState();
@@ -193,7 +193,7 @@ LSQUnit<Impl>::resetState()
 
     storeHead = storeWBIdx = storeTail = 0;
 
-    usedPorts = 0;
+    usedStorePorts = 0;
 
     retryPkt = NULL;
     memDepViolator = NULL;
@@ -792,7 +792,7 @@ LSQUnit<Impl>::writebackStores()
            storeQueue[storeWBIdx].inst &&
            storeQueue[storeWBIdx].canWB &&
            ((!needsTSO) || (!storeInFlight)) &&
-           usedPorts < cachePorts) {
+           usedStorePorts < cacheStorePorts) {
 
         if (isStoreBlocked) {
             DPRINTF(LSQUnit, "Unable to write back any more stores, cache"
@@ -810,7 +810,7 @@ LSQUnit<Impl>::writebackStores()
             continue;
         }
 
-        ++usedPorts;
+        ++usedStorePorts;
 
         if (storeQueue[storeWBIdx].inst->isDataPrefetch()) {
             incrStIdx(storeWBIdx);
@@ -950,8 +950,8 @@ LSQUnit<Impl>::writebackStores()
                 assert(snd_data_pkt);
 
                 // Ensure there are enough ports to use.
-                if (usedPorts < cachePorts) {
-                    ++usedPorts;
+                if (usedStorePorts < cacheStorePorts) {
+                    ++usedStorePorts;
                     if (sendStore(snd_data_pkt)) {
                         storePostSend(snd_data_pkt);
                     } else {
@@ -975,7 +975,7 @@ LSQUnit<Impl>::writebackStores()
     }
 
     // Not sure this should set it to 0.
-    usedPorts = 0;
+    usedStorePorts = 0;
 
     assert(stores >= 0 && storesToWB >= 0);
 }