cpu: Fix retries on barrier/store in Minor's store buffer

This patch fixes a case where a store in Minor's store buffer never
leaves the store buffer as it is pre-maturely counted as having been
issued, leading to the store buffer idling.

LSQ::StoreBuffer::numUnissuedAccesses should count the number of accesses
either in memory, or still in the store buffer after being completed.

For stores which are also barriers, the store will stay in the store
buffer for a cycle after it is completed and will be cleaned up by the
barrier clearing code (to ensure that barriers are completed in-order).
To acheive this, numUnissuedAccesses is not decremented when a store-barrier
is issued to memory, but when its barrier effect is cleared.

Without this patch, the correct behaviour happens when a memory transaction
is immediately accepted, but not if it needs a retry.

2 files changed, 17 insertions, 6 deletions

diff --git a/src/cpu/minor/lsq.cc b/src/cpu/minor/lsq.cc
index 75dbb804a..501620746 100644
--- a/src/cpu/minor/lsq.cc
+++ b/src/cpu/minor/lsq.cc
@@ -730,6 +730,15 @@ LSQ::StoreBuffer::forwardStoreData(LSQRequestPtr load,
 }
 
 void
+LSQ::StoreBuffer::countIssuedStore(LSQRequestPtr request)
+{
+    /* Barriers are accounted for as they are cleared from
+     *  the queue, not after their transfers are complete */
+    if (!request->isBarrier())
+        numUnissuedAccesses--;
+}
+
+void
 LSQ::StoreBuffer::step()
 {
     DPRINTF(MinorMem, "StoreBuffer step numUnissuedAccesses: %d\n",
@@ -785,10 +794,7 @@ LSQ::StoreBuffer::step()
                     " system\n", *(request->inst));
 
                 if (lsq.tryToSend(request)) {
-                    /* Barrier are accounted for as they are cleared from
-                     *  the queue, not after their transfers are complete */
-                    if (!request->isBarrier())
-                        numUnissuedAccesses--;
+                    countIssuedStore(request);
                     issue_count++;
                 } else {
                     /* Don't step on to the next store buffer entry if this
@@ -1198,7 +1204,7 @@ LSQ::recvTimingResp(PacketPtr response)
         /* Response to a request from the store buffer */
         request->retireResponse(response);
 
-        /* Remove completed requests unless they are barrier (which will
+        /* Remove completed requests unless they are barriers (which will
          *  need to be removed in order */
         if (request->isComplete()) {
             if (!request->isBarrier()) {
@@ -1265,7 +1271,7 @@ LSQ::recvRetry()
             break;
           case LSQRequest::StoreBufferIssuing:
             /* In the store buffer */
-            storeBuffer.numUnissuedAccesses--;
+            storeBuffer.countIssuedStore(retryRequest);
             break;
           default:
             assert(false);
diff --git a/src/cpu/minor/lsq.hh b/src/cpu/minor/lsq.hh
index 7da2fd694..7936ae8f0 100644
--- a/src/cpu/minor/lsq.hh
+++ b/src/cpu/minor/lsq.hh
@@ -514,6 +514,11 @@ class LSQ : public Named
          *  completely issued to the memory system */
         unsigned int numUnissuedStores() { return numUnissuedAccesses; }
 
+        /** Count a store being issued to memory by decrementing
+         *  numUnissuedAccesses.  Does not count barrier requests as they
+         *  will be handles as barriers are cleared from the buffer */
+        void countIssuedStore(LSQRequestPtr request);
+
         /** Drained if there is absolutely nothing left in the buffer */
         bool isDrained() const { return slots.empty(); }