mem: Make returning snoop responses occupy response layer

This patch introduces a mirrored internal snoop port to facilitate
easy addition of flow control for the snoop responses that are turned
into normal responses on their return. To perform this, the slave
ports of the coherent bus are wrapped in internal master ports that
are passed as the source ports to the response layer in question.

As a result of this patch, there is more contention for the response
resources, and as such system performance will decrease slightly.

A consequence of the mirrored internal port is that the port the bus
tells to retry (the internal one) and the port actually retrying (the
mirrored) one are not the same. Thus, the existing check in tryTiming
is not longer correct. In fact, the test is redundant as the layer is
only in the retry state while calling sendRetry on the waiting port,
and if the latter does not immediately call the bus then the retry
state is left. Consequently the check is removed.

1 files changed, 51 insertions, 0 deletions

diff --git a/src/mem/coherent_bus.hh b/src/mem/coherent_bus.hh
index d8ddd507c..49f7ae698 100644
--- a/src/mem/coherent_bus.hh
+++ b/src/mem/coherent_bus.hh
@@ -219,6 +219,57 @@ class CoherentBus : public BaseBus
 
     };
 
+    /**
+     * Internal class to bridge between an incoming snoop response
+     * from a slave port and forwarding it through an outgoing slave
+     * port. It is effectively a dangling master port.
+     */
+    class SnoopRespPort : public MasterPort
+    {
+
+      private:
+
+        /** The port which we mirror internally. */
+        SlavePort& slavePort;
+
+        /** The bus to which this port belongs. */
+        CoherentBus &bus;
+
+      public:
+
+        /**
+         * Create a snoop response port that mirrors a given slave port.
+         */
+        SnoopRespPort(SlavePort& slave_port, CoherentBus& _bus) :
+            MasterPort(slave_port.name() + ".snoopRespPort", &_bus),
+            slavePort(slave_port), bus(_bus) { }
+
+        /**
+         * Override the sending of retries and pass them on through
+         * the mirrored slave port.
+         */
+        void sendRetry() {
+            slavePort.sendRetry();
+        }
+
+        /**
+         * Provided as necessary.
+         */
+        void recvRetry() { panic("SnoopRespPort should never see retry\n"); }
+
+        /**
+         * Provided as necessary.
+         */
+        bool recvTimingResp(PacketPtr pkt)
+        {
+            panic("SnoopRespPort should never see timing response\n");
+            return false;
+        }
+
+    };
+
+    std::vector<SnoopRespPort*> snoopRespPorts;
+
     std::vector<SlavePort*> snoopPorts;
 
     /**