mem: Move the point of coherency to the coherent crossbar

This patch introduces the ability of making the coherent crossbar the
point of coherency. If so, the crossbar does not forward packets where
a cache with ownership has already committed to responding, and also
does not forward any coherency-related packets that are not intended
for a downstream memory controller. Thus, invalidations and upgrades
are turned around in the crossbar, and the memory controller only sees
normal reads and writes.

In addition this patch moves the express snoop promotion of a packet
to the crossbar, thus allowing the downstream cache to check the
express snoop flag (as it should) for bypassing any blocking, rather
than relying on whether a cache is responding or not.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/mem/coherent_xbar.hh b/src/mem/coherent_xbar.hh
index 24d00628b..8c55b59da 100644
--- a/src/mem/coherent_xbar.hh
+++ b/src/mem/coherent_xbar.hh
@@ -273,6 +273,9 @@ class CoherentXBar : public BaseXBar
     /** Cycles of snoop response latency.*/
     const Cycles snoopResponseLatency;
 
+    /** Is this crossbar the point of coherency? **/
+    const bool pointOfCoherency;
+
     /**
      * Upstream caches need this packet until true is returned, so
      * hold it for deletion until a subsequent call
@@ -384,6 +387,12 @@ class CoherentXBar : public BaseXBar
      */
     void forwardFunctional(PacketPtr pkt, PortID exclude_slave_port_id);
 
+    /**
+     * Determine if the crossbar should sink the packet, as opposed to
+     * forwarding it, or responding.
+     */
+    bool sinkPacket(const PacketPtr pkt) const;
+
     Stats::Scalar snoops;
     Stats::Distribution snoopFanout;