mem: Make the coherent crossbar account for timing snoops

This patch introduces the concept of a snoop latency. Given the
requirement to snoop and forward packets in zero time (due to the
coherency mechanism), the latency is accounted for later.

On a snoop, we establish the latency, and later add it to the header
delay of the packet. To allow multiple caches to contribute to the
snoop latency, we use a separate variable in the packet, and then take
the maximum before adding it to the header delay.

1 files changed, 12 insertions, 5 deletions

diff --git a/src/mem/cache/cache.hh b/src/mem/cache/cache.hh
index 3d439caa7..577eab664 100644
--- a/src/mem/cache/cache.hh
+++ b/src/mem/cache/cache.hh
@@ -300,12 +300,19 @@ class Cache : public BaseCache
                                 bool already_copied, bool pending_inval);
 
     /**
-     * Sets the blk to the new state.
-     * @param blk The cache block being snooped.
-     * @param new_state The new coherence state for the block.
+     * Perform an upward snoop if needed, and update the block state
+     * (possibly invalidating the block). Also create a response if required.
+     *
+     * @param pkt Snoop packet
+     * @param blk Cache block being snooped
+     * @param is_timing Timing or atomic for the response
+     * @param is_deferred Is this a deferred snoop or not?
+     * @param pending_inval Do we have a pending invalidation?
+     *
+     * @return The snoop delay incurred by the upwards snoop
      */
-    void handleSnoop(PacketPtr ptk, CacheBlk *blk,
-                     bool is_timing, bool is_deferred, bool pending_inval);
+    uint32_t handleSnoop(PacketPtr pkt, CacheBlk *blk,
+                         bool is_timing, bool is_deferred, bool pending_inval);
 
     /**
      * Create a writeback request for the given block.