Mem: Fix issue with dirty block being lost when entire block transferred to non-cache.

This change fixes the problem for all the cases we actively use. If you want to try
more creative I/O device attachments (E.g. sharing an L2), this won't work. You
would need another level of caching between the I/O device and the cache
(which you actually need anyway with our current code to make sure writes
propagate). This is required so that you can mark the cache in between as
top level and it won't try to send ownership of a block to the I/O device.
Asserts have been added that should catch any issues.

4 files changed, 8 insertions, 1 deletions

diff --git a/src/mem/cache/BaseCache.py b/src/mem/cache/BaseCache.py
index dffac2234..5c7ae5274 100644
--- a/src/mem/cache/BaseCache.py
+++ b/src/mem/cache/BaseCache.py
@@ -48,6 +48,7 @@ class BaseCache(MemObject):
     size = Param.MemorySize("capacity in bytes")
     forward_snoops = Param.Bool(True,
         "forward snoops from mem side to cpu side")
+    is_top_level = Param.Bool(False, "Is this cache at the top level (e.g. L1)")
     subblock_size = Param.Int(0,
         "Size of subblock in IIC used for compression")
     tgts_per_mshr = Param.Int("max number of accesses per MSHR")
diff --git a/src/mem/cache/base.cc b/src/mem/cache/base.cc
index 9166e1a09..b7e331d54 100644
--- a/src/mem/cache/base.cc
+++ b/src/mem/cache/base.cc
@@ -58,6 +58,7 @@ BaseCache::BaseCache(const Params *p)
       hitLatency(p->latency),
       numTarget(p->tgts_per_mshr),
       forwardSnoops(p->forward_snoops),
+      isTopLevel(p->is_top_level),
       blocked(0),
       noTargetMSHR(NULL),
       missCount(p->max_miss_count),
diff --git a/src/mem/cache/base.hh b/src/mem/cache/base.hh
index e8a644296..28ddf5054 100644
--- a/src/mem/cache/base.hh
+++ b/src/mem/cache/base.hh
@@ -194,6 +194,11 @@ class BaseCache : public MemObject
     /** Do we forward snoops from mem side port through to cpu side port? */
     bool forwardSnoops;
 
+    /** Is this cache a toplevel cache (e.g. L1, I/O cache). If so we should
+     * never try to forward ownership and similar optimizations to the cpu
+     * side */
+    bool isTopLevel;
+
     /**
      * Bit vector of the blocking reasons for the access path.
      * @sa #BlockedCause
diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index e4e4a3c92..0b2b273f9 100644
--- a/src/mem/cache/cache_impl.hh
+++ b/src/mem/cache/cache_impl.hh
@@ -216,7 +216,7 @@ Cache<TagStore>::satisfyCpuSideRequest(PacketPtr pkt, BlkType *blk,
                 
                 if (blk->isDirty()) {
                     // special considerations if we're owner:
-                    if (!deferred_response) {
+                    if (!deferred_response && !isTopLevel) {
                         // if we are responding immediately and can
                         // signal that we're transferring ownership
                         // along with exclusivity, do so