mem-cache: Allow clflush's uncacheable requests on classic cache (#205)

When a linux kernel changes a page property, it flushes the related
cache lines. The kernel might change the page property before flushing
the cache lines. This results in the clflush might occur in an
uncacheable region.

Currently, an uncacheable request must be a read or a write. However,
clflush request is neither of them.

This change aims to allow clflush requests to work on uncacheable
regions. Since there is no straightforward way to check if a packet is
from a clflush instruction, this change permits all Clean Invalidate
Requests, which is the type of request produced by clflush, to work on
uncacheable regions.

src/mem/cache/cache.cc

@@ -338,15 +338,22 @@ Cache::handleTimingReqMiss(PacketPtr pkt, CacheBlk *blk, Tick forward_time,
 
         if (pkt->isWrite()) {
             allocateWriteBuffer(pkt, forward_time);
-        } else {
-            assert(pkt->isRead());
-
+        } else if (pkt->isRead()) {
             // uncacheable accesses always allocate a new MSHR
 
             // Here we are using forward_time, modelling the latency of
             // a miss (outbound) just as forwardLatency, neglecting the
             // lookupLatency component.
             allocateMissBuffer(pkt, forward_time);
+        } else {
+            // When a linux kernel wants to change a page property,
+            // it flushes the related cache lines. The kernel might change
+            // the page property before flushing the cache. This results in
+            // the clflush might occur in an uncacheable region.
+            // clflush results in a CleanInvalidReq, which is neither read
+            // nor write.
+            assert(pkt->isCleanInvalidateRequest());
+            allocateWriteBuffer(pkt, forward_time);
         }
 
         return;

src/mem/packet.hh

@@ -1437,6 +1437,15 @@ class Packet : public Printable, public Extensible<Packet>
         return cmd == MemCmd::CleanEvict || cmd == MemCmd::WritebackClean;
     }
 
+    /**
+     * Is this packet a clean invalidate request, e.g., clflush/clflushopt?
+     */
+    bool
+    isCleanInvalidateRequest() const
+    {
+        return cmd == MemCmd::CleanInvalidReq;
+    }
+
     bool
     isMaskedWrite() const
     {