prefetch: don't panic on requests w/o contextID (e.g., writebacks).

src/mem/cache/prefetch/ghb.cc

@@ -34,13 +34,18 @@
  * GHB Prefetcher implementation.
  */
 
+#include "base/trace.hh"
 #include "mem/cache/prefetch/ghb.hh"
-#include "arch/isa_traits.hh"
 
 void
 GHBPrefetcher::calculatePrefetch(PacketPtr &pkt, std::list<Addr> &addresses,
                                  std::list<Tick> &delays)
 {
+    if (useContextId && !pkt->req->hasContextId()) {
+        DPRINTF(HWPrefetch, "ignoring request with no context ID");
+        return;
+    }
+
     Addr blk_addr = pkt->getAddr() & ~(Addr)(blkSize-1);
     int ctx_id = useContextId ? pkt->req->contextId() : 0;
     assert(ctx_id < Max_Contexts);

src/mem/cache/prefetch/stride.cc

@@ -46,6 +46,11 @@ StridePrefetcher::calculatePrefetch(PacketPtr &pkt, std::list<Addr> &addresses,
         return;
     }
 
+    if (useContextId && !pkt->req->hasContextId()) {
+        DPRINTF(HWPrefetch, "ignoring request with no context ID");
+        return;
+    }
+
     Addr blk_addr = pkt->getAddr() & ~(Addr)(blkSize-1);
     int ctx_id = useContextId ? pkt->req->contextId() : 0;
     Addr pc = pkt->req->getPC();

src/mem/request.hh

@@ -409,6 +409,12 @@ class Request : public FastAlloc
         flags.set(VALID_EXTRA_DATA);
     }
 
+    bool
+    hasContextId() const
+    {
+        return flags.isSet(VALID_CONTEXT_ID);
+    }
+
     /** Accessor function for context ID.*/
     int
     contextId() const
@@ -425,13 +431,13 @@ class Request : public FastAlloc
         return _threadId;
     }
 
-    /** Accessor function for pc.*/
     bool
     hasPC() const
     {
         return flags.isSet(VALID_PC);
     }
 
+    /** Accessor function for pc.*/
     Addr
     getPC() const
     {