x86: Delay X86 table walk on receiving walker response

This patch fixes a minor issue in the X86 page table walker where it
ended up sending new request packets to the crossbar before the
response processing was finished (recvTimingResp is directly calling
sendTimingReq). Under certain conditions this caused the crossbar to
see illegal combinations of request/response overlap, in turn causing
problems with a slightly modified crossbar implementation.

src/arch/x86/pagetable_walker.cc

@@ -124,8 +124,10 @@ Walker::recvTimingResp(PacketPtr pkt)
         delete senderWalk;
         // Since we block requests when another is outstanding, we
         // need to check if there is a waiting request to be serviced
-        if (currStates.size())
-            startWalkWrapper();
+        if (currStates.size() && !startWalkWrapperEvent.scheduled())
+            // delay sending any new requests until we are finished
+            // with the responses
+            schedule(startWalkWrapperEvent, clockEdge());
     }
     return true;
 }

src/arch/x86/pagetable_walker.hh

@@ -183,6 +183,11 @@ namespace X86ISA
         // Wrapper for checking for squashes before starting a translation.
         void startWalkWrapper();
 
+        /**
+         * Event used to call startWalkWrapper.
+         **/
+        EventWrapper<Walker, &Walker::startWalkWrapper> startWalkWrapperEvent;
+
         // Functions for dealing with packets.
         bool recvTimingResp(PacketPtr pkt);
         void recvRetry();
@@ -207,7 +212,8 @@ namespace X86ISA
             MemObject(params), port(name() + ".port", this),
             funcState(this, NULL, NULL, true), tlb(NULL), sys(params->system),
             masterId(sys->getMasterId(name())),
-            numSquashable(params->num_squash_per_cycle)
+            numSquashable(params->num_squash_per_cycle),
+            startWalkWrapperEvent(this)
         {
         }
     };