x86: Squash outstanding walks when instructions are squashed.

This is the x86 version of the ARM changeset baa17ba80e06. In case an
instruction has been squashed by the o3 cpu, this patch allows page
table walker to avoid carrying out a pending translation that the
instruction requested for.

src/arch/x86/X86TLB.py

@@ -47,6 +47,8 @@ class X86PagetableWalker(MemObject):
     cxx_header = 'arch/x86/pagetable_walker.hh'
     port = MasterPort("Port for the hardware table walker")
     system = Param.System(Parent.any, "system object")
+    num_squash_per_cycle = Param.Unsigned(4,
+            "Number of outstanding walks that can be squashed per cycle")
 
 class X86TLB(BaseTLB):
     type = 'X86TLB'

src/arch/x86/pagetable_walker.cc

@@ -139,11 +139,8 @@ 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()) {
-            WalkerState * newState = currStates.front();
-            if (!newState->wasStarted())
-                newState->startWalk();
-        }
+        if (currStates.size())
+            startWalkWrapper();
     }
     return true;
 }
@@ -192,6 +189,36 @@ Walker::WalkerState::initState(ThreadContext * _tc,
     timing = _isTiming;
 }
 
+void
+Walker::startWalkWrapper()
+{
+    unsigned num_squashed = 0;
+    WalkerState *currState = currStates.front();
+    while ((num_squashed < numSquashable) && currState &&
+        currState->translation->squashed()) {
+        currStates.pop_front();
+        num_squashed++;
+
+        DPRINTF(PageTableWalker, "Squashing table walk for address %#x\n",
+            currState->req->getVaddr());
+
+        // finish the translation which will delete the translation object
+        currState->translation->finish(new UnimpFault("Squashed Inst"),
+                currState->req, currState->tc, currState->mode);
+
+        // delete the current request
+        delete currState;
+
+        // check the next translation request, if it exists
+        if (currStates.size())
+            currState = currStates.front();
+        else
+            currState = NULL;
+    }
+    if (currState && !currState->wasStarted())
+        currState->startWalk();
+}
+
 Fault
 Walker::WalkerState::startWalk()
 {

src/arch/x86/pagetable_walker.hh

@@ -87,6 +87,7 @@ namespace X86ISA
         // State to track each walk of the page table
         class WalkerState
         {
+          friend class Walker;
           private:
             enum State {
                 Ready,
@@ -176,6 +177,12 @@ namespace X86ISA
         System * sys;
         MasterID masterId;
 
+        // The number of outstanding walks that can be squashed per cycle.
+        unsigned numSquashable;
+
+        // Wrapper for checking for squashes before starting a translation.
+        void startWalkWrapper();
+
         // Functions for dealing with packets.
         bool recvTimingResp(PacketPtr pkt);
         void recvRetry();
@@ -199,7 +206,8 @@ namespace X86ISA
         Walker(const Params *params) :
             MemObject(params), port(name() + ".port", this),
             funcState(this, NULL, NULL, true), tlb(NULL), sys(params->system),
-            masterId(sys->getMasterId(name()))
+            masterId(sys->getMasterId(name())),
+            numSquashable(params->num_squash_per_cycle)
         {
         }
     };