mem, cpu: Add a separate flag for strictly ordered memory

The Request::UNCACHEABLE flag currently has two different
functions. The first, and obvious, function is to prevent the memory
system from caching data in the request. The second function is to
prevent reordering and speculation in CPU models.

This changeset gives the order/speculation requirement a separate flag
(Request::STRICT_ORDER). This flag prevents CPU models from doing the
following optimizations:

    * Speculation: CPU models are not allowed to issue speculative
      loads.

    * Write combining: CPU models and caches are not allowed to merge
      writes to the same cache line.

Note: The memory system may still reorder accesses unless the
UNCACHEABLE flag is set. It is therefore expected that the
STRICT_ORDER flag is combined with the UNCACHEABLE flag to prevent
this behavior.

5 files changed, 31 insertions, 28 deletions

diff --git a/src/cpu/o3/comm.hh b/src/cpu/o3/comm.hh
index a425484f5..4da251104 100644
--- a/src/cpu/o3/comm.hh
+++ b/src/cpu/o3/comm.hh
@@ -185,8 +185,9 @@ struct TimeBufStruct {
         /// Instruction that caused the a non-mispredict squash
         DynInstPtr squashInst; // *F
 
-        /// Hack for now to send back an uncached access to the IEW stage.
-        DynInstPtr uncachedLoad; // *I
+        /// Hack for now to send back a strictly ordered access to the
+        /// IEW stage.
+        DynInstPtr strictlyOrderedLoad; // *I
 
         /// Communication specifically to the IQ to tell the IQ that it can
         /// schedule a non-speculative instruction.
@@ -216,8 +217,9 @@ struct TimeBufStruct {
         /// If the interrupt ended up being cleared before being handled
         bool clearInterrupt; // *F
 
-        /// Hack for now to send back an uncached access to the IEW stage.
-        bool uncached; // *I
+        /// Hack for now to send back an strictly ordered access to
+        /// the IEW stage.
+        bool strictlyOrdered; // *I
 
     };
 
diff --git a/src/cpu/o3/commit_impl.hh b/src/cpu/o3/commit_impl.hh
index aa1948602..5323e1413 100644
--- a/src/cpu/o3/commit_impl.hh
+++ b/src/cpu/o3/commit_impl.hh
@@ -1145,7 +1145,7 @@ DefaultCommit<Impl>::commitHead(DynInstPtr &head_inst, unsigned inst_num)
         // think are possible.
         assert(head_inst->isNonSpeculative() || head_inst->isStoreConditional()
                || head_inst->isMemBarrier() || head_inst->isWriteBarrier() ||
-               (head_inst->isLoad() && head_inst->uncacheable()));
+               (head_inst->isLoad() && head_inst->strictlyOrdered()));
 
         DPRINTF(Commit, "Encountered a barrier or non-speculative "
                 "instruction [sn:%lli] at the head of the ROB, PC %s.\n",
@@ -1162,11 +1162,11 @@ DefaultCommit<Impl>::commitHead(DynInstPtr &head_inst, unsigned inst_num)
         // it is executed.
         head_inst->clearCanCommit();
 
-        if (head_inst->isLoad() && head_inst->uncacheable()) {
-            DPRINTF(Commit, "[sn:%lli]: Uncached load, PC %s.\n",
+        if (head_inst->isLoad() && head_inst->strictlyOrdered()) {
+            DPRINTF(Commit, "[sn:%lli]: Strictly ordered load, PC %s.\n",
                     head_inst->seqNum, head_inst->pcState());
-            toIEW->commitInfo[tid].uncached = true;
-            toIEW->commitInfo[tid].uncachedLoad = head_inst;
+            toIEW->commitInfo[tid].strictlyOrdered = true;
+            toIEW->commitInfo[tid].strictlyOrderedLoad = head_inst;
         } else {
             ++commitNonSpecStalls;
         }
diff --git a/src/cpu/o3/iew_impl.hh b/src/cpu/o3/iew_impl.hh
index 4741df634..730eb0cfe 100644
--- a/src/cpu/o3/iew_impl.hh
+++ b/src/cpu/o3/iew_impl.hh
@@ -1418,9 +1418,9 @@ DefaultIEW<Impl>::writebackInsts()
 
         // Some instructions will be sent to commit without having
         // executed because they need commit to handle them.
-        // E.g. Uncached loads have not actually executed when they
+        // E.g. Strictly ordered loads have not actually executed when they
         // are first sent to commit.  Instead commit must tell the LSQ
-        // when it's ready to execute the uncached load.
+        // when it's ready to execute the strictly ordered load.
         if (!inst->isSquashed() && inst->isExecuted() && inst->getFault() == NoFault) {
             int dependents = instQueue.wakeDependents(inst);
 
@@ -1522,9 +1522,10 @@ DefaultIEW<Impl>::tick()
         if (fromCommit->commitInfo[tid].nonSpecSeqNum != 0) {
 
             //DPRINTF(IEW,"NonspecInst from thread %i",tid);
-            if (fromCommit->commitInfo[tid].uncached) {
-                instQueue.replayMemInst(fromCommit->commitInfo[tid].uncachedLoad);
-                fromCommit->commitInfo[tid].uncachedLoad->setAtCommit();
+            if (fromCommit->commitInfo[tid].strictlyOrdered) {
+                instQueue.replayMemInst(
+                    fromCommit->commitInfo[tid].strictlyOrderedLoad);
+                fromCommit->commitInfo[tid].strictlyOrderedLoad->setAtCommit();
             } else {
                 instQueue.scheduleNonSpec(
                     fromCommit->commitInfo[tid].nonSpecSeqNum);
diff --git a/src/cpu/o3/lsq_unit.hh b/src/cpu/o3/lsq_unit.hh
index 6fe832bf6..e356dd442 100644
--- a/src/cpu/o3/lsq_unit.hh
+++ b/src/cpu/o3/lsq_unit.hh
@@ -559,15 +559,15 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
 
     assert(!load_inst->isExecuted());
 
-    // Make sure this isn't an uncacheable access
-    // A bit of a hackish way to get uncached accesses to work only if they're
-    // at the head of the LSQ and are ready to commit (at the head of the ROB
-    // too).
-    if (req->isUncacheable() &&
+    // Make sure this isn't a strictly ordered load
+    // A bit of a hackish way to get strictly ordered accesses to work
+    // only if they're at the head of the LSQ and are ready to commit
+    // (at the head of the ROB too).
+    if (req->isStrictlyOrdered() &&
         (load_idx != loadHead || !load_inst->isAtCommit())) {
         iewStage->rescheduleMemInst(load_inst);
         ++lsqRescheduledLoads;
-        DPRINTF(LSQUnit, "Uncachable load [sn:%lli] PC %s\n",
+        DPRINTF(LSQUnit, "Strictly ordered load [sn:%lli] PC %s\n",
                 load_inst->seqNum, load_inst->pcState());
 
         // Must delete request now that it wasn't handed off to
@@ -579,7 +579,7 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
             delete sreqHigh;
         }
         return std::make_shared<GenericISA::M5PanicFault>(
-            "Uncachable load [sn:%llx] PC %s\n",
+            "Strictly ordered load [sn:%llx] PC %s\n",
             load_inst->seqNum, load_inst->pcState());
     }
 
@@ -653,7 +653,7 @@ LSQUnit<Impl>::read(Request *req, Request *sreqLow, Request *sreqHigh,
 
         if (store_size == 0)
             continue;
-        else if (storeQueue[store_idx].inst->uncacheable())
+        else if (storeQueue[store_idx].inst->strictlyOrdered())
             continue;
 
         assert(storeQueue[store_idx].inst->effAddrValid());
diff --git a/src/cpu/o3/lsq_unit_impl.hh b/src/cpu/o3/lsq_unit_impl.hh
index 9c500443e..3019e80d2 100644
--- a/src/cpu/o3/lsq_unit_impl.hh
+++ b/src/cpu/o3/lsq_unit_impl.hh
@@ -471,7 +471,7 @@ LSQUnit<Impl>::checkSnoop(PacketPtr pkt)
     while (load_idx != loadTail) {
         DynInstPtr ld_inst = loadQueue[load_idx];
 
-        if (!ld_inst->effAddrValid() || ld_inst->uncacheable()) {
+        if (!ld_inst->effAddrValid() || ld_inst->strictlyOrdered()) {
             incrLdIdx(load_idx);
             continue;
         }
@@ -528,7 +528,7 @@ LSQUnit<Impl>::checkViolations(int load_idx, DynInstPtr &inst)
      */
     while (load_idx != loadTail) {
         DynInstPtr ld_inst = loadQueue[load_idx];
-        if (!ld_inst->effAddrValid() || ld_inst->uncacheable()) {
+        if (!ld_inst->effAddrValid() || ld_inst->strictlyOrdered()) {
             incrLdIdx(load_idx);
             continue;
         }
@@ -617,15 +617,15 @@ LSQUnit<Impl>::executeLoad(DynInstPtr &inst)
     // along to commit without the instruction completing.
     if (load_fault != NoFault || !inst->readPredicate()) {
         // Send this instruction to commit, also make sure iew stage
-        // realizes there is activity.
-        // Mark it as executed unless it is an uncached load that
-        // needs to hit the head of commit.
+        // realizes there is activity.  Mark it as executed unless it
+        // is a strictly ordered load that needs to hit the head of
+        // commit.
         if (!inst->readPredicate())
             inst->forwardOldRegs();
         DPRINTF(LSQUnit, "Load [sn:%lli] not executed from %s\n",
                 inst->seqNum,
                 (load_fault != NoFault ? "fault" : "predication"));
-        if (!(inst->hasRequest() && inst->uncacheable()) ||
+        if (!(inst->hasRequest() && inst->strictlyOrdered()) ||
             inst->isAtCommit()) {
             inst->setExecuted();
         }