cpu: Fix o3 drain bug

For X86, the o3 CPU would get stuck with the commit stage not being
drained if an interrupt arrived while drain was pending. isDrained()
makes sure that pcState.microPC() == 0, thus ensuring that we are at
an instruction boundary. However, when we take an interrupt we
execute:

    pcState.upc(romMicroPC(entry));
    pcState.nupc(romMicroPC(entry) + 1);
    tc->pcState(pcState);

As a result, the MicroPC is no longer zero. This patch ensures the drain is
delayed until no interrupts are present.  Once draining, non-synchronous
interrupts are deffered until after the switch.

src/cpu/o3/commit_impl.hh

@@ -104,6 +104,7 @@ DefaultCommit<Impl>::DefaultCommit(O3CPU *_cpu, DerivO3CPUParams *params)
       commitWidth(params->commitWidth),
       numThreads(params->numThreads),
       drainPending(false),
+      drainImminent(false),
       trapLatency(params->trapLatency),
       canHandleInterrupts(true),
       avoidQuiesceLiveLock(false)
@@ -406,6 +407,7 @@ void
 DefaultCommit<Impl>::drainResume()
 {
     drainPending = false;
+    drainImminent = false;
 }
 
 template <class Impl>
@@ -816,8 +818,10 @@ template <class Impl>
 void
 DefaultCommit<Impl>::propagateInterrupt()
 {
+    // Don't propagate intterupts if we are currently handling a trap or
+    // in draining and the last observable instruction has been committed.
     if (commitStatus[0] == TrapPending || interrupt || trapSquash[0] ||
-            tcSquash[0])
+            tcSquash[0] || drainImminent)
         return;
 
     // Process interrupts if interrupts are enabled, not in PAL
@@ -1089,10 +1093,15 @@ DefaultCommit<Impl>::commitInsts()
                     squashAfter(tid, head_inst);
 
                 if (drainPending) {
-                    DPRINTF(Drain, "Draining: %i:%s\n", tid, pc[tid]);
-                    if (pc[tid].microPC() == 0 && interrupt == NoFault) {
+                    if (pc[tid].microPC() == 0 && interrupt == NoFault &&
+                        !thread[tid]->trapPending) {
+                        // Last architectually committed instruction.
+                        // Squash the pipeline, stall fetch, and use
+                        // drainImminent to disable interrupts
+                        DPRINTF(Drain, "Draining: %i:%s\n", tid, pc[tid]);
                         squashAfter(tid, head_inst);
                         cpu->commitDrained(tid);
+                        drainImminent = true;
                     }
                 }