commit 642a563cf08171f1a89ae67229e93187cae28bd8
author Tuan Ta <qtt2@cornell.edu> Mon Apr 02 15:18:57 2018 -0400
committer Tuan Ta <qtt2@cornell.edu> Thu Jun 14 22:42:16 2018 +0000
tree 980800270fd015745c9ba8914a88e4ef2d96ad70
parent 7341f14f147337810325b19bac20db095b162911

cpu: Prevent suspended TimingSimple CPUs from fetching next instructions

In TimingSimpleCPU model, when a CPU is suspended by a syscall (e.g.,
futex(FUTEX_WAIT)), the CPU waits for another CPU to wake it up
(e.g., FUTEX_WAKE operation). While staying Idle, the suspended CPU
should not try to fetch next instructions after the syscall.

This patch added a status check before a CPU schedule a fetch event
after a fault is handled.

Change-Id: I0cc953135686c9b35afe94942aa1d0b245ec60a2
Reviewed-on: https://gem5-review.googlesource.com/8181
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>

src/cpu/simple/timing.cc

diff --git a/src/cpu/simple/timing.cc b/src/cpu/simple/timing.cc
index 14e760a..b5450cf 100644
--- a/src/cpu/simple/timing.cc
+++ b/src/cpu/simple/timing.cc
@@ -680,25 +680,32 @@
         return;
 
     if (fault != NoFault) {
-        DPRINTF(SimpleCPU, "Fault occured, scheduling fetch event\n");
+        DPRINTF(SimpleCPU, "Fault occured. Handling the fault\n");
 
         advancePC(fault);
 
-        Tick stall = dynamic_pointer_cast<SyscallRetryFault>(fault) ?
-                     clockEdge(syscallRetryLatency) : clockEdge();
+        // A syscall fault could suspend this CPU (e.g., futex_wait)
+        // If the _status is not Idle, schedule an event to fetch the next
+        // instruction after 'stall' ticks.
+        // If the cpu has been suspended (i.e., _status == Idle), another
+        // cpu will wake this cpu up later.
+        if (_status != Idle) {
+            DPRINTF(SimpleCPU, "Scheduling fetch event after the Fault\n");
 
-        reschedule(fetchEvent, stall, true);
+            Tick stall = dynamic_pointer_cast<SyscallRetryFault>(fault) ?
+                         clockEdge(syscallRetryLatency) : clockEdge();
+            reschedule(fetchEvent, stall, true);
+            _status = Faulting;
+        }
 
-        _status = Faulting;
         return;
     }
 
-
     if (!t_info.stayAtPC)
         advancePC(fault);
 
     if (tryCompleteDrain())
-            return;
+        return;
 
     if (_status == BaseSimpleCPU::Running) {
         // kick off fetch of next instruction... callback from icache