dev: Exit correctly in dist-gem5 for SE mode

Do not allow the exit() syscall to terminate gem5 when running in dist-gem5
mode.  The exit must be coordinated by the distributed interface instead.

Change-Id: I57f47610b59fe9e18ba3a1667fb5e45cecac1a81
Reviewed-on: https://gem5-review.googlesource.com/10461
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Mohammad Alian <m.alian1369@gmail.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

src/sim/syscall_emul.cc

@@ -43,6 +43,7 @@
 #include "base/trace.hh"
 #include "config/the_isa.hh"
 #include "cpu/thread_context.hh"
+#include "dev/net/dist_iface.hh"
 #include "mem/page_table.hh"
 #include "sim/process.hh"
 #include "sim/sim_exit.hh"
@@ -102,6 +103,20 @@ exitImpl(SyscallDesc *desc, int callnum, Process *p, ThreadContext *tc,
     for (auto &system: sys->systemList)
         activeContexts += system->numRunningContexts();
     if (activeContexts == 1) {
+        /**
+         * Even though we are terminating the final thread context, dist-gem5
+         * requires the simulation to remain active and provide
+         * synchronization messages to the switch process. So we just halt
+         * the last thread context and return. The simulation will be
+         * terminated by dist-gem5 in a coordinated manner once all nodes
+         * have signaled their readiness to exit. For non dist-gem5
+         * simulations, readyToExit() always returns true.
+         */
+        if (!DistIface::readyToExit(0)) {
+            tc->halt();
+            return status;
+        }
+
         exitSimLoop("exiting with last active thread context", status & 0xff);
         return status;
     }