diff --git a/arch/sim/src/up_idle.c b/arch/sim/src/up_idle.c
index d3009028f95..93d850dfe76 100644
--- a/arch/sim/src/up_idle.c
+++ b/arch/sim/src/up_idle.c
@@ -48,10 +48,6 @@
 
 #include "up_internal.h"
 
-/****************************************************************************
- * Pre-processor Definitions
- ****************************************************************************/
-
 /****************************************************************************
  * Private Data
  ****************************************************************************/
@@ -71,10 +67,6 @@ extern void up_x11update(void);
 #endif
 #endif
 
-/****************************************************************************
- * Private Functions
- ****************************************************************************/
-
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -197,9 +189,12 @@ void up_idle(void)
 
   lock = SP_UNLOCKED;
 
+#if !defined(CONFIG_SIM_WALLTIME) && !defined(CONFIG_SIM_X11FB)
   /* Give other pthreads/CPUs a shot */
 
   //pthread_yield();
-  usleep(25*1000);
+  up_hostusleep(25*1000);
+
+#endif
 #endif
 }
diff --git a/arch/sim/src/up_simsmp.c b/arch/sim/src/up_simsmp.c
index da9ccd6e46a..c54144678d5 100644
--- a/arch/sim/src/up_simsmp.c
+++ b/arch/sim/src/up_simsmp.c
@@ -96,6 +96,10 @@ static void *sim_idle_trampoline(void *arg)
       return NULL;
     }
 
+  /* Let up_cpustart() continue */
+
+  (void)pthread_mutex_unlock(&cpuinfo->mutex);
+
   /* Give control to the IDLE task */
 
   (void)cpuinfo->idletask(0, (char **)0);