Since cpu emulation happens on a separate thread, it is necessary to
pause/resume it upon certain events such as reset, debug exception,
live migration, etc.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Index: trunk/cpu-defs.h
===================================================================
--- trunk.orig/cpu-defs.h
+++ trunk/cpu-defs.h
@@ -172,6 +172,8 @@ typedef struct CPUWatchpoint {
     target_ulong mem_io_vaddr; /* target virtual addr at which the      \
                                      memory was accessed */             \
     uint32_t halted; /* Nonzero if the CPU is in suspend state */       \
+    uint32_t stop;   /* Stop request */                                 \
+    uint32_t stopped; /* Artificially stopped */                        \
     uint32_t interrupt_request;                                         \
     volatile sig_atomic_t exit_request;                                 \
     /* The meaning of the MMU modes is defined in the target code. */   \
Index: trunk/vl.c
===================================================================
--- trunk.orig/vl.c
+++ trunk/vl.c
@@ -292,6 +292,11 @@ QemuCond qemu_cpu_cond;
 /* system init */
 QemuCond qemu_system_cond;
 
+QemuCond qemu_pause_cond;
+
+static void pause_all_vcpus(void);
+static void resume_all_vcpus(void);
+
 /***********************************************************/
 /* x86 ISA bus support */
 
@@ -3539,6 +3544,7 @@ void vm_start(void)
     if (!vm_running) {
         cpu_enable_ticks();
         vm_running = 1;
+        resume_all_vcpus();
         vm_state_notify(1, 0);
         qemu_rearm_alarm_timer(alarm_timer);
     }
@@ -3549,6 +3555,7 @@ void vm_stop(int reason)
     if (vm_running) {
         cpu_disable_ticks();
         vm_running = 0;
+        pause_all_vcpus();
         vm_state_notify(0, reason);
     }
 }
@@ -3737,8 +3744,29 @@ static void host_main_loop_wait(int *tim
 }
 #endif
 
+/* Q: is it allowed to enter cpu_exec */
+static int cpu_can_run(CPUState *env)
+{
+    if (env->stop)
+        return 0;
+    if (env->stopped)
+        return 0;
+    if (shutdown_requested)
+        return 0;
+    if (powerdown_requested)
+        return 0;
+    if (reset_requested)
+        return 0;
+    return 1;
+}
+
+/* Q: should break out of the wait loop */
 static int cpu_has_work(CPUState *env)
 {
+    if (env->stop)
+        return 1;
+    if (env->stopped)
+        return 0;
     if (!env->halted)
         return 1;
     if (qemu_cpu_has_work(env))
@@ -3771,6 +3799,11 @@ static void qemu_wait_io_event(CPUState 
     qemu_mutex_unlock(&qemu_fair_mutex);
 
     qemu_mutex_lock(&qemu_global_mutex);
+    if (env->stop) {
+        env->stop = 0;
+        env->stopped = 1;
+        qemu_cond_signal(&qemu_pause_cond);
+    }
 }
 
 void qemu_cpu_kick(void *_env)
@@ -3837,6 +3870,53 @@ static void qemu_signal_lock(unsigned in
     qemu_mutex_unlock(&qemu_fair_mutex);
 }
 
+static int all_vcpus_paused(void)
+{
+    CPUState *penv = first_cpu;
+
+    while (penv) {
+        if (penv->stop)
+            return 0;
+        penv = (CPUState *)penv->next_cpu;
+    }
+
+    return 1;
+}
+
+static void pause_all_vcpus(void)
+{
+    CPUState *penv = first_cpu;
+
+    while (penv) {
+        penv->stop = 1;
+        qemu_thread_signal(penv->thread, SIGUSR1);
+        qemu_cpu_kick(penv);
+        penv = (CPUState *)penv->next_cpu;
+    }
+
+    while (!all_vcpus_paused()) {
+        qemu_cond_timedwait(&qemu_pause_cond, &qemu_global_mutex, 100);
+        penv = first_cpu;
+        while (penv) {
+            qemu_thread_signal(penv->thread, SIGUSR1);
+            penv = (CPUState *)penv->next_cpu;
+        }
+    }
+}
+
+static void resume_all_vcpus(void)
+{
+    CPUState *penv = first_cpu;
+
+    while (penv) {
+        penv->stop = 0;
+        penv->stopped = 0;
+        qemu_thread_signal(penv->thread, SIGUSR1);
+        qemu_cpu_kick(penv);
+        penv = (CPUState *)penv->next_cpu;
+    }
+}
+
 void main_loop_wait(int timeout)
 {
     IOHandlerRecord *ioh;
@@ -3999,7 +4079,9 @@ static void *cpu_main_loop(void *arg)
                     env->icount_decr.u16.low = decr;
                     env->icount_extra = count;
                 }
-                ret = cpu_exec(env);
+                ret = EXCP_HALTED;
+                if (cpu_can_run(env))
+                    ret = cpu_exec(env);
 #ifdef CONFIG_PROFILER
                 qemu_time += profile_getclock() - ti;
 #endif
@@ -4113,6 +4195,7 @@ void qemu_init_vcpu(void *_env)
 
 static void qemu_init_state(void)
 {
+    qemu_cond_init(&qemu_pause_cond);
     qemu_mutex_init(&qemu_fair_mutex);
     qemu_mutex_init(&qemu_global_mutex);
     qemu_mutex_lock(&qemu_global_mutex);
@@ -4131,14 +4214,18 @@ static void main_loop(void)
     while (1) {
         main_loop_wait(1000);
         if (qemu_shutdown_requested()) {
+            pause_all_vcpus();
             if (no_shutdown)
                 no_shutdown = 0;
             else
                     break;
         } else if (qemu_powerdown_requested())
             qemu_system_powerdown();
-        else if (qemu_reset_requested())
+        else if (qemu_reset_requested()) {
+            pause_all_vcpus();
             qemu_system_reset();
+            resume_all_vcpus();
+        }
     }
 }