vl: allow other threads to do qemu_system_vmstop_request

There patch protects vmstop_requested with a lock and introduces
qemu_system_vmstop_request_prepare.

Together with the new call to qemu_vmstop_requested in vm_start,
qemu_system_vmstop_request_prepare avoids a race where the VM could remain
stopped even though the iostatus of a block device has already been set
(for example).

qemu_system_vmstop_request_prepare however also lets the caller thread
delay observation of the state change until it has itself communicated
that change to the user.  This delay avoids any possibility of a wrong
reordering of the BLOCK_IO_ERROR event and the subsequent STOP event.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

cpus.c

@@ -1206,6 +1206,7 @@ void cpu_stop_current(void)
 int vm_stop(RunState state)
 {
     if (qemu_in_vcpu_thread()) {
+        qemu_system_vmstop_request_prepare();
         qemu_system_vmstop_request(state);
         /*
          * FIXME: should not return to device code in case

include/sysemu/sysemu.h

@@ -62,6 +62,7 @@ void qemu_system_powerdown_request(void);
 void qemu_register_powerdown_notifier(Notifier *notifier);
 void qemu_system_debug_request(void);
 void qemu_system_vmstop_request(RunState reason);
+void qemu_system_vmstop_request_prepare(void);
 int qemu_shutdown_requested_get(void);
 int qemu_reset_requested_get(void);
 void qemu_system_killed(int signal, pid_t pid);

target-lm32/op_helper.c

@@ -42,7 +42,7 @@ void HELPER(ill)(CPULM32State *env)
     fprintf(stderr, "VM paused due to illegal instruction. "
             "Connect a debugger or switch to the monitor console "
             "to find out more.\n");
-    qemu_system_vmstop_request(RUN_STATE_PAUSED);
+    vm_stop(RUN_STATE_PAUSED);
     cs->halted = 1;
     raise_exception(env, EXCP_HALTED);
 #endif

vl.c

@@ -574,6 +574,10 @@ static int default_driver_check(QemuOpts *opts, void *opaque)
 
 static RunState current_run_state = RUN_STATE_PRELAUNCH;
 
+/* We use RUN_STATE_MAX but any invalid value will do */
+static RunState vmstop_requested = RUN_STATE_MAX;
+static QemuMutex vmstop_lock;
+
 typedef struct {
     RunState from;
     RunState to;
@@ -650,10 +654,11 @@ static void runstate_init(void)
     const RunStateTransition *p;
 
     memset(&runstate_valid_transitions, 0, sizeof(runstate_valid_transitions));
-
     for (p = &runstate_transitions_def[0]; p->from != RUN_STATE_MAX; p++) {
         runstate_valid_transitions[p->from][p->to] = true;
     }
+
+    qemu_mutex_init(&vmstop_lock);
 }
 
 /* This function will abort() on invalid state transitions */
@@ -693,6 +698,54 @@ StatusInfo *qmp_query_status(Error **errp)
     return info;
 }
 
+static bool qemu_vmstop_requested(RunState *r)
+{
+    qemu_mutex_lock(&vmstop_lock);
+    *r = vmstop_requested;
+    vmstop_requested = RUN_STATE_MAX;
+    qemu_mutex_unlock(&vmstop_lock);
+    return *r < RUN_STATE_MAX;
+}
+
+void qemu_system_vmstop_request_prepare(void)
+{
+    qemu_mutex_lock(&vmstop_lock);
+}
+
+void qemu_system_vmstop_request(RunState state)
+{
+    vmstop_requested = state;
+    qemu_mutex_unlock(&vmstop_lock);
+    qemu_notify_event();
+}
+
+void vm_start(void)
+{
+    RunState requested;
+
+    qemu_vmstop_requested(&requested);
+    if (runstate_is_running() && requested == RUN_STATE_MAX) {
+        return;
+    }
+
+    /* Ensure that a STOP/RESUME pair of events is emitted if a
+     * vmstop request was pending.  The BLOCK_IO_ERROR event, for
+     * example, according to documentation is always followed by
+     * the STOP event.
+     */
+    if (runstate_is_running()) {
+        monitor_protocol_event(QEVENT_STOP, NULL);
+    } else {
+        cpu_enable_ticks();
+        runstate_set(RUN_STATE_RUNNING);
+        vm_state_notify(1, RUN_STATE_RUNNING);
+        resume_all_vcpus();
+    }
+
+    monitor_protocol_event(QEVENT_RESUME, NULL);
+}
+
+
 /***********************************************************/
 /* real time host monotonic timer */
 
@@ -1658,17 +1711,6 @@ void vm_state_notify(int running, RunState state)
     }
 }
 
-void vm_start(void)
-{
-    if (!runstate_is_running()) {
-        cpu_enable_ticks();
-        runstate_set(RUN_STATE_RUNNING);
-        vm_state_notify(1, RUN_STATE_RUNNING);
-        resume_all_vcpus();
-        monitor_protocol_event(QEVENT_RESUME, NULL);
-    }
-}
-
 /* reset/shutdown handler */
 
 typedef struct QEMUResetEntry {
@@ -1693,7 +1735,6 @@ static NotifierList suspend_notifiers =
 static NotifierList wakeup_notifiers =
     NOTIFIER_LIST_INITIALIZER(wakeup_notifiers);
 static uint32_t wakeup_reason_mask = ~(1 << QEMU_WAKEUP_REASON_NONE);
-static RunState vmstop_requested = RUN_STATE_MAX;
 
 int qemu_shutdown_requested_get(void)
 {
@@ -1761,18 +1802,6 @@ static int qemu_debug_requested(void)
     return r;
 }
 
-/* We use RUN_STATE_MAX but any invalid value will do */
-static bool qemu_vmstop_requested(RunState *r)
-{
-    if (vmstop_requested < RUN_STATE_MAX) {
-        *r = vmstop_requested;
-        vmstop_requested = RUN_STATE_MAX;
-        return true;
-    }
-
-    return false;
-}
-
 void qemu_register_reset(QEMUResetHandler *func, void *opaque)
 {
     QEMUResetEntry *re = g_malloc0(sizeof(QEMUResetEntry));
@@ -1922,12 +1951,6 @@ void qemu_system_debug_request(void)
     qemu_notify_event();
 }
 
-void qemu_system_vmstop_request(RunState state)
-{
-    vmstop_requested = state;
-    qemu_notify_event();
-}
-
 static bool main_loop_should_exit(void)
 {
     RunState r;