replay: allow runstate shutdown->running when replaying trace

When replaying a trace, it is possible to go from shutdown to running
with a reverse-debugging step. This can be useful if the problem being
debugged triggers a reset or shutdown.

This can be tested by making a recording of a machine that shuts down,
then using -action shutdown=pause when replaying it. Continuing to the
end of the trace then reverse-stepping in gdb crashes due to invalid
runstate transition.

Just permitting the transition seems to be all that's necessary for
reverse-debugging to work well in such a state.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20240813050638.446172-5-npiggin@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240813202329.1237572-13-alex.bennee@linaro.org>

include/sysemu/runstate.h

@@ -9,6 +9,7 @@ void runstate_set(RunState new_state);
 RunState runstate_get(void);
 bool runstate_is_running(void);
 bool runstate_needs_reset(void);
+void runstate_replay_enable(void);
 
 typedef void VMChangeStateHandler(void *opaque, bool running, RunState state);
 

replay/replay.c

@@ -385,6 +385,8 @@ static void replay_enable(const char *fname, int mode)
         replay_fetch_data_kind();
     }
 
+    runstate_replay_enable();
+
     replay_init_events();
 }
 

system/runstate.c

@@ -181,6 +181,12 @@ static const RunStateTransition runstate_transitions_def[] = {
     { RUN_STATE__MAX, RUN_STATE__MAX },
 };
 
+static const RunStateTransition replay_play_runstate_transitions_def[] = {
+    { RUN_STATE_SHUTDOWN, RUN_STATE_RUNNING},
+
+    { RUN_STATE__MAX, RUN_STATE__MAX },
+};
+
 static bool runstate_valid_transitions[RUN_STATE__MAX][RUN_STATE__MAX];
 
 bool runstate_check(RunState state)
@@ -188,14 +194,33 @@ bool runstate_check(RunState state)
     return current_run_state == state;
 }
 
-static void runstate_init(void)
+static void transitions_set_valid(const RunStateTransition *rst)
 {
     const RunStateTransition *p;
 
-    memset(&runstate_valid_transitions, 0, sizeof(runstate_valid_transitions));
-    for (p = &runstate_transitions_def[0]; p->from != RUN_STATE__MAX; p++) {
+    for (p = rst; p->from != RUN_STATE__MAX; p++) {
         runstate_valid_transitions[p->from][p->to] = true;
     }
+}
+
+void runstate_replay_enable(void)
+{
+    assert(replay_mode != REPLAY_MODE_NONE);
+
+    if (replay_mode == REPLAY_MODE_PLAY) {
+        /*
+         * When reverse-debugging, it is possible to move state from
+         * shutdown to running.
+         */
+        transitions_set_valid(&replay_play_runstate_transitions_def[0]);
+    }
+}
+
+static void runstate_init(void)
+{
+    memset(&runstate_valid_transitions, 0, sizeof(runstate_valid_transitions));
+
+    transitions_set_valid(&runstate_transitions_def[0]);
 
     qemu_mutex_init(&vmstop_lock);
 }