[RFC,v2,48/49] replay: replay_seek_step command

diff --git a/hmp-commands.hx b/hmp-commands.hx
index 3eaa80e..57ce5da 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -1815,6 +1815,20 @@  Stops replaying at the specified @var{step}.
 
 ETEXI
 
+    {
+        .name       = "replay_seek",
+        .args_type  = "step:l",
+        .params     = "step",
+        .help       = "seeks the replay log to the specified step",
+        .mhandler.cmd = do_replay_seek,
+    },
+
+STEXI
+@item replay_seek @var{step}
+Seeks the replay log to the specified @var{step}.
+
+ETEXI
+
 STEXI
 @end table
 ETEXI
diff --git a/monitor.c b/monitor.c
index 4710a59..031decf 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1205,6 +1205,23 @@  static void do_replay_break(Monitor *mon, const QDict *qdict)
     }
 }
 
+static void do_replay_seek(Monitor *mon, const QDict *qdict)
+{
+    if (replay_mode == REPLAY_PLAY) {
+        uint64_t step = qdict_get_int(qdict, "step");
+        if (replay_seek_step(step)) {
+            monitor_printf(mon, "Seeking for step: %" PRId64 "\n", step);
+            if (!runstate_is_running()) {
+                vm_start();
+            }
+        } else {
+            monitor_printf(mon, "Cannot seek to the specified step.\n");
+        }
+    } else {
+        monitor_printf(mon, "You can go to the specific step only in PLAY mode.\n");
+    }
+}
+
 static void monitor_printc(Monitor *mon, int c)
 {
     monitor_printf(mon, "'");
diff --git a/qapi-schema.json b/qapi-schema.json
index d0a4651..9d3fda3 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -3529,3 +3529,14 @@ 
 # Since:  2.2
 ##
 { 'command': 'replay_break', 'data': { 'step': 'uint64' } }
+
+##
+# @replay_seek
+#
+# Seeks the replay log to the specified step
+#
+# @step: destination replay step 
+#
+# Since:  2.2
+##
+{ 'command': 'replay_seek', 'data': { 'step': 'uint64' } }
diff --git a/qmp-commands.hx b/qmp-commands.hx
index 575db76..a144327 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -3792,3 +3792,26 @@  Example:
 <- { "return": {} }
 
 EQMP
+
+    {
+        .name       = "replay_seek",
+        .args_type  = "step:l",
+        .mhandler.cmd_new = qmp_marshal_input_replay_seek,
+    },
+
+SQMP
+replay_seek
+-----------
+
+Seeks the replay log to the specified step.
+
+Arguments:
+
+- "step": destination replay step
+
+Example:
+
+-> { "execute": "replay_seek", "arguments": { "step": 1024 } }
+<- { "return": {} }
+
+EQMP
diff --git a/replay/replay-internal.h b/replay/replay-internal.h
index 56b6ad9..c7655e7 100755
--- a/replay/replay-internal.h
+++ b/replay/replay-internal.h
@@ -135,6 +135,9 @@  void skip_async_events_until(unsigned int kind);
     If the parameter is -1, the clock value is read to the cache anyway. */
 void replay_read_next_clock(unsigned int kind);
 
+/*! Finds saved state info which is nearest before the specified step. */
+SavedStateInfo *find_nearest_state(uint64_t step);
+
 /* Asynchronous events queue */
 
 /*! Initializes events' processing internals */
diff --git a/replay/replay-qmp.c b/replay/replay-qmp.c
index a9f30da..5cb04fb 100755
--- a/replay/replay-qmp.c
+++ b/replay/replay-qmp.c
@@ -40,3 +40,18 @@  void qmp_replay_break(uint64_t step, Error **errp)
         error_setg(errp, "replay_break can be used only in PLAY mode");
     }
 }
+
+void qmp_replay_seek(uint64_t step, Error **errp)
+{
+    if (replay_mode == REPLAY_PLAY) {
+        if (replay_seek_step(step)) {
+            if (!runstate_is_running()) {
+                vm_start();
+            }
+        } else {
+            error_setg(errp, "Cannot seek to the specified step");
+        }
+    } else {
+        error_setg(errp, "replay_seek can be used only in PLAY mode");
+    }
+}
diff --git a/replay/replay.c b/replay/replay.c
index d6949d6..e6b3e39 100755
--- a/replay/replay.c
+++ b/replay/replay.c
@@ -685,3 +685,46 @@  void replay_set_break(uint64_t step)
 {
     replay_break_step = step;
 }
+
+SavedStateInfo *find_nearest_state(uint64_t step)
+{
+    SavedStateInfo *first = saved_states;
+    SavedStateInfo *last = saved_states + saved_states_count;
+    while (first < last - 1) {
+        SavedStateInfo *next = first + (last - first) / 2;
+        if (next->step > step) {
+            last = next;
+        } else {
+            first = next;
+        }
+    }
+
+    return first;
+}
+
+int replay_seek_step(uint64_t step)
+{
+    if (replay_mode != REPLAY_PLAY) {
+        return 0;
+    }
+
+    /* load VM state, if possible */
+    if (saved_states_count > 0) {
+        /* find VM state to load */
+        SavedStateInfo *first = find_nearest_state(step);
+
+        if (first->step <= step 
+            && (replay_get_current_step() > step 
+                || replay_get_current_step() < first->step)) {
+            replay_loadvm(first - saved_states);
+        }
+    }
+
+    /* setup the breakpoint */
+    if (step >= replay_get_current_step()) {
+        replay_set_break(step);
+        return 1;
+    }
+
+    return 0;
+}
diff --git a/replay/replay.h b/replay/replay.h
index 9a20a3b..356e99b 100755
--- a/replay/replay.h
+++ b/replay/replay.h
@@ -67,6 +67,10 @@  void replay_init_timer(void);
 void replay_finish(void);
 /*! Sets step where execution should be stopped. */
 void replay_set_break(uint64_t step);
+/*! Seeks to the specified step. 
+    Loads VM state, if possible, and sets break to specified step.
+    Returns nonzero, when seeking was successful. */
+int replay_seek_step(uint64_t step);
 
 /* Processing the instructions */