replay: fix recursive checkpoints

Record/replay uses checkpoints to synchronize the execution
of the threads and timers. Hardware events such as BH are
processed at the checkpoints too.
Event processing can cause refreshing the virtual timers
and calling the icount-related functions, that also use checkpoints.
This patch prevents recursive processing of such checkpoints,
because they have their own records in the log and should be
processed later.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgalyuk@ispras.ru>
Message-Id: <161700476500.1140362.10108444973730452257.stgit@pasha-ThinkPad-X280>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

replay/replay.c

@@ -180,12 +180,13 @@ bool replay_checkpoint(ReplayCheckpoint checkpoint)
     }
 
     if (in_checkpoint) {
-        /* If we are already in checkpoint, then there is no need
-           for additional synchronization.
+        /*
            Recursion occurs when HW event modifies timers.
-           Timer modification may invoke the checkpoint and
-           proceed to recursion. */
-        return true;
+           Prevent performing icount warp in this case and
+           wait for another invocation of the checkpoint.
+        */
+        g_assert(replay_mode == REPLAY_MODE_PLAY);
+        return false;
     }
     in_checkpoint = true;