cpu-exec: Print to console if the guest is late

If the align option is enabled, we print to the user whenever
the guest clock is behind the host clock in order for he/she
to have a hint about the actual performance. The maximum
print interval is 2s and we limit the number of messages to 100.
If desired, this can be changed in cpu-exec.c

Signed-off-by: Sebastian Tanase <sebastian.tanase@openwide.fr>
Tested-by: Camille Bégué <camille.begue@openwide.fr>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

cpu-exec.c

@@ -29,6 +29,7 @@
 typedef struct SyncClocks {
     int64_t diff_clk;
     int64_t last_cpu_icount;
+    int64_t realtime_clock;
 } SyncClocks;
 
 #if !defined(CONFIG_USER_ONLY)
@@ -37,6 +38,9 @@ typedef struct SyncClocks {
  * oscillate around 0.
  */
 #define VM_CLOCK_ADVANCE 3000000
+#define THRESHOLD_REDUCE 1.5
+#define MAX_DELAY_PRINT_RATE 2000000000LL
+#define MAX_NB_PRINTS 100
 
 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
 {
@@ -68,16 +72,43 @@ static void align_clocks(SyncClocks *sc, const CPUState *cpu)
     }
 }
 
+static void print_delay(const SyncClocks *sc)
+{
+    static float threshold_delay;
+    static int64_t last_realtime_clock;
+    static int nb_prints;
+
+    if (icount_align_option &&
+        sc->realtime_clock - last_realtime_clock >= MAX_DELAY_PRINT_RATE &&
+        nb_prints < MAX_NB_PRINTS) {
+        if ((-sc->diff_clk / (float)1000000000LL > threshold_delay) ||
+            (-sc->diff_clk / (float)1000000000LL <
+             (threshold_delay - THRESHOLD_REDUCE))) {
+            threshold_delay = (-sc->diff_clk / 1000000000LL) + 1;
+            printf("Warning: The guest is now late by %.1f to %.1f seconds\n",
+                   threshold_delay - 1,
+                   threshold_delay);
+            nb_prints++;
+            last_realtime_clock = sc->realtime_clock;
+        }
+    }
+}
+
 static void init_delay_params(SyncClocks *sc,
                               const CPUState *cpu)
 {
     if (!icount_align_option) {
         return;
     }
+    sc->realtime_clock = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     sc->diff_clk = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) -
-                   qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +
+                   sc->realtime_clock +
                    cpu_get_clock_offset();
     sc->last_cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
+
+    /* Print every 2s max if the guest is late. We limit the number
+       of printed messages to NB_PRINT_MAX(currently 100) */
+    print_delay(sc);
 }
 #else
 static void align_clocks(SyncClocks *sc, const CPUState *cpu)