Exit loop if we have been there too long

Checking each 64 pages is a random magic number as good as any other.
We don't want to test too many times, but on the other hand,
qemu_get_clock_ns() is not so expensive either.  We want to be sure
that we spent less than 50ms (half of buffered_file timer), if we
spent more than 100ms, all the accounting got wrong.

Signed-off-by: Juan Quintela <quintela@redhat.com>

arch_init.c

@@ -309,12 +309,15 @@ static void migration_end(void)
     memory_global_dirty_log_stop();
 }
 
+#define MAX_WAIT 50 /* ms, half buffered_file limit */
+
 int ram_save_live(QEMUFile *f, int stage, void *opaque)
 {
     ram_addr_t addr;
     uint64_t bytes_transferred_last;
     double bwidth = 0;
     int ret;
+    int i;
 
     if (stage < 0) {
         migration_end();
@@ -354,6 +357,7 @@ int ram_save_live(QEMUFile *f, int stage, void *opaque)
     bytes_transferred_last = bytes_transferred;
     bwidth = qemu_get_clock_ns(rt_clock);
 
+    i = 0;
     while ((ret = qemu_file_rate_limit(f)) == 0) {
         int bytes_sent;
 
@@ -362,6 +366,20 @@ int ram_save_live(QEMUFile *f, int stage, void *opaque)
         if (bytes_sent == 0) { /* no more blocks */
             break;
         }
+        /* we want to check in the 1st loop, just in case it was the 1st time
+           and we had to sync the dirty bitmap.
+           qemu_get_clock_ns() is a bit expensive, so we only check each some
+           iterations
+        */
+        if ((i & 63) == 0) {
+            uint64_t t1 = (qemu_get_clock_ns(rt_clock) - bwidth) / 1000000;
+            if (t1 > MAX_WAIT) {
+                DPRINTF("big wait: " PRIu64 " milliseconds, %d iterations\n",
+                        t1, i);
+                break;
+            }
+        }
+        i++;
     }
 
     if (ret < 0) {