migration/dirtyrate: Fix precision losses and g_usleep overshoot

Signed-off-by: Andrei Gudkov <gudkov.andrei@huawei.com>
Reviewed-by: Hyman Huang <yong.huang@smartx.com>
Message-Id: <8ddb0d40d143f77aab8f602bd494e01e5fa01614.1691161009.git.gudkov.andrei@huawei.com>
Signed-off-by: Hyman Huang <yong.huang@smartx.com>

migration/dirtyrate.c

@@ -57,6 +57,8 @@ static int64_t dirty_stat_wait(int64_t msec, int64_t initial_time)
         msec = current_time - initial_time;
     } else {
         g_usleep((msec + initial_time - current_time) * 1000);
+        /* g_usleep may overshoot */
+        msec = qemu_clock_get_ms(QEMU_CLOCK_REALTIME) - initial_time;
     }
 
     return msec;
@@ -77,9 +79,13 @@ static int64_t do_calculate_dirtyrate(DirtyPageRecord dirty_pages,
 {
     uint64_t increased_dirty_pages =
         dirty_pages.end_pages - dirty_pages.start_pages;
-    uint64_t memory_size_MiB = qemu_target_pages_to_MiB(increased_dirty_pages);
 
-    return memory_size_MiB * 1000 / calc_time_ms;
+    /*
+     * multiply by 1000ms/s _before_ converting down to megabytes
+     * to avoid losing precision
+     */
+    return qemu_target_pages_to_MiB(increased_dirty_pages * 1000) /
+        calc_time_ms;
 }
 
 void global_dirty_log_change(unsigned int flag, bool start)