The migration test cases that actually exercise live migration want to
ensure there is a minimum of two iterations of pre-copy, in order to
exercise the dirty tracking code.
Historically we've queried the migration status, looking for the
'dirty-sync-count' value to increment to track iterations. This was
not entirely reliable because often all the data would get transferred
quickly enough that the migration would finish before we wanted it
to. So we massively dropped the bandwidth and max downtime to
guarantee non-convergance. This had the unfortunate side effect
@@ -445,6 +459,91 @@ static void migrate_ensure_converge(QTestState *who)
migrate_set_parameter_int(who, "downtime-limit", 30 * 1000);
}
+/*
+ * Our goal is to ensure that we run a single full migration
+ * iteration, and also dirty memory, ensuring that at least
+ * one further iteration is required.
+ *
+ * We can't directly synchronize with the start of a migration
+ * so we have to apply some tricks monitoring memory that is
+ * transferred.
+ *
+ * Initially we set the migration bandwidth to an insanely
+ * low value, with tiny max downtime too. This basically
+ * guarantees migration will never complete.
+ *
+ * This will result in a test that is unacceptably slow though,
+ * so we can't let the entire migration pass run at this speed.
+ * Our intent is to let it run just long enough that we can
+ * prove data prior to the marker has been transferred *AND*
+ * also prove this transferred data is dirty again.
+ *
+ * Before migration starts, we write a 64-bit magic marker
+ * into a fixed location in the src VM RAM.
+ *
+ * Then watch dst memory until the marker appears. This is
+ * proof that start_address -> MAGIC_OFFSET_BASE has been
+ * transferred.
+ *
+ * Finally we go back to the source and read a byte just
+ * before the marker untill we see it flip in value. This