docs/devel: Mention post_load hook restrictions where we document the hook

Accessing another device in a post_load hook is a bad idea, because
the order of device save/restore is not fixed, and so this
cross-device access makes the save/restore non-deterministic.

We previously only flagged up this requirement in the
record-and-replay developer docs; repeat it in the main migration
documentation, where a developer trying to implement a post_load hook
is more likely to see it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/devel/migration/main.rst

@@ -465,6 +465,12 @@ Examples of such API functions are:
   - portio_list_set_address()
   - portio_list_set_enabled()
 
+Since the order of device save/restore is not defined, you must
+avoid accessing or changing any other device's state in one of these
+callbacks. (For instance, don't do anything that calls ``update_irq()``
+in a ``post_load`` hook.) Otherwise, restore will not be deterministic,
+and this will break execution record/replay.
+
 Iterative device migration
 --------------------------
 

docs/devel/replay.rst

@@ -202,6 +202,9 @@ into the log.
 Saving/restoring the VM state
 -----------------------------
 
+Record/replay relies on VM state save and restore being complete and
+deterministic.
+
 All fields in the device state structure (including virtual timers)
 should be restored by loadvm to the same values they had before savevm.