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>
---
This came up in an IRC discussion.
docs/devel/migration/main.rst | 6 ++++++
docs/devel/replay.rst | 3 +++
2 files changed, 9 insertions(+)
diff --git a/docs/devel/migration/main.rst b/docs/devel/migration/main.rst
index 784c899dca6..c2857fc2446 100644
--- a/docs/devel/migration/main.rst
+++ b/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
--------------------------
diff --git a/docs/devel/replay.rst b/docs/devel/replay.rst
index effd856f0c6..40f58d9d4fc 100644
--- a/docs/devel/replay.rst
+++ b/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.