[v8,20/20] replay: document development rules

Commit Message

Pavel Dovgalyuk Dec. 18, 2018, 11:22 a.m. UTC

This patch introduces docs/devel/replay.txt which describes the rules
that should be followed to make virtual devices usable in record/replay mode.

Signed-off-by: Pavel Dovgalyuk <[e-mail]>
---
 docs/devel/replay.txt |   45 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 docs/devel/replay.txt

Comments

Artem Pisarenko Dec. 18, 2018, 5:23 p.m. UTC | #1

> +Virtual devices can also use realtime clock for the events that do not
change
> +the guest state directly. When the clock ticking should depend on VM
execution
> +speed, use virtual ext clock. It is not deterministic, but its speed
depends
> +on the guest execution. This clock is used by the virtual devices (e.g.,
> +slirp routing device) that lie outside the replayed guest.

Virtual ext clock replaced with virtual clock timers having EXTERNAL
attribute.

Patch

diff --git a/docs/devel/replay.txt b/docs/devel/replay.txt
new file mode 100644
index 0000000..61dac1b
--- /dev/null
+++ b/docs/devel/replay.txt
@@ -0,0 +1,45 @@ 
+Record/replay mechanism, that could be enabled through icount mode, expects
+the virtual devices to satisfy the following requirements.
+
+The main idea behind this document is that everything that affects
+the guest state during execution in icount mode should be deterministic.
+
+Timers
+======
+
+All virtual devices should use virtual clock for timers that change the guest
+state. Virtual clock is deterministic, therefore such timers are deterministic
+too.
+
+Virtual devices can also use realtime clock for the events that do not change
+the guest state directly. When the clock ticking should depend on VM execution
+speed, use virtual ext clock. It is not deterministic, but its speed depends
+on the guest execution. This clock is used by the virtual devices (e.g.,
+slirp routing device) that lie outside the replayed guest.
+
+Bottom halves
+=============
+
+Bottom half callbacks, that affect the guest state, should be invoked through
+replay_bh_schedule_event or replay_bh_schedule_oneshot_event functions.
+Their invocations are saved in record mode and synchronized with the existing
+log in replay mode.
+
+Saving/restoring the VM state
+=============================
+
+All fields in the device state structure (including virtual timers)
+should be restored by loadvm to the same values they had before savevm.
+
+Avoid accessing other devices' state, because the order of saving/restoring
+is not defined. It means that you should not call functions like
+'update_irq' in post_load callback. Save everything explicitly to avoid
+the dependencies that may make restoring the VM state non-deterministic.
+
+Stopping the VM
+===============
+
+Stopping the guest should not interfere with its state (with the exception
+of the network connections, that could be broken by the remote timeouts).
+VM can be stopped at any moment of replay by the user. Restarting the VM
+after that stop should not break the replay by the unneeded guest state change.