diff options
Diffstat (limited to 'docs/devel')
| -rw-r--r-- | docs/devel/migration.rst | 2 | ||||
| -rw-r--r-- | docs/devel/replay.txt | 46 |
2 files changed, 47 insertions, 1 deletions
diff --git a/docs/devel/migration.rst b/docs/devel/migration.rst index 2200596..f7668ae 100644 --- a/docs/devel/migration.rst +++ b/docs/devel/migration.rst @@ -314,7 +314,7 @@ For example: a) Add a new property using ``DEFINE_PROP_BOOL`` - e.g. support-foo and default it to true. - b) Add an entry to the ``HW_COMPAT_`` for the previous version that sets + b) Add an entry to the ``hw_compat_`` for the previous version that sets the property to false. c) Add a static bool support_foo function that tests the property. d) Add a subsection with a .needed set to the support_foo function diff --git a/docs/devel/replay.txt b/docs/devel/replay.txt new file mode 100644 index 0000000..e641c35 --- /dev/null +++ b/docs/devel/replay.txt @@ -0,0 +1,46 @@ +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 clock with EXTERNAL attribute. 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. |