docs/devel/replay.rst

23 E.g. network packets are written into the log when they arrive into the virtual
54 of the virtual machine. We also use icount to control the occurrence of the
84 Replaying the execution of virtual machine is bound by sources of
90 of the virtual machine. Reads produce non-deterministic data taken from
95 Checkpoints here do not refer to virtual machine snapshots. They are just
103 Two other checkpoints govern the "warping" of the virtual clock.
104 While the virtual machine is idle, the virtual clock increments at
106 (called the warp timer) on the virtual real time clock, so that the
107 timer fires at the next deadline of the virtual clock; the virtual clock
108 is then incremented (which is called "warping" the virtual clock) as
111 virtual machine state and must be deterministic, each of them creates a
113 starts accounting real time to virtual clock. ``icount_account_warp_timer``
115 and it warps the virtual clock by the amount of real time that has passed
118 Virtual devices
122 the virtual devices to satisfy the following requirement:
133    do not change the virtual machine state. For this reason real time
135  * Virtual clock. These timers run only during the emulation. In icount
136    mode virtual clock value is calculated using executed instructions counter.
142  * Virtual real time clock. This clock is similar to real time clock but
143    it is used only for increasing virtual clock while virtual machine is
147 All virtual devices should use virtual clock for timers that change the guest
148 state. Virtual clock is deterministic, therefore such timers are deterministic
151 Virtual devices can also use realtime clock for the events that do not change
153 speed, use virtual clock with EXTERNAL attribute. It is not deterministic,
155 the virtual devices (e.g., slirp routing device) that lie outside the
179 in both record and replay modes we start virtual machine from the same
180 disk state. But callbacks that virtual disk controller uses for reading and
189 affect the virtual machine state (like user interaction with monitor).
208 All fields in the device state structure (including virtual timers)
232 The sequence of the events describes virtual machine state changes.
237 that perform operations with virtual hardware. These operations may change
257        callbacks that affect virtual machine state, but normally called