This patch clarifies the description of the record/replay feature in docs/replay.txt Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru> Message-Id: <20180227095333.1060.1331.stgit@pasha-VirtualBox> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
358 lines
17 KiB
Plaintext
358 lines
17 KiB
Plaintext
Copyright (c) 2010-2015 Institute for System Programming
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of the Russian Academy of Sciences.
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This work is licensed under the terms of the GNU GPL, version 2 or later.
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See the COPYING file in the top-level directory.
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Record/replay
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-------------
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Record/replay functions are used for the deterministic replay of qemu execution.
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Execution recording writes a non-deterministic events log, which can be later
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used for replaying the execution anywhere and for unlimited number of times.
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It also supports checkpointing for faster rewind to the specific replay moment.
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Execution replaying reads the log and replays all non-deterministic events
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including external input, hardware clocks, and interrupts.
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Deterministic replay has the following features:
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* Deterministically replays whole system execution and all contents of
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the memory, state of the hardware devices, clocks, and screen of the VM.
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* Writes execution log into the file for later replaying for multiple times
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on different machines.
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* Supports i386, x86_64, and ARM hardware platforms.
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* Performs deterministic replay of all operations with keyboard and mouse
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input devices.
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Usage of the record/replay:
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* First, record the execution with the following command line:
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qemu-system-i386 \
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-icount shift=7,rr=record,rrfile=replay.bin \
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-drive file=disk.qcow2,if=none,id=img-direct \
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-drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay \
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-device ide-hd,drive=img-blkreplay \
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-netdev user,id=net1 -device rtl8139,netdev=net1 \
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-object filter-replay,id=replay,netdev=net1
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* After recording, you can replay it by using another command line:
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qemu-system-i386 \
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-icount shift=7,rr=replay,rrfile=replay.bin \
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-drive file=disk.qcow2,if=none,id=img-direct \
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-drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay \
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-device ide-hd,drive=img-blkreplay \
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-netdev user,id=net1 -device rtl8139,netdev=net1 \
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-object filter-replay,id=replay,netdev=net1
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The only difference with recording is changing the rr option
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from record to replay.
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* Block device images are not actually changed in the recording mode,
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because all of the changes are written to the temporary overlay file.
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This behavior is enabled by using blkreplay driver. It should be used
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for every enabled block device, as described in 'Block devices' section.
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* '-net none' option should be specified when network is not used,
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because QEMU adds network card by default. When network is needed,
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it should be configured explicitly with replay filter, as described
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in 'Network devices' section.
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* Interaction with audio devices and serial ports are recorded and replayed
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automatically when such devices are enabled.
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Academic papers with description of deterministic replay implementation:
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http://www.computer.org/csdl/proceedings/csmr/2012/4666/00/4666a553-abs.html
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http://dl.acm.org/citation.cfm?id=2786805.2803179
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Modifications of qemu include:
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* wrappers for clock and time functions to save their return values in the log
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* saving different asynchronous events (e.g. system shutdown) into the log
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* synchronization of the bottom halves execution
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* synchronization of the threads from thread pool
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* recording/replaying user input (mouse, keyboard, and microphone)
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* adding internal checkpoints for cpu and io synchronization
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* network filter for recording and replaying the packets
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* block driver for making block layer deterministic
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* serial port input record and replay
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Locking and thread synchronisation
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----------------------------------
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Previously the synchronisation of the main thread and the vCPU thread
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was ensured by the holding of the BQL. However the trend has been to
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reduce the time the BQL was held across the system including under TCG
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system emulation. As it is important that batches of events are kept
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in sequence (e.g. expiring timers and checkpoints in the main thread
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while instruction checkpoints are written by the vCPU thread) we need
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another lock to keep things in lock-step. This role is now handled by
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the replay_mutex_lock. It used to be held only for each event being
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written but now it is held for a whole execution period. This results
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in a deterministic ping-pong between the two main threads.
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As the BQL is now a finer grained lock than the replay_lock it is almost
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certainly a bug, and a source of deadlocks, to take the
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replay_mutex_lock while the BQL is held. This is enforced by an assert.
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While the unlocks are usually in the reverse order, this is not
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necessary; you can drop the replay_lock while holding the BQL, without
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doing a more complicated unlock_iothread/replay_unlock/lock_iothread
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sequence.
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Non-deterministic events
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------------------------
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Our record/replay system is based on saving and replaying non-deterministic
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events (e.g. keyboard input) and simulating deterministic ones (e.g. reading
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from HDD or memory of the VM). Saving only non-deterministic events makes
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log file smaller and simulation faster.
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The following non-deterministic data from peripheral devices is saved into
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the log: mouse and keyboard input, network packets, audio controller input,
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serial port input, and hardware clocks (they are non-deterministic
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too, because their values are taken from the host machine). Inputs from
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simulated hardware, memory of VM, software interrupts, and execution of
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instructions are not saved into the log, because they are deterministic and
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can be replayed by simulating the behavior of virtual machine starting from
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initial state.
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We had to solve three tasks to implement deterministic replay: recording
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non-deterministic events, replaying non-deterministic events, and checking
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that there is no divergence between record and replay modes.
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We changed several parts of QEMU to make event log recording and replaying.
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Devices' models that have non-deterministic input from external devices were
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changed to write every external event into the execution log immediately.
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E.g. network packets are written into the log when they arrive into the virtual
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network adapter.
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All non-deterministic events are coming from these devices. But to
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replay them we need to know at which moments they occur. We specify
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these moments by counting the number of instructions executed between
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every pair of consecutive events.
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Instruction counting
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--------------------
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QEMU should work in icount mode to use record/replay feature. icount was
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designed to allow deterministic execution in absence of external inputs
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of the virtual machine. We also use icount to control the occurrence of the
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non-deterministic events. The number of instructions elapsed from the last event
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is written to the log while recording the execution. In replay mode we
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can predict when to inject that event using the instruction counter.
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Timers
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------
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Timers are used to execute callbacks from different subsystems of QEMU
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at the specified moments of time. There are several kinds of timers:
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* Real time clock. Based on host time and used only for callbacks that
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do not change the virtual machine state. For this reason real time
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clock and timers does not affect deterministic replay at all.
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* Virtual clock. These timers run only during the emulation. In icount
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mode virtual clock value is calculated using executed instructions counter.
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That is why it is completely deterministic and does not have to be recorded.
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* Host clock. This clock is used by device models that simulate real time
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sources (e.g. real time clock chip). Host clock is the one of the sources
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of non-determinism. Host clock read operations should be logged to
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make the execution deterministic.
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* Virtual real time clock. This clock is similar to real time clock but
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it is used only for increasing virtual clock while virtual machine is
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sleeping. Due to its nature it is also non-deterministic as the host clock
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and has to be logged too.
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Checkpoints
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-----------
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Replaying of the execution of virtual machine is bound by sources of
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non-determinism. These are inputs from clock and peripheral devices,
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and QEMU thread scheduling. Thread scheduling affect on processing events
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from timers, asynchronous input-output, and bottom halves.
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Invocations of timers are coupled with clock reads and changing the state
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of the virtual machine. Reads produce non-deterministic data taken from
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host clock. And VM state changes should preserve their order. Their relative
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order in replay mode must replicate the order of callbacks in record mode.
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To preserve this order we use checkpoints. When a specific clock is processed
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in record mode we save to the log special "checkpoint" event.
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Checkpoints here do not refer to virtual machine snapshots. They are just
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record/replay events used for synchronization.
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QEMU in replay mode will try to invoke timers processing in random moment
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of time. That's why we do not process a group of timers until the checkpoint
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event will be read from the log. Such an event allows synchronizing CPU
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execution and timer events.
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Two other checkpoints govern the "warping" of the virtual clock.
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While the virtual machine is idle, the virtual clock increments at
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1 ns per *real time* nanosecond. This is done by setting up a timer
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(called the warp timer) on the virtual real time clock, so that the
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timer fires at the next deadline of the virtual clock; the virtual clock
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is then incremented (which is called "warping" the virtual clock) as
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soon as the timer fires or the CPUs need to go out of the idle state.
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Two functions are used for this purpose; because these actions change
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virtual machine state and must be deterministic, each of them creates a
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checkpoint. qemu_start_warp_timer checks if the CPUs are idle and if so
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starts accounting real time to virtual clock. qemu_account_warp_timer
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is called when the CPUs get an interrupt or when the warp timer fires,
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and it warps the virtual clock by the amount of real time that has passed
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since qemu_start_warp_timer.
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Bottom halves
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-------------
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Disk I/O events are completely deterministic in our model, because
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in both record and replay modes we start virtual machine from the same
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disk state. But callbacks that virtual disk controller uses for reading and
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writing the disk may occur at different moments of time in record and replay
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modes.
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Reading and writing requests are created by CPU thread of QEMU. Later these
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requests proceed to block layer which creates "bottom halves". Bottom
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halves consist of callback and its parameters. They are processed when
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main loop locks the global mutex. These locks are not synchronized with
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replaying process because main loop also processes the events that do not
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affect the virtual machine state (like user interaction with monitor).
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That is why we had to implement saving and replaying bottom halves callbacks
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synchronously to the CPU execution. When the callback is about to execute
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it is added to the queue in the replay module. This queue is written to the
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log when its callbacks are executed. In replay mode callbacks are not processed
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until the corresponding event is read from the events log file.
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Sometimes the block layer uses asynchronous callbacks for its internal purposes
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(like reading or writing VM snapshots or disk image cluster tables). In this
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case bottom halves are not marked as "replayable" and do not saved
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into the log.
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Block devices
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-------------
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Block devices record/replay module intercepts calls of
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bdrv coroutine functions at the top of block drivers stack.
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To record and replay block operations the drive must be configured
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as following:
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-drive file=disk.qcow2,if=none,id=img-direct
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-drive driver=blkreplay,if=none,image=img-direct,id=img-blkreplay
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-device ide-hd,drive=img-blkreplay
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blkreplay driver should be inserted between disk image and virtual driver
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controller. Therefore all disk requests may be recorded and replayed.
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All block completion operations are added to the queue in the coroutines.
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Queue is flushed at checkpoints and information about processed requests
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is recorded to the log. In replay phase the queue is matched with
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events read from the log. Therefore block devices requests are processed
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deterministically.
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Snapshotting
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------------
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New VM snapshots may be created in replay mode. They can be used later
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to recover the desired VM state. All VM states created in replay mode
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are associated with the moment of time in the replay scenario.
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After recovering the VM state replay will start from that position.
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Default starting snapshot name may be specified with icount field
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rrsnapshot as follows:
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-icount shift=7,rr=record,rrfile=replay.bin,rrsnapshot=snapshot_name
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This snapshot is created at start of recording and restored at start
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of replaying. It also can be loaded while replaying to roll back
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the execution.
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Use QEMU monitor to create additional snapshots. 'savevm <name>' command
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created the snapshot and 'loadvm <name>' restores it. To prevent corruption
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of the original disk image, use overlay files linked to the original images.
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Therefore all new snapshots (including the starting one) will be saved in
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overlays and the original image remains unchanged.
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Network devices
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---------------
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Record and replay for network interactions is performed with the network filter.
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Each backend must have its own instance of the replay filter as follows:
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-netdev user,id=net1 -device rtl8139,netdev=net1
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-object filter-replay,id=replay,netdev=net1
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Replay network filter is used to record and replay network packets. While
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recording the virtual machine this filter puts all packets coming from
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the outer world into the log. In replay mode packets from the log are
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injected into the network device. All interactions with network backend
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in replay mode are disabled.
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Audio devices
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-------------
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Audio data is recorded and replay automatically. The command line for recording
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and replaying must contain identical specifications of audio hardware, e.g.:
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-soundhw ac97
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Serial ports
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------------
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Serial ports input is recorded and replay automatically. The command lines
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for recording and replaying must contain identical number of ports in record
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and replay modes, but their backends may differ.
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E.g., '-serial stdio' in record mode, and '-serial null' in replay mode.
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Replay log format
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-----------------
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Record/replay log consits of the header and the sequence of execution
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events. The header includes 4-byte replay version id and 8-byte reserved
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field. Version is updated every time replay log format changes to prevent
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using replay log created by another build of qemu.
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The sequence of the events describes virtual machine state changes.
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It includes all non-deterministic inputs of VM, synchronization marks and
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instruction counts used to correctly inject inputs at replay.
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Synchronization marks (checkpoints) are used for synchronizing qemu threads
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that perform operations with virtual hardware. These operations may change
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system's state (e.g., change some register or generate interrupt) and
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therefore should execute synchronously with CPU thread.
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Every event in the log includes 1-byte event id and optional arguments.
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When argument is an array, it is stored as 4-byte array length
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and corresponding number of bytes with data.
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Here is the list of events that are written into the log:
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- EVENT_INSTRUCTION. Instructions executed since last event.
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Argument: 4-byte number of executed instructions.
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- EVENT_INTERRUPT. Used to synchronize interrupt processing.
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- EVENT_EXCEPTION. Used to synchronize exception handling.
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- EVENT_ASYNC. This is a group of events. They are always processed
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together with checkpoints. When such an event is generated, it is
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stored in the queue and processed only when checkpoint occurs.
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Every such event is followed by 1-byte checkpoint id and 1-byte
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async event id from the following list:
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- REPLAY_ASYNC_EVENT_BH. Bottom-half callback. This event synchronizes
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callbacks that affect virtual machine state, but normally called
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asyncronously.
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Argument: 8-byte operation id.
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- REPLAY_ASYNC_EVENT_INPUT. Input device event. Contains
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parameters of keyboard and mouse input operations
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(key press/release, mouse pointer movement).
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Arguments: 9-16 bytes depending of input event.
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- REPLAY_ASYNC_EVENT_INPUT_SYNC. Internal input synchronization event.
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- REPLAY_ASYNC_EVENT_CHAR_READ. Character (e.g., serial port) device input
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initiated by the sender.
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Arguments: 1-byte character device id.
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Array with bytes were read.
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- REPLAY_ASYNC_EVENT_BLOCK. Block device operation. Used to synchronize
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operations with disk and flash drives with CPU.
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Argument: 8-byte operation id.
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- REPLAY_ASYNC_EVENT_NET. Incoming network packet.
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Arguments: 1-byte network adapter id.
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4-byte packet flags.
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Array with packet bytes.
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- EVENT_SHUTDOWN. Occurs when user sends shutdown event to qemu,
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e.g., by closing the window.
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- EVENT_CHAR_WRITE. Used to synchronize character output operations.
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Arguments: 4-byte output function return value.
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4-byte offset in the output array.
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- EVENT_CHAR_READ_ALL. Used to synchronize character input operations,
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initiated by qemu.
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Argument: Array with bytes that were read.
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- EVENT_CHAR_READ_ALL_ERROR. Unsuccessful character input operation,
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initiated by qemu.
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Argument: 4-byte error code.
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- EVENT_CLOCK + clock_id. Group of events for host clock read operations.
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Argument: 8-byte clock value.
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- EVENT_CHECKPOINT + checkpoint_id. Checkpoint for synchronization of
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CPU, internal threads, and asynchronous input events. May be followed
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by one or more EVENT_ASYNC events.
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- EVENT_END. Last event in the log.
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