On 02/27/2015 06:09 AM, Pavel Dovgalyuk wrote: > This patch adds global variables, defines, functions declarations,
s/functions/function/ > and function stubs for deterministic VM replay used by external modules. > > Reviewed-by: Paolo Bonzini <pbonz...@redhat.com> > Reviewed-by: Eric Blake <ebl...@redhat.com> > > Signed-off-by: Pavel Dovgalyuk <pavel.dovga...@ispras.ru> > --- > +++ b/docs/replay.txt > @@ -0,0 +1,161 @@ > +Record/replay > +------------- It might be nice to include an explicit copyright and license statement at the top of this file (without one, you have inherited the top-level default of GPLv2+). > + > +Record/replay functions are used for the reverse execution and deterministic > +replay of qemu execution. This implementation of deterministic replay can > +be used for deterministic debugging of guest code through gdb remote s/through/through a/ > +interface. > + > +Execution recording writes non-deterministic events log, which can be later s/writes/writes a/ > +used for replaying the execution anywhere and for unlimited number of times. > +It also supports checkpointing for faster rewinding during reverse > debugging. > +Execution replaying reads the log and replays all non-deterministic events > +including external input, hardware clocks, and interrupts. > + > +Deterministic replay has the following features: > + * Deterministically replays whole system execution and all contents of > + the memory, state of the hadrware devices, clocks, and screen of the VM. s/hadrware/hardware/ > + * Writes execution log into the file for latter replaying for multiple > times s/latter/later/ > + on different machines. > + * Supports i386, x86_64, and ARM hardware platforms. > + * Performs deterministic replay of all operations with keyboard and mouse > + input devices. > + > +Usage of the record/replay: > + * First, record the execution, by adding the following string to the > command line: s/string/arguments/ (it is not 1 string argument with 3 embedded spaces, but four separate arguments) > + '-icount shift=7,rr=record,rrfile=replay.bin -net none'. > + Block devices' images are not actually changed in the recording mode, > + because all of the changes are written to the temporary overlay file. > + * Then you can replay it for the multiple times by using another command s/for the multiple times// > + line option: '-icount shift=7,rr=replay,rrfile=replay.bin -net none' > + * '-net none' option should also be specified if network replay patches > + are not applied. > + > +Paper with short description of deterministic replay implementation: > +http://www.computer.org/csdl/proceedings/csmr/2012/4666/00/4666a553-abs.html > + > +Modifications of qemu include: > + * wrappers for clock and time functions to save their return values in the > log > + * saving different asynchronous events (e.g. system shutdown) into the log > + * synchronization of the bottom halves execution > + * synchronization of the threads from thread pool > + * recording/replaying user input (mouse and keyboard) > + * adding internal checkpoints for cpu and io synchronization > + > +Non-deterministic events > +------------------------ > + > +Our record/replay system is based on saving and replaying non-deterministic > +events (e.g. keyboard input) and simulating deterministic ones (e.g. reading > +from HDD or memory of the VM). Saving only non-deterministic events makes > +log file smaller, simulation faster, and allows using reverse debugging even > +for realtime applications. > + > +The following non-deterministic data from peripheral devices is saved into > +the log: mouse and keyboard input, network packets, audio controller input, > +USB packets, serial port input, and hardware clocks (they are > non-deterministic > +too, because their values are taken from the host machine). Inputs from > +simulated hardware, memory of VM, software interrupts, and execution of > +instructions are not saved into the log, because they are deterministic and > +can be replayed by simulating the behavior of virtual machine starting from > +initial state. > + > +We had to solve three tasks to implement deterministic replay: recording > +non-deterministic events, replaying non-deterministic events, and checking > +that there is no divergence between record and replay modes. > + > +We changed several parts of QEMU to make event log recording and replaying. > +Devices' models that have non-deterministic input from external devices were > +changed to write every external event into the execution log immediately. > +E.g. network packets are written into the log when they arrive into the > virtual > +network adapter. > + > +All non-deterministic events are coming from these devices. But to > +replay them we need to know at which moments they occur. We specify > +these moments by counting the number of instructions executed between > +every pair of consecutive events. > + > +Instructions counting s/Instructions/Instruction/ > +--------------------- > + > +QEMU should work in icount mode to use record/replay feature. icount was > +designed to allow deterministic execution in absense of external inputs s/absense/absence/ > +of the virtual machine. We also use icount to control the occurence of the s/occurence/occurrence/ > +non-deterministic events. Number of instruction passed from the last event s/Number/The number/ s/instruction passed/instructions elapsed/ > +is written to the log while recording the execution. In replay mode we > +can predict when to inject that event using the instructions counter. s/instructions/instruction/ > + > +Timers > +------ > + > +Timers are used to execute callbacks from different subsystems of QEMU > +at the specified moments of time. There are several kinds of timers: > + * Real time clock. Based on host time and used only for callbacks that > + do not change the virtual machine state. For this reason real time > + clock and timers does not affect deterministic replay at all; s/;/./ > + * Virtual clock. These timers run only during the emulation. In icount > + mode virtual clock value is calculated using executed instructions > counter. > + That is why it is completely deterministic and does not have to be > recorded; > + * Host clock. This clock is used by device models that simulate real time > + sources (e.g. real time clock chip). Host clock is the one of the sources > + of non-determinism. Host clock read operations should be logged to > + make the execution deterministic. > + * Real time clock for icount. This clock is similar to real time clock but > + it is used only for increasing virtual clock while virtual machine is > + sleeping. Due to its nature it is also non-deterministic as the host clock > + and has to be logged too. > + > +Checkpoints > +----------- > + > +Replaying of the execution of virtual machine is bound by sources of > +non-determinism. These are inputs from clock and peripheral devices, > +and QEMU thread scheduling. Thread scheduling affect on processing events > +from timers, asynchronous input-output, and bottom halves. > + > +Invocations of timers are coupled with clock reads and changing the state > +of the virtual machine. Reads produce non-deterministic data taken from > +host clock. And VM state changes should preserve their order. Their relative > +order in replay mode must replicate the order of callbacks in record mode. > +To preserve this order we use checkpoints. When specific clock is processed s/When/When a/ > +in record mode we save to the log special ''checkpoint`` event. s/``/''/ or even s/''\(.*\)``/"\1"/ > +Checkpoints here do not refer to virtual machine snapshots. They are just > +record/replay events used for synchronization. > + > +QEMU in replay mode will try to invoke timers processing in random moment > +of time. That's why we do not process group of timers until the checkpoint s/group/a group/ > +event will be read from the log. Such an event allows synchronizing CPU > +execution and timer events. > + > +Another checkpoints application in record/replay is instructions counting s/instructions/instruction/ > +while the virtual machine is idle. This function (qemu_clock_warp) is called > +from the wait loop. It changes virtual machine state and must be > deterministic > +then. That is why we added checkpoint to this function to prevent its > +operation in replay mode when it does not correspond to record mode. > + > +Bottom halves > +------------- > + > +Disk I/O events are completely deterministic in our model, because > +in both record and replay modes we start virtual machine from the same > +disk state. But callbacks that virtual disk controller uses for reading and > +writing the disk may occur at different moments of time in record and replay > +modes. > + > +Reading and writing requests are created by CPU thread of QEMU. Later these > +requests proceed to block layer which creates ''bottom halves``. Bottom Another instance of your odd quoting style. Make it consistent with the solution you chose above. > +halves consist of callback and its parameters. They are processed when > +main loop locks the global mutex. These locking are not synchronized with s/locking/locks/ > +replaying process because main loop also processes the events that do not > +affect the virtual machine state (like user interation with monitor). s/interation/interaction/ > + > +That is why we had to implement saving and replaying bottom halves callbacks > +synchronously to the CPU execution. When the callback is about to execute > +it is added to the queue in the replay module. This queue is written to the > +log when its callbacks are executed. In replay mode callbacks do not > processed s/do/are/ > +until the corresponding event is read from the events log file. > + > +Sometimes block layer uses asynchronous callbacks for its internal purposes s/block/the block/ > +(like reading or writing VM snapshots or disk image cluster tables). In this > +case bottom halves are not marked as ''replayable`` and do not saved No trailing whitespace. Another instance of odd quoting style. > +into the log. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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