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Re: [Qemu-devel] [RFC PATCH v10 02/24] replay: global variables and func

From: Eric Blake
Subject: Re: [Qemu-devel] [RFC PATCH v10 02/24] replay: global variables and function stubs
Date: Fri, 13 Mar 2015 13:07:21 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 02/27/2015 06:09 AM, Pavel Dovgalyuk wrote:
> This patch adds global variables, defines, functions declarations,


> and function stubs for deterministic VM replay used by external modules.
> Reviewed-by: Paolo Bonzini <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> Signed-off-by: Pavel Dovgalyuk <address@hidden>
> ---

> +++ 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.


> + * Writes execution log into the file for latter replaying for multiple 
> times 


> +   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


> +---------------------
> +
> +QEMU should work in icount mode to use record/replay feature. icount was
> +designed to allow deterministic execution in absense of external inputs


> +of the virtual machine. We also use icount to control the occurence of the


> +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.


> +
> +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;


> + * 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


> +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


> +replaying process because main loop also processes the events that do not
> +affect the virtual machine state (like user interation with monitor).


> +
> +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


> +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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