Re: [Qemu-devel] [RFC PATCH v11 00/23] Deterministic replay core

[Paolo Bonzini]

On 27/04/2015 09:32, Pavel Dovgalyuk wrote:
> This set of patches is related to 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
> interface.
> 
> These patches include only core function of the replay,
> excluding the support for replaying serial, audio, network, and USB devices'
> operations. Reverse debugging and monitor commands were also excluded to
> be submitted later as separate patches.
> 
> Execution recording writes non-deterministic events log, which can be later 
> 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.
>  * Supports auto-checkpointing for convenient reverse debugging.
> 
> Usage of the record/replay:
>  * First, record the execution, by adding the following string to the command 
> line:
>    '-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
>    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 events for cpu and io synchronization
> 
> v11 changes:
>  * Fixed instructions event processing.
>  * Added some mutex protection calls for replay.
>  * Fixed replaying read operations for qcow2.
>  * Fixed rtc reads on initializations stage.
>  * Eliminated some warnings in replay module.
>  * Fixed misprints in documentation for replay (as suggested by Eric Blake)

This has the same problem as before, namely that the block changes are
too intrusive and, likely, no one is going to review them.

I strongly suggest dropping them and only supporting synchronous I/O
devices for now.  Instead, I would like to see patches for the other
sources of non-determinism, especially character devices.

Paolo