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

["Pavel Dovgaluk" <Pavel.Dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> 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.  

Ok, I'll remove them in the next iteration.

> Instead, I would like to see patches for the other
> sources of non-determinism, especially character devices.

I'll release these patches after applying the core, because they need
reworking for the newest versions of QEMU and replay.

Pavel Dovgalyuk