Re: [Qemu-devel] [RFC PATCH v6 00/14] Reverse execution.

[Frederic Konrad <fred.konrad@greensocs.com>]

On 01/09/2014 18:22, Paolo Bonzini wrote:
> Il 03/07/2014 16:33, fred.konrad@greensocs.com ha scritto:
>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>
>> Hi everybody,
>>
>> This is the sixth version of this RFC (see the changes below).
>>
>> Those are the two first patch-set we have been worked on for reverse execution.
>>
>> The first part is fully reviewed except the "icount: introduce icount timer"
>> patch maybe we can merge them?
>>
>> The first series:
>>    icount: put icount variables into TimerState.
>>    migration: migrate icount fields.
>>    migration: make qemu_savevm_state public.
>>    icount: introduce icount timer.
>>    icount: check for icount clock deadline when cpu loop exits.
>>    icount: make icount extra computed on icount clock as well.
>>    timer: add cpu_icount_to_ns function.
>>
>> are various preparation patches for reverse execution.
>>
>> The last patches:
>>    trace-events: add reverse-execution events.
>>    introduce reverse execution mechanism.
>>    gdbstub: allow reverse execution in gdb stub.
>>    cpu-exec: trigger a debug request when rexec stops.
>>    rexec: synchronize icount on the next event.
>>    rexec: allow to enable reverse execution.
>>
>> are reverse execution introduction.
>>
>> They can be clone at: git://git.greensocs.com/qemu_cexe.git:cexe_2_3_v6
>>
>> The third series will be sent as soon as possible and have some issues with
>> QEMU's thread as it use fork.
>>
>> This implementation of reverse execution works with instruction counting:
>>
>> A new clock is implemented which is icount clock. It grows each time an
>> instruction is executed and is totally independant of host clock.
>>
>> Snapshots are taken regularly (based on icount clock) with help of migration
>> code and written on the disk.
>>
>> When user wants to use reverse-stepi:
>>   * Last snapshot is reloaded.
>>   * A stop callback is created to be triggered at the previous instruction.
>>
>> This stop callback generates a debug exception so QEMU stops in debug mode.
>>
>> Command line:
>>   * rexec suboption is added to icount to enable reverse execution, it needs
>>     icount=N and doesn't support auto mode.
>>
>> About non determinism in QEMU:
>>   * This implementation doesn't take IO in account so any IO will cause non
>>     determinism and break reverse execution.
>>
>>   * The icount warp mechanism have been disabled when reverse execution is
>>     enabled so the time grow differently inside the VM.
>>
>> Testing:
>>   * It has been tested on ARM without any IO such as network or asynchronous file
>>     access to keep the deterministic behaviour of icount.
>>
>> Known issues:
>>   * On ARM stepi seems to do some additional steps which are added to icount
>>     counter so reverse-stepi just after stepi is broken.
>>
>>   * The IO replay explained above.
> Hi, can you rebase and repost these patches?  It would be nice to have a
> discussion of the different approaches to record/replay in your patches
> and Pavel's (perhaps before Pavel presents at KVM Forum).
>
> Paolo
Hi Paolo,

Sorry for the delay.
I'll rebase and resend these patches this week.

Thanks,
Fred