Re: [Qemu-devel] [RFC PATCH] vl: fix migration when watchdog expires

[Paolo Bonzini]

On 14/08/2018 15:03, Zhoujian (jay) wrote:
>> -----Original Message-----
>> From: Paolo Bonzini [mailto:address@hidden
>> Sent: Tuesday, August 14, 2018 8:02 PM
>> To: Dr. David Alan Gilbert <address@hidden>
>> Cc: Zhoujian (jay) <address@hidden>; address@hidden;
>> address@hidden; wangxin (U) <address@hidden>
>> Subject: Re: [RFC PATCH] vl: fix migration when watchdog expires
>>
>> On 14/08/2018 13:52, Dr. David Alan Gilbert wrote:
>>>  a) Should the watchdog expire when the VM is stopped; I think it
>>> shouldn't - hw/acpi/tco.c uses a virtual timer as does i6300esb; so is
>>> the bug here that the watchdog being used didn't use a virtual timer?
>>
>> All watchdogs do.
>>
>>>  b) If the watchdog expires just before the VM gets stopped, is there
>>> a race which could hit this?  Possibly.
>>
>> Yes, I think it is a race that happens just before vm_stop, but I don't
>> understand why the "qemu_clock_enable" in pause_all_vcpus does not prevent 
>> it.
> 
> Hi Paolo,
> The sequence is like this I think
> 
>          |
>          |  <-----  watchdog expired, which set reset_requested to 
> SHUTDOWN_CAUSE_GUEST_RESET
>          | 
>          |  <-----  migration thread sets to RUN_STATE_FINISH_MIGRATE, it 
> will disable QEMU_CLOCK_VIRTUAL clock,
>          |          but it is done after the setting of reset_requested

So the fix would be to process the reset request here?  (In do_vm_stop
or pause_all_vcpus).  The code is currently in main_loop_should_exit().

Paolo

>          |  <-----  main loop thread sets to RUN_STATE_PRELAUNCH since it 
> detected a reset request
>          |
>          |  <-----  migration thread sets to RUN_STATE_POSTMIGRATE
> 
> 
> Regards,
> Jay Zhou
> 
>>
>> It should be possible to write a deterministic testcase with qtest...
>>
>> Paolo