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

[Zhoujian (jay)]

> -----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
         |
         |  <-----  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