Re: [PATCH] spapr: Migrate CAS reboot flag

[Laurent Vivier]

On 15/01/2020 19:10, Laurent Vivier wrote:
> Hi,
> 
> On 15/01/2020 18:48, Greg Kurz wrote:
>> Migration can potentially race with CAS reboot. If the migration thread
>> completes migration after CAS has set spapr->cas_reboot but before the
>> mainloop could pick up the reset request and reset the machine, the
>> guest is migrated unrebooted and the destination doesn't reboot it
>> either because it isn't aware a CAS reboot was needed (eg, because a
>> device was added before CAS). This likely result in a broken or hung
>> guest.
>>
>> Even if it is small, the window between CAS and CAS reboot is enough to
>> re-qualify spapr->cas_reboot as state that we should migrate. Add a new
>> subsection for that and always send it when a CAS reboot is pending.
>> This may cause migration to older QEMUs to fail but it is still better
>> than end up with a broken guest.
>>
>> The destination cannot honour the CAS reboot request from a post load
>> handler because this must be done after the guest is fully restored.
>> It is thus done from a VM change state handler.
>>
>> Reported-by: Lukáš Doktor <address@hidden>
>> Signed-off-by: Greg Kurz <address@hidden>
>> ---
>>
> 
> I'm wondering if the problem can be related with the fact that
> main_loop_should_exit() could release qemu_global_mutex in
> pause_all_vcpus() in the reset case?
> 
> 1602 static bool main_loop_should_exit(void)
> 1603 {
> ...
> 1633     request = qemu_reset_requested();
> 1634     if (request) {
> 1635         pause_all_vcpus();
> 1636         qemu_system_reset(request);
> 1637         resume_all_vcpus();
> 1638         if (!runstate_check(RUN_STATE_RUNNING) &&
> 1639                 !runstate_check(RUN_STATE_INMIGRATE)) {
> 1640             runstate_set(RUN_STATE_PRELAUNCH);
> 1641         }
> 1642     }
> ...
> 
> I already sent a patch for this kind of problem (in current Juan pull
> request):
> 
> "runstate: ignore finishmigrate -> prelaunch transition"
> 
> but I don't know if it could fix this one.

I think it should be interesting to have the state transition on source
and destination when the problem occurs (with something like "-trace
runstate_set").

Thanks,
Laurent