Re: [PATCH] spapr: Migrate CAS reboot flag

[Laurent Vivier <lvivier@redhat.com>]

On 16/01/2020 09:48, Greg Kurz wrote:
> On Wed, 15 Jan 2020 19:10:37 +0100
> Laurent Vivier <lvivier@redhat.com> 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 <ldoktor@redhat.com>
>>> Signed-off-by: Greg Kurz <groug@kaod.org>
>>> ---
>>>
>>
>> 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"
>>
> 
> IIUC your patch avoids an invalid 'prelaunch' -> 'postmigrate' runstate
> transition that can happen if the migration thread sets the runstate to
> 'finishmigrate' when pause_all_vcpus() releases the main loop mutex.
> 
> ie. symptom of the problem is QEMU aborting, correct ? The issue I'm
> trying to fix is a guest breakage caused by a discrepancy between
> QEMU and the guest after migration has succeeded.
> 
>> but I don't know if it could fix this one.
>>
> 
> I don't think so and your patch kinda illustrates it. If the runstate
> is 'finishmigrate' when returning from pause_all_vcpus(), this means
> that state was sent to the destination before we could actually reset
> the machine.

Yes, you're right.

But the question behind my comment was: is it expected to have a pending
reset while we are migrating?

Perhaps H_CAS can return H_BUSY and wait the end of the migration and
then be fully executed on destination?

Thanks,
Laurent