Re: [PATCH] spapr: Migrate CAS reboot flag

[Greg Kurz]

On Wed, 15 Jan 2020 19:10:37 +0100
Laurent Vivier <address@hidden> 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"
> 

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.

> Thanks,
> Laurent
>