Re: [PATCH] spapr: Migrate CAS reboot flag

[Laurent Vivier <lvivier@redhat.com>]

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"

but I don't know if it could fix this one.

Thanks,
Laurent