Re: [PATCH] spapr: Migrate CAS reboot flag

[Greg Kurz]

On Thu, 16 Jan 2020 11:37:24 +0100
Laurent Vivier <address@hidden> wrote:

> On 16/01/2020 09:48, Greg Kurz wrote:
> > 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.
> 
> Yes, you're right.
> 
> But the question behind my comment was: is it expected to have a pending
> reset while we are migrating?
> 

Nothing prevents qemu_system_reset_request() to be called when migration
is active. 

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

And so we would need to teach SLOF to try H_CAS again until it stops
returning H_BUSY ? It seems safer to migrate the CAS reboot flag IMHO.

> Thanks,
> Laurent
>