Re: [PATCH] spapr: Migrate CAS reboot flag

[Greg Kurz <groug@kaod.org>]

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On Tue, 21 Jan 2020 14:41:26 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Jan 15, 2020 at 07:10:47PM +0100, Cédric Le Goater wrote:
> > On 1/15/20 6:48 PM, 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>
> > 
> > Cédric Le Goater <clg@kaod.org>
> > 
> > Nice work ! That was quite complex to catch !
> 
> It is a very nice analysis.  However, I'm disinclined to merge this
> for the time being.
> 
> My preferred approach would be to just eliminate CAS reboots
> altogether, since that has other benefits.  I'm feeling like this
> isn't super-urgent, since CAS reboots are extremely rare in practice,
> now that we've eliminated the one for the irq switchover.
> 

Yeah. The only _true_ need for CAS rebooting now seems to be hotplug
before CAS, which is likely not something frequent.

> However, if it's not looking like we'll be ready to do that as the
> qemu-5.0 release approaches, then I'll be more than willing to
> reconsider this.
> 

I hope we can drop CAS reboot in time.

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