Re: [PATCH] spapr: Migrate CAS reboot flag

[Greg Kurz <groug@kaod.org>]

On Wed, 15 Jan 2020 19:10:47 +0100
Cédric Le Goater <clg@kaod.org> 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>
> 

I guess you mean:

Reviewed-by: Cédric Le Goater <clg@kaod.org>

?

> Nice work ! That was quite complex to catch !
> 
> Thanks,
> 
> C.
> 
> > ---
> > 
> > This patch is supposed to fix the interrupt controller mode inconsistency
> > between QEMU and the guest reported in this BZ:
> > 
> > https://bugzilla.redhat.com/show_bug.cgi?id=1781315 (requires auth)
> > 
> > Even if interrupt controller selection doesn't involve CAS reboot anymore,
> > we still have other conditions that require CAS reboot.
> > ---
> >  hw/ppc/spapr.c |   43 +++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 43 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 30a5fbd3bea6..bf2763aa16e5 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -1959,6 +1959,31 @@ static const VMStateDescription vmstate_spapr_dtb = {
> >      },
> >  };
> >  
> > +static bool spapr_cas_reboot_needed(void *opaque)
> > +{
> > +    SpaprMachineState *spapr = SPAPR_MACHINE(opaque);
> > +
> > +    /*
> > +     * This causes the "spapr_cas_reboot" subsection to always be
> > +     * sent if migration raced with CAS. This causes older QEMUs
> > +     * that don't know about the subsection to fail migration but
> > +     * it is still better than end up with a broken guest on the
> > +     * destination.
> > +     */
> > +    return spapr->cas_reboot;
> > +}
> > +
> > +static const VMStateDescription vmstate_spapr_cas_reboot = {
> > +    .name = "spapr_cas_reboot",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .needed = spapr_cas_reboot_needed,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_BOOL(cas_reboot, SpaprMachineState),
> > +        VMSTATE_END_OF_LIST()
> > +    },
> > +};
> > +
> >  static const VMStateDescription vmstate_spapr = {
> >      .name = "spapr",
> >      .version_id = 3,
> > @@ -1992,6 +2017,7 @@ static const VMStateDescription vmstate_spapr = {
> >          &vmstate_spapr_dtb,
> >          &vmstate_spapr_cap_large_decr,
> >          &vmstate_spapr_cap_ccf_assist,
> > +        &vmstate_spapr_cas_reboot,
> >          NULL
> >      }
> >  };
> > @@ -2577,6 +2603,21 @@ static PCIHostState *spapr_create_default_phb(void)
> >      return PCI_HOST_BRIDGE(dev);
> >  }
> >  
> > +static void spapr_change_state_handler(void *opaque, int running,
> > +                                       RunState state)
> > +{
> > +    SpaprMachineState *spapr = opaque;
> > +
> > +    if (running && spapr->cas_reboot) {
> > +        /*
> > +         * This happens when resuming from migration if the source
> > +         * processed a CAS but didn't have time to trigger the CAS
> > +         * reboot. Do it now.
> > +         */
> > +        qemu_system_reset_request(SHUTDOWN_CAUSE_SUBSYSTEM_RESET);
> > +    }
> > +}
> > +
> >  /* pSeries LPAR / sPAPR hardware init */
> >  static void spapr_machine_init(MachineState *machine)
> >  {
> > @@ -2970,6 +3011,8 @@ static void spapr_machine_init(MachineState *machine)
> >  
> >          kvmppc_spapr_enable_inkernel_multitce();
> >      }
> > +
> > +    qemu_add_vm_change_state_handler(spapr_change_state_handler, spapr);
> >  }
> >  
> >  static int spapr_kvm_type(MachineState *machine, const char *vm_type)
> > 
>