Re: [PATCH] spapr: Migrate CAS reboot flag

["Cédric Le Goater" <clg@kaod.org>]

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 !

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)
>