Re: [Qemu-devel] [PULL 1/2] spapr: Error out when CPU hotplug is attempted on older pseries machines

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Wed, Aug 03, 2016 at 03:25:50PM +1000, David Gibson wrote:
> From: Bharata B Rao <bharata@linux.vnet.ibm.com>
> 
> CPU hotplug and coldplug aren't supported prior to pseries-2.7.  Further,
> earlier machine types don't use CPU core objects at all.  These mean that
> query-hotpluggable-cpus and coldplug on older pseries machines will crash
> QEMU.  It also means that hotpluggable_cpus flag in query-machines will
> be incorrectly set to true for pseries < 2.7, since it is based on the
> presence of the query_hotpluggable_cpus hook.
> 
> - Don't assign the query_hotpluggable_cpus hook for pseries < 2.7
> - query_hotpluggable_cpus should therefore never be called on pseries <
>   2.7, so add an assert
> - spapr_core_pre_plug() should fail hot/cold plug attempts for pseries <
>   2.7, since core objects are never used there
> - spapr_core_plug() should therefore never be called for pseries < 2.7, so
>   add an assert.
> 
> Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
> [dwg: Change from query_hotpluggable_cpus returning NULL for pseries < 2.7
>  to not being called at all, reword commit message for accuracy]
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  hw/ppc/spapr.c          |  7 ++++++-
>  hw/ppc/spapr_cpu_core.c | 19 ++++++-------------
>  2 files changed, 12 insertions(+), 14 deletions(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index fbbd051..bce2371 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2376,8 +2376,11 @@ static HotpluggableCPUList *spapr_query_hotpluggable_cpus(MachineState *machine)
>      int i;
>      HotpluggableCPUList *head = NULL;
>      sPAPRMachineState *spapr = SPAPR_MACHINE(machine);
> +    sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(machine);
>      int spapr_max_cores = max_cpus / smp_threads;
> 
> +    g_assert(smc->dr_cpu_enabled);
> +
>      for (i = 0; i < spapr_max_cores; i++) {
>          HotpluggableCPUList *list_item = g_new0(typeof(*list_item), 1);
>          HotpluggableCPU *cpu_item = g_new0(typeof(*cpu_item), 1);
> @@ -2432,7 +2435,9 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
>      hc->plug = spapr_machine_device_plug;
>      hc->unplug = spapr_machine_device_unplug;
>      mc->cpu_index_to_socket_id = spapr_cpu_index_to_socket_id;
> -    mc->query_hotpluggable_cpus = spapr_query_hotpluggable_cpus;
> +    if (smc->dr_cpu_enabled) {
> +        mc->query_hotpluggable_cpus = spapr_query_hotpluggable_cpus;
> +    }
> 
>      smc->dr_lmb_enabled = true;
>      smc->dr_cpu_enabled = true;

Unfortunately smc->dr_cpu_enabled is always false when you set
mc->query_hotpluggable_cpus and will be set to true immediately
afterwards as seen in above hunk.

This leads to query-hotpluggable-cpus being unavailable for all
machine type versions. Your first version of setting
mc->query_hotpluggable_cpus to NULL explicitly for 2.6 and downwards
was correct.

Regards,
Bharata.