Re: [Qemu-devel] CPU hotplug, again

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

On Tue, Feb 23, 2016 at 04:24:31PM +1100, David Gibson wrote:
> Hi Andreas,
> 
> I've now found (with Thomas' help) your RFC series for socket/core
> based cpu hotplug on x86
> (https://github.com/afaerber/qemu-cpu/compare/qom-cpu-x86).  It seems
> sensible enough as far as it goes, but doesn't seem to address a bunch
> of the things that I was attempting to do with the cpu-package
> proposal - and which we absolutely need for cpu hotplug on Power.
> 
> 1) What interface do you envisage beyond cpu_add?
> 
> The patches I see just construct extra socket and core objects, but
> still control hotplug (for x86) through the cpu_add interface.  That
> interface is absolutely unusable on Power, since it operates on a
> per-thread basis, whereas the PAPR guest<->host interfaces can only
> communicate information at a per-core granularity.
> 
> 2) When hotplugging at core or socket granularity, where would the
>    code to construct the individual thread objects sit?
> 
> Your series has the construction done in both the machine init path
> and the hotplug path.  The latter works because hotplug occurs at
> thread granularity.  If we're hotplugging at core or socket
> granularity what would do the construct?  The core/socket object
> itself (in instance_init?  in realize?); the hotplug handler?
> something else?
> 
> 3) How does the management layer determine what is pluggable?
> 
> Both the number of pluggable slots, and what it will need to do to
> populate them.
> 
> 4) How do we enforce that toplogies illegal for the platform can't be
>    constructed?

5) QOM-links

Andreas, You have often talked about setting up links from machine object
to the CPU objects. Would the below code correctly capture that idea of
yours ?

#define SPAPR_MACHINE_CPU_CORE_PROP "core"

/* MachineClass.init for sPAPR */
static void ppc_spapr_init(MachineState *machine)
{
    sPAPRMachineState *spapr = SPAPR_MACHINE(machine);
    int spapr_smp_cores = smp_cpus / smp_threads;
    int spapr_max_cores = max_cpus / smp_threads;

    ...
    for (i = 0; i < spapr_max_cores; i++) {
        Object *obj = object_new(TYPE_SPAPR_CPU_CORE);
        sPAPRCPUCore *core = SPAPR_CPU_CORE(obj);
        char name[32];

        snprintf(name, sizeof(name), "%s[%d]", SPAPR_MACHINE_CPU_CORE_PROP, i);

        /*
         * Create links from machine objects to all possible cores.
         */
        object_property_add_link(OBJECT(spapr), name, TYPE_SPAPR_CPU_CORE,
                                 (Object **)&spapr->core[i],
                                 NULL, NULL, &error_abort); 

        /*
         * Set the QOM link from machine object to core object for all
         * boot time CPUs specified with -smp. For rest of the hotpluggable
         * cores this is done from the core hotplug path.
         */
        if (i < spapr_smp_cores) {
            object_property_set_link(OBJECT(spapr), OBJECT(core),
                                     SPAPR_MACHINE_CPU_CORE_PROP, &error_abort);
        }
    ...
}