Re: [Qemu-devel] [PATCH v4 05/16] pc: enforce adding CPUs contiguously and removing them in opposit order

[Bandan Das <bsd@redhat.com>]

Igor Mammedov <imammedo@redhat.com> writes:

> On Thu, 14 Jul 2016 14:10:24 -0400
> Bandan Das <bsd@redhat.com> wrote:
>
>> Igor Mammedov <imammedo@redhat.com> writes:
>> 
>> > it will still allow us to use cpu_index as migration instance_id
>> > since when CPUs are added contiguously (from the first to the last)
>> > and removed in opposite order, cpu_index stays stable and it's
>> > reproducable on destination side.
>> >
>> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>> > ---
>> > While there is work in progress to support migration when there are holes
>> > in cpu_index range resulting from out-of-order plug or unplug, this patch
>> > is intended as a last resort if no easy, risk-free and elegant solution
>> > emerges before 2.7 dev cycle ends.  
>> 
>> I think this (or a modified version) is appropriate comment
>> material to accompany the changes. Ok if you are sure this code
>> is short-lived, but if it stays longer, a comment is definitely
>> helpful. Maybe a bit of reasoning added to the error message is
>> fine too.
> dwg is looking at cpu_index refactoring but that's not 2.7 material,
> this patch is doing what the similar spapr patch did
> (which David applied to his ppc queue).
>
> Perhaps moving comment under --- to commit message itself would
> be better as to leave trace of future refactoring plans.

Right, sounds good.

>> > ---
>> >  hw/i386/pc.c | 34 ++++++++++++++++++++++++++++++++++
>> >  1 file changed, 34 insertions(+)
>> >
>> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> > index 33c5f97..75a92d0 100644
>> > --- a/hw/i386/pc.c
>> > +++ b/hw/i386/pc.c
>> > @@ -1762,6 +1762,23 @@ static void pc_cpu_unplug_request_cb(HotplugHandler *hotplug_dev,
>> >          goto out;
>> >      }
>> >  
>> > +    if (idx < pcms->possible_cpus->len - 1 &&
>> > +        pcms->possible_cpus->cpus[idx + 1].cpu != NULL) {
>> > +        X86CPU *cpu;
>> > +
>> > +        for (idx = pcms->possible_cpus->len - 1;
>> > +             pcms->possible_cpus->cpus[idx].cpu == NULL; idx--) {
>> > +            ;;
>> > +        }
>> > +
>> > +        cpu = X86_CPU(pcms->possible_cpus->cpus[idx].cpu);
>> > +        error_setg(&local_err, "CPU [socket-id: %u, core-id: %u,"
>> > +                   " thread-id: %u] should be removed first",
>> > +                   cpu->socket_id, cpu->core_id, cpu->thread_id);
>> > +        goto out;
>> > +
>> > +    }
>> > +
>> >      hhc = HOTPLUG_HANDLER_GET_CLASS(pcms->acpi_dev);
>> >      hhc->unplug_request(HOTPLUG_HANDLER(pcms->acpi_dev), dev, &local_err);
>> >  
>> > @@ -1860,6 +1877,23 @@ static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
>> >          return;
>> >      }
>> >  
>> > +    if (idx != 0 && pcms->possible_cpus->cpus[idx - 1].cpu == NULL) {
>> > +        PCMachineClass *pcmc = PC_MACHINE_GET_CLASS(pcms);
>> > +
>> > +        for (idx = 1; pcms->possible_cpus->cpus[idx].cpu != NULL; idx++) {
>> > +            ;;
>> > +        }
>> > +
>> > +        x86_topo_ids_from_apicid(pcms->possible_cpus->cpus[idx].arch_id,
>> > +                                 smp_cores, smp_threads, &topo);
>> > +
>> > +        if (!pcmc->legacy_cpu_hotplug) {
>> > +            error_setg(errp, "CPU [socket: %u, core: %u, thread: %u] should be"
>> > +                       " added first", topo.pkg_id, topo.core_id, topo.smt_id);
>> > +            return;
>> > +        }
>> > +    }
>> > +
>> >      /* if 'address' properties socket-id/core-id/thread-id are not set, set them
>> >       * so that query_hotpluggable_cpus would show correct values
>> >       */