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

[Igor Mammedov]

On Thu, 14 Jul 2016 14:10:24 -0400
Bandan Das <address@hidden> wrote:

> Igor Mammedov <address@hidden> 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 <address@hidden>
> > ---
> > 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.

> > ---
> >  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
> >       */