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:

> 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.

> ---
>  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
>       */