Re: [Qemu-devel] [PATCH 5/8] pc: init CPUState->cpu_index with index in possible_cpus[]

[David Gibson]

On Thu, Jul 21, 2016 at 05:54:36PM +0200, Igor Mammedov wrote:
> It will enshure that cpu_index for a given cpu stays the same
> regardless of the order cpus has been created/deleted.
> 
> No compat code is needed as for initial cpus index in
> possible_cpus[] matches cpu_index that's been auto-allocated
> in cpu_exec_init().
> 
> Tha same applies for hotplug with cpu-add command if cpus are
> added sequentially in increasing order as 'id' matches cpu_index.
> 
> If cpu-add had been used for creating out-of-order cpus,
> that created unmigratable instance since it were not possible
> to start target with the same cpu_index using old way
> of migrating instance with hotplugged cpus:
> 
> * source QEMU with CLI (-smp 1,maxcpus=3 and cpu-add id=2)
>   following set of cpu_index is allocated [0, 1] with
>   apics set [0, 2] respectivelly
> * target QEMU is started with CLI -smp 2,maxcpus=3
>   resulting in set of cpu_index [0, 1] but with
>   set of apics [0, 1] wich doesn't match source.
> 
> So we don't need compat code in this case as it's never worked
> and newelly added device_add support would use stable cpu_index
> set by machine to begin with, so it won't have above limitation
> and source QEMU could be migrated to destination regardless
> of the order cpus were created.
> 
> Signed-off-by: Igor Mammedov <address@hidden>

Reviewed-by: David Gibson <address@hidden>

> ---
>  hw/i386/pc.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index ac7a4d5..316fb43 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1872,6 +1872,7 @@ static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
>                              DeviceState *dev, Error **errp)
>  {
>      int idx;
> +    CPUState *cs;
>      CPUArchId *cpu_slot;
>      X86CPUTopoInfo topo;
>      X86CPU *cpu = X86_CPU(dev);
> @@ -1972,6 +1973,9 @@ static void pc_cpu_pre_plug(HotplugHandler *hotplug_dev,
>          return;
>      }
>      cpu->thread_id = topo.smt_id;
> +
> +    cs = CPU(cpu);
> +    cs->cpu_index = idx;
>  }
>  
>  static void pc_machine_device_pre_plug_cb(HotplugHandler *hotplug_dev,

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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