Re: [Qemu-ppc] [RFC PATCH v0 2/5] cpu: Optionally use arch_id instead of cpu_index in cpu vmstate_register()

[Bharata B Rao]

On Tue, Jul 05, 2016 at 02:56:53PM +1000, David Gibson wrote:
> On Tue, Jul 05, 2016 at 10:12:49AM +0530, Bharata B Rao wrote:
> > Introduce CPUState.prefer_arch_id_over_cpu_index and
> > MachineClass.prefer_arch_id_over_cpu_index that allow target
> > machines to optionally switch to using arch_id instead of cpu_index
> > as instance_id in vmstate_register(). This will help allow successful
> > migration in cases where holes are introduced in cpu_index range
> > after CPU hot removals.
> > 
> > Whether to use arch_id or cpu_index is based on machine type version
> > and hence added MachineClass.prefer_arch_id_over_cpu_index. However the
> > enforcement is via and during CPU creation and hence added
> > CPUState.prefer_arch_id_over_cpu_index. So it becomes a two step
> > process for the target to enable the use of arch_id:
> > 
> > 1. Set MachineClass.prefer_arch_id_over_cpu_index.
> > 2. Ensure CPUState.prefer_arch_id_over_cpu_index is set for all CPUs
> >    based on 1. above.
> > 
> > Suggested-by: Igor Mammedov <address@hidden>
> > Signed-off-by: Bharata B Rao <address@hidden>
> 
> How is migration between a version with this flag and a version
> without this flag handled?

There can't be such a case as this flag is enabled for a given machine version.

Regards,
Bharata.