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

[Igor Mammedov]

On Tue,  5 Jul 2016 10:12:49 +0530
Bharata B Rao <address@hidden> 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>
> ---
>  exec.c              | 10 ++++++++--
>  include/hw/boards.h |  1 +
>  include/qom/cpu.h   |  4 ++++
>  3 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index 8ce8e90..7cc1d06 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -616,15 +616,21 @@ static void cpu_release_index(CPUState *cpu)
>      bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
>  }
>  
> +/*
> + * TODO: cpu_index and instance_id are of type int
> while .get_arch_id()is
> + * of type int64_t. What is the consequence of changing instance_id
> to int64_t ?
> + */
ARM potentially could have get_arch_id() == MPIDR (return 64bit int)
that would get truncated in this case.

I wonder if we could add "int CPUState::migration_id" and let machine
code set it to what it uses for stable cpu id and then cpu_realize
could call vmstate_register with it.