Re: [Qemu-devel] [PATCH v7 01/16] hw/cpu: introduce CPU clusters

[Eduardo Habkost]

Hi,

Sorry for not reviewing this series earlier.  I just stumbled
upon this part of the code:

On Fri, Nov 23, 2018 at 10:17:14AM +0100, Luc Michel wrote:
> This commit adds the cpu-cluster type. It aims at gathering CPUs from
> the same cluster in a machine.
> 
> For now it only has a `cluster-id` property.
> 
> Signed-off-by: Luc Michel <address@hidden>
> Reviewed-by: Alistair Francis <address@hidden>
> Reviewed-by: Philippe Mathieu-Daudé <address@hidden>
> Tested-by: Philippe Mathieu-Daudé <address@hidden>
> Reviewed-by: Edgar E. Iglesias <address@hidden>
[...]
> +static void cpu_cluster_init(Object *obj)
> +{
> +    static uint32_t cluster_id_auto_increment;
> +    CPUClusterState *cluster = CPU_CLUSTER(obj);
> +
> +    cluster->cluster_id = cluster_id_auto_increment++;

I see that you implemented this after a suggestion from Philippe,
but I'm worried about this kind of side-effect on object/device
code.  I'm afraid this will bite us back in the future.  We were
bitten by problems caused by automatic cpu_index assignment on
CPU instance_init, and we took a while to fix that.

If you really want to do this and assign cluster_id
automatically, please do it on realize, where it won't have
unexpected side-effects after a simple `qom-list-properties` QMP
command.

I would also add a huge warning above the cluster_id field
declaration, mentioning that the field is not supposed to be used
for anything except debugging.  I think there's a large risk of
people trying to reuse the field incorrectly, just like cpu_index
was reused for multiple (conflicting) purposes in the past.


> +}
> +
> +static Property cpu_cluster_properties[] = {
> +    DEFINE_PROP_UINT32("cluster-id", CPUClusterState, cluster_id, 0),
> +    DEFINE_PROP_END_OF_LIST()
> +};
[...]

-- 
Eduardo