Re: [PATCH v1] target/s390x: filter deprecated features based on model expansion type

[David Hildenbrand <david@redhat.com>]

On 11.07.24 22:32, Collin Walling wrote:
> It is beneficial to provide an interface to retrieve *all* deprecated
> features in one go. Management applications will need this information
> to determine which features need to be disabled regardless of the
> host-model's capabilities.
> 
> To remedy this, deprecated features are only filtered during a static
> expansion. All deperecated features are reported on a full expansion.
> 
> Suggested-by: Jiri Denemark <jdenemar@redhat.com>
> Signed-off-by: Collin Walling <walling@linux.ibm.com>
> ---
>   target/s390x/cpu_models_sysemu.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/target/s390x/cpu_models_sysemu.c b/target/s390x/cpu_models_sysemu.c
> index 977fbc6522..76d15f2e4d 100644
> --- a/target/s390x/cpu_models_sysemu.c
> +++ b/target/s390x/cpu_models_sysemu.c
> @@ -211,7 +211,15 @@ static void cpu_info_from_model(CpuModelInfo *info, const S390CPUModel *model,
>       bitmap_zero(bitmap, S390_FEAT_MAX);
>       s390_get_deprecated_features(bitmap);
>   
> -    bitmap_and(bitmap, bitmap, model->def->full_feat, S390_FEAT_MAX);
> +    /*
> +     * For static model expansion, filter out deprecated features that are
> +     * not a subset of the model's feature set. Otherwise, report the entire
> +     * deprecated features list.
> +     */
> +    if (delta_changes) {
> +        bitmap_and(bitmap, bitmap, model->def->full_feat, S390_FEAT_MAX);
> +    }
> +

This would likely be the only interface where we expose "more" features 
than a CPU model actually understands? I guess it wouldn't be that bad 
because disabling unsupported features will just work, even if the CPU 
model is not aware of them.

So no strong opinion.

Just noting that libvirt cannot really rely on that information because 
the behavior would change between QEMU versions? Maybe not an issue.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb