Re: [PATCH v3 13/14] hw/arm: Prefer arm_feature(AARCH64) over object_property_find(aarch64)

[Philippe Mathieu-Daudé]

On 10/1/24 20:53, Philippe Mathieu-Daudé wrote:
> The "aarch64" property is added to ARMCPU when the
> ARM_FEATURE_AARCH64 feature is available. Rather than
> checking whether the QOM property is present, directly
> check the feature.
>
> Suggested-by: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   hw/arm/virt.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 49ed5309ff..a43e87874c 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -2140,7 +2140,7 @@ static void machvirt_init(MachineState *machine)
>           numa_cpu_pre_plug(&possible_cpus->cpus[cs->cpu_index], DEVICE(cpuobj),
>                             &error_fatal);
>   
> -        aarch64 &= object_property_get_bool(cpuobj, "aarch64", NULL);
> +        aarch64 &= arm_feature(cpu_env(cs), ARM_FEATURE_AARCH64);

So after this patch there are no more use of the ARMCPU "aarch64"
property from code. Still it is exposed via the qom-tree. Thus it
can be set (see aarch64_cpu_set_aarch64). I could understand one
flip this feature to create a custom CPU (as a big-LITTLE setup
as Marc mentioned on IRC), but I don't understand what is the
expected behavior when this is flipped at runtime. Can that
happen in real hardware (how could the guest react to that...)?

Thanks,

Phil.