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

[Marc Zyngier]

On Thu, 11 Jan 2024 09:39:18 +0000,
Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> 
> 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...)?

I don't think it makes any sense to do that while a guest is running
(and no HW I'm aware of would do this). However, it all depends what
you consider "run time". You could imagine creating a skeletal VM with
all features, and then apply a bunch of changes before the guest
actually runs.

I don't know enough about the qom-tree and dynamic manipulation of
these properties though, and I'm likely to be wrong about the expected
usage model.

Thanks,

        M.

-- 
Without deviation from the norm, progress is not possible.