Re: [Qemu-devel] [PATCH 6/6] arm: drop intermadiate cpu_model -> cpu type parsing and use cpu type directly

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Sep 05, 2017 at 03:46:07PM -0700, Alistair Francis wrote:
> On Tue, Sep 5, 2017 at 3:12 PM, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > On Tue, Sep 05, 2017 at 02:47:52PM -0700, Alistair Francis wrote:
> >> On Tue, Sep 5, 2017 at 2:31 PM, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > [...]
> >> >> diff --git a/hw/arm/stm32f205_soc.c b/hw/arm/stm32f205_soc.c
> >> >> index f61e735..1cd6374 100644
> >> >> --- a/hw/arm/stm32f205_soc.c
> >> >> +++ b/hw/arm/stm32f205_soc.c
> >> >> @@ -112,7 +112,7 @@ static void stm32f205_soc_realize(DeviceState *dev_soc, Error **errp)
> >> >>
> >> >>      armv7m = DEVICE(&s->armv7m);
> >> >>      qdev_prop_set_uint32(armv7m, "num-irq", 96);
> >> >> -    qdev_prop_set_string(armv7m, "cpu-model", s->cpu_model);
> >> >> +    qdev_prop_set_string(armv7m, "cpu-type", s->cpu_type);
> >> >>      object_property_set_link(OBJECT(&s->armv7m), OBJECT(get_system_memory()),
> >> >>                                       "memory", &error_abort);
> >> >>      object_property_set_bool(OBJECT(&s->armv7m), true, "realized", &err);
> >> >> @@ -200,7 +200,7 @@ static void stm32f205_soc_realize(DeviceState *dev_soc, Error **errp)
> >> >>  }
> >> >>
> >> >>  static Property stm32f205_soc_properties[] = {
> >> >> -    DEFINE_PROP_STRING("cpu-model", STM32F205State, cpu_model),
> >> >> +    DEFINE_PROP_STRING("cpu-type", STM32F205State, cpu_type),
> >> >
> >> > Same as armv7m: are we 100% sure users are not setting this
> >> > manually?
> >>
> >> In an embedded board like this it really doesn't make sense to let the
> >> user overwrite the CPU. The SoC will take it as an option, but the
> >> board (which creates the SoC) just blindly always uses the same CPU.
> >> That feature is more for QOMificatoion then any real reason though.
> >>
> >
> > I'm not talking about -cpu (no user-visible change in the
> > handling of -cpu should result from this patch), but about
> > possible cases where the user set the "cpu-model" property using
> > another mechanism, like -global.  Probably it's impossible for an
> > user to override the property successfully, but I would like to
> > be sure.
> 
> Ah, that is trickier.
> 
> I guess that is possible to do, but the object setting logic should
> handle the error gracefully and inform the user of the error.

After looking at the code more closely, I think we can be 100%
sure the user doesn't rely on the property, because:

* TYPE_ARMV7M and TYPE_STM32F205_SOC are both sysbus devices
  with user_creatable=false, so the user can't instantiate them
  directly;
* The only places where those objects are realized inside the
  code are:
  * mps2_common_init()
  * netduino2_init()
  * stm32f205_soc_realize()
  * armv7m_init()
  Those functions always set the "cpu-model" property immediately
  before realize.

This means any value set by the user (e.g. using -global) would
be always overwritten before realize.

However, I have a suggestion for Igor: making a separate patch
that renames the existing property to "x-cpu-model", and using
"x-cpu-type" in this series.  This way we will explicitly
document the fact that the property is not a stable
user/management interface.

> 
> >
> >
> >> In saying that I think a warning if the user tries to set the CPU
> >> would make sense. I know that this issues comes up in other ARM boards
> >> (Zynq-7000 has the same issue as well) so maybe a machine property
> >> saying that the board doesn't accept custom CPUs would be a good idea.
> >
> > Yeah, there are multiple cases in this patch where boards are
> > validating the CPU model, but not all boards do that.  A generic
> > MachineClass::valid_cpu_types[] field would be useful.
> >
> >>
> >> Overall I think this patch is moving in the right direction though and
> >> this CPU option being ignored existed before this series.
> >
> > I agree this is going on the right direction.  However, I don't
> > see any board that ignore the CPU option: all of them seem to use
> > cpu_model when creating the CPUs, already.
> 
> The Netduino2 will ignore any CPU options and always use a Cortex-m3.
> I was wrong about Zynq-7000 though, it does respect the -cpu option.
> 
> Thanks,
> Alistair
> 
> >
> > --
> > Eduardo

-- 
Eduardo