Re: [RFC v6 10/11] accel: introduce AccelCPUClass extending CPUClass

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Nov 26, 2020 at 11:32:17PM +0100, Claudio Fontana wrote:
> add a new optional interface to CPUClass,
> which allows accelerators to extend the CPUClass
> with additional accelerator-specific initializations.
> 
> Signed-off-by: Claudio Fontana <cfontana@suse.de>
> ---
[...]
> +static void accel_init_cpu_int_aux(ObjectClass *klass, void *opaque)
> +{
> +    CPUClass *cc = CPU_CLASS(klass);
> +    AccelCPUClass *accel_cpu_interface = opaque;
> +
> +    cc->accel_cpu_interface = accel_cpu_interface;
> +    if (accel_cpu_interface->cpu_class_init) {
> +        accel_cpu_interface->cpu_class_init(cc);
> +    }
> +}

So, now that the approach we're following to trigger the
accel_init_cpu*() call is less controversial (thanks for your
patience!), we can try to address the monkey patching issue:

Monkey patching classes like this is acceptable as an initial
solution, but I'd like us to have a plan to eventually get rid of
it.  Monkey patching CPU classes makes querying of CPU model
information less predictable and subtly dependent on QEMU
initialization state.

Removing CPUClass.accel_cpu_interface may be easy, because it
should be possible to just call current_accel() when realizing
CPUs.  Getting rid of CPUClass.cpu_class_init might be more
difficult, depending on what the ->cpu_class_init() function is
doing.

> +
> +/* initialize the arch-specific accel CpuClass interfaces */
> +static void accel_init_cpu_interfaces(AccelClass *ac, const char *cpu_type)
> +{
> +    const char *ac_name; /* AccelClass name */
> +    char *acc_name;      /* AccelCPUClass name */
> +    ObjectClass *acc;    /* AccelCPUClass */
> +
> +    ac_name = object_class_get_name(OBJECT_CLASS(ac));
> +    g_assert(ac_name != NULL);
> +
> +    acc_name = g_strdup_printf("%s-%s", ac_name, CPU_RESOLVING_TYPE);
> +    acc = object_class_by_name(acc_name);
> +    g_free(acc_name);
> +
> +    if (acc) {
> +        object_class_foreach(accel_init_cpu_int_aux, cpu_type, false, acc);
> +    }
> +}
> +
>  void accel_init_interfaces(AccelClass *ac, const char *cpu_type)
>  {
>  #ifndef CONFIG_USER_ONLY
>      accel_init_ops_interfaces(ac);
>  #endif /* !CONFIG_USER_ONLY */
> +
> +    accel_init_cpu_interfaces(ac, cpu_type);
>  }
[...]

-- 
Eduardo