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

[Claudio Fontana]

On 11/27/20 7:13 PM, Eduardo Habkost wrote:
> On Fri, Nov 27, 2020 at 06:58:22PM +0100, Claudio Fontana wrote:
>> On 11/27/20 6:06 PM, Eduardo Habkost wrote:
>>> 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.
>>
>>
>> The question of QEMU initialization state and the querying of supported 
>> functionality, also in relationship with the loadable modules, is I think a 
>> larger discussion.
>>
>> Regardless of the amount of glue code and lipstick, this is hiding the fact 
>> that the fundamentals of the object hierarchy for cpus are wrong,
>> and are (unfortunately) codified as part of the external interface.
> 
> That's probably right, and removal of monkey patching might force
> us to change our external interfaces.
> 
>>
>>
>>>
>>> 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.
>>
>>
>> This seems to be for a next step to me.
> 
> Agreed, although I'd like to understand what makes
> AccelCPUClass.cpu_class_init() so important in the first version
> (considering that existing x86_cpu_class_init() has zero
> tcg_enabled() calls today).
> 

currently x86_cpu_common_class_init() has

#ifdef CONFIG_TCG

and

#ifdef CONFIG_USER_ONLY

I move this code to a tcg specific module,
and I also move the parts that should have been CONFIG_TCG before but were 
probably just missed.

Ciao,

Claudio