Re: [PATCH 01/27] include/qemu/target-info-qom.h: declare TYPE_TARGET_SPECIFIC interface

On 8/6/2026 7:52 AM, Philippe Mathieu-Daudé wrote:
> On 6/8/26 12:21, Daniel P. Berrangé wrote:
>> On Wed, Aug 05, 2026 at 12:10:27PM -0700, Pierrick Bouvier wrote:
>>> On 8/5/2026 9:53 AM, Daniel P. Berrangé wrote:
>>>> On Wed, Aug 05, 2026 at 09:16:20AM -0700, Pierrick Bouvier wrote:
>>>>> On 8/5/2026 7:05 AM, Daniel P. Berrangé wrote:
>>>>>> On Fri, Jul 24, 2026 at 12:09:21AM +0000, Pierrick Bouvier wrote:
>>>>>>> In the next commits, We'll replace the logic to filter QOM types per
>>>>>>> target from a static one (based on INTERFACES) to a runtime one,
>>>>>>> based
>>>>>>> on is_available() function, that can be overriden per class.
>>>>>>>
>>>>>>> Introduce the new interface we'll use for that.
>>>>>>>
>>>>>>> Signed-off-by: Pierrick Bouvier <[email protected]>
>>>>>>> ---
>>>>>>>   include/qemu/target-info-qom.h | 15 +++++++++++++++
>>>>>>>   target-info-qom.c              |  5 +++++
>>>>>>>   2 files changed, 20 insertions(+)
>>>>>>>
>>>>>>> diff --git a/include/qemu/target-info-qom.h b/include/qemu/
>>>>>>> target-info-qom.h
>>>>>>> index 91be415ed33..83eb537333b 100644
>>>>>>> --- a/include/qemu/target-info-qom.h
>>>>>>> +++ b/include/qemu/target-info-qom.h
>>>>>>> @@ -14,6 +14,21 @@
>>>>>>>     #define TYPE_TARGET_INFO "target-info"
>>>>>>>   +#define TYPE_TARGET_SPECIFIC "target-specific"
>>>>>>> +
>>>>>>> +typedef struct TargetSpecific TargetSpecific;
>>>>>>> +
>>>>>>> +typedef struct TargetSpecificClass {
>>>>>>> +    InterfaceClass parent_class;
>>>>>>> +
>>>>>>> +    bool (*is_available)(void);
>>>>>>> +} TargetSpecificClass;
>>>>>>> +
>>>>>>> +#define TARGET_SPECIFIC(obj) \
>>>>>>> +    INTERFACE_CHECK(TargetSpecific, (obj), TYPE_TARGET_SPECIFIC)
>>>>>>> +DECLARE_CLASS_CHECKERS(TargetSpecificClass, TARGET_SPECIFIC,
>>>>>>> +                       TYPE_TARGET_SPECIFIC)
>>>>>>
>>>>>> Looking through the series,I don't really see the point
>>>>>> in this interface.   Why is this not possible to do by
>>>>>> adding 'is_available' to MachineClass.  It would make
>>>>>> the rest of the series simpler and especially avoid the
>>>>>> need to introduced yet more series of macros for defining
>>>>>> machine classes.
>>>>>>
>>>>>
>>>>> We'll need the exact same interface for cpus, and devices also.
>>>>> IMHO, it
>>>>> makes sense to have this in an external interface, instead of
>>>>> forcing it
>>>>> to be present in all cpus/devices/machines. I also considered
>>>>> adding it
>>>>> directly in Object class directly (would be the simplest), but I
>>>>> felt it
>>>>> would be hard to motivate it.
>>>>
>>>> I don't see a need for the common interface across cpus/devices/etc as
>>>> as code that's filtering only cares about the specific types. It also
>>>> definitely doesn't beloong in Object class, but the Object class could
>>>> be changed to make it simpler.
>>>>
>>>
>>> We agree on this.
>>>
>>>> The object_class_get_list() method could get a 'bool
>>>> filter(ObjectClass *cl)'
>>>> callback which could be invoked on each class to filter it.
>>>>
>>>> That said I find it pretty undesirable as an approach that we're
>>>> registering classes that can't then be used in a given situation.
>>>> This has a ripple effect where every bit of code that iterates over
>>>> classes needs changing to add filtering after the fact. It is also
>>>> not great for scalability, as it means every QEMU process will have
>>>> the union of all classes for all targets registered, most of which
>>>> have to be discarded / ignored at runtime.
>>>>
>>>
>>> This is inherent to the nature of having a single binary.
>>> We need to cover those two requirements:
>>> 1. having a filter mechanism (per target)
>>> 2. have all the classes accessible for the heterogeneous machines that
>>> will be coming in the future
>>>
>>> 1. could be covered by what you describe, however, it breaks 2. For
>>> this, you need to be able to register all types by design.
>>
>>
>> Even the heterogeneous machines aren't going to need all the
>> classes from all 30+ targets that QEMU supports.
>>
>> IIUC, the current approach relies on '--target <blah>' to select
>> which target we need. Would the heterogeneous machines not just
>> change that to allow "--target <this> --target <that>". It still
>> looks like we should be able to significantly limit  what we
>> register for heterogeneous machines.
> 
> Heterogeneous binary won't filter anything at runtime (if we want
> to filter components we already have Kconfig at compile time).
> 
> "--target <foo>" is only needed to have a single binary backward
> compatible. If you use it, you fall back to single architecture
> (our current binaries). If you want anything heterogeneous, you
> can not use it. This will be by design.

I've been thinking about it yesterday, and we could apply the paradigm
"register only classes that will be used". For heterogeneous machine,
either we'll provide a "none" target, which enables all target, or use
--target aarch64,riscv64, like Daniel proposed.

I will implement what Daniel asked for v2.
However, please be aware it will be probably be more invasive and
verbose, since we'll need to add macros and stuff to have conditional
type_register_static.

I would like to avoid doing a change you asked, and have no further
comment after that or something like "it was better before".
Do we agree on this Daniel?

Regards,
Pierrick