Re: [PATCH 00/18] qapi/qom: QAPIfy object-add

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/12/20 18:35, Kevin Wolf wrote:
>> Could we have an intermediate state that doesn't require any
>> duplication and thus have no separate code paths that could
>> diverge?
> 
> The one requirement we have for an intermediate state is that it
> supports both interfaces: The well-know create/set properties/realize
> dance, and a new DeviceClass method, say .create(), that takes the
> configuration in parameters instead of relying on previously set
> properties.
> 
> I assumed two separate implementations of transferring the configuration
> into the internal state. On second thought, this assumption is maybe
> wrong.
> 
> You can implement the new method as wrapper around the old way: It could
> just set all the properties and call realize. Of course, you don't win
> much in terms of improving the class implementation this way, but just
> support the new interface, but I guess it can be a reasonable
> intermediate step to resolve complicated dependencies etc.

I sketched something at https://wiki.qemu.org/Features/QOM-QAPI_integration.

The main difference with what we discussed so far is that it doesn't 
subsume the complete/realize step, only the setting of properties.  The 
idea is that user_creatable_add_type does the following:

- calls an oc->configure method on every superclass of the object

- takes what's left of the input visitor and uses it to set properties

- then calls ucc->complete

So in the end the only new step is the first.  If the first two steps 
are bundled in a new function object_configure, they can be reused for 
devices, machines and accelerators.

The QAPI code generator can also easily wrap them into a C API for QOM 
object creation.

I glossed completely over the generation of properties within the QAPI 
code generator.  Making properties read-only (or, in the 
field-properties world, having a default allow_set of "return false") is 
already a nice improvement over

> It would be much nicer to do the wrapper the other way round, i.e.
> setting properties before the device is realized would update a
> configuration struct and realize would then call .create() with that
> struct. To me, this sounds much harder, though also a more useful state.

This sounds much harder.  However, based on the RngEgd sample, going 
from a basic QAPI struct to a full conversion is not too hard and makes 
code shorter.  So it's win-win even though it's human work.

Paolo