Re: [PATCH 00/54] qom/qdev: associate properties with QAPI schema types

Hi

On Wed, Jul 29, 2026 at 4:43 PM Markus Armbruster <[email protected]> wrote:
>
> Marc-André Lureau <[email protected]> writes:
>
> > Hi
> >
> > On Wed, Jul 29, 2026 at 11:29 AM Markus Armbruster via qemu
> > development <[email protected]> wrote:
> >> Before I dive into individual patches, let me try to work out what the
> >> series does as a whole.
> >>
> >> > This series adds:
> >> >   - A new QAPITypeInfo struct that pairs a property with its QAPI schema
> >> >     type name, enum lookup table, and list-element type.
> >>
> >> Peeking at the code, I see that ObjectProperty gains a member @qapi_type
> >> pointing to its QAPITypeInfo.
> >>
> >> It is null when the ObjectProperty doesn't have a QAPI type.
> >>
> >> If it's non-null, then ObjectProperty members @name and @type are
> >> redundant with qapi_type.name and .type.
> >>
> >> Correct?
> >
> > Almost. @name is the property name (e.g. "policy"), not the type name.
> > But @type becomes redundant with qapi_type->name when qapi_type is set
> > (object_property_add_qapi derive prop->type from qapi_type->name).
> >
> > Also @type carries additional information for some property kinds that
> > @qapi_type doesn't cover: child<T> and link<T> embed the linked
> > object's QOM type name in the type string. qapi_type can't represent
> > that atm.
>
> Can child and link properties have non-null @qapi_type?

No. Both object_property_add_child() and object_property_add_link()
use the base object_property_add(), not the _qapi variants, so
qapi_type stays NULL.

On the qdev side, qdev_prop_link.qapi_type is &str_type_info, but
create_link_property() calls object_class_property_add_link() which
bypasses that, so the ObjectProperty ends up with qapi_type == NULL
and type = "link<T>".

>
> >> Would "every ObjectProperty has a QAPI type" be a reasonable goal for
> >> the future?
> >
> > Maybe? we would need to address the child<>/link<> gap. At least we
> > can make raw object_property_add() deprecated/static in object.c after
> > this series
>
> Interesting!
>
> >> >   - A QAPI code generator (qapi-type-infos) that emits a QAPITypeInfo
> >> >     instance for every schema-defined type, including the mapping
> >> >     between internal C names and the schema name visible to clients.
> >>
> >> Peeking at the code, I find:
> >>
> >> * The type
> >>
> >>     typedef struct QAPITypeInfo {
> >>         const char *name;
> >>         const char *schema_name;
> >>         const QEnumLookup *lookup;
> >>         const struct QAPITypeInfo *list;
> >>     } QAPITypeInfo;
> >>
> >> * A T_type_info for each QAPI type T, including built-in types.
> >>
> >> * T_type_info member @name is T's QAPI name, i.e. "T".
> >>
> >> * T_type_info member @schema_name is T's masked name used in
> >>   query-qmp-schema output, null when T is elided there.
> >>
> >> * T_type_info member @list points to TList_type_info when that exists,
> >>   else it's null.
> >>
> >> * T_type_info member @lookup points to T_lookup when T is an enum, else
> >>   it's null.
> >>
> >> Correct?
> >
> > Yes
> >
> >> >   - A "qapi-type" field in the ObjectPropertyInfo and
> >> >     ObjectPropertyValue QMP structs, populated from the QAPITypeInfo
> >> >     when present giving clients a cross-reference into query-qmp-schema
> >> >     output.
> >>
> >> To be precise: when ObjectPropertyInfo member @type is "T", then member
> >> @qapi-type is T_type_info.qapi-type.  Correct?
> >
> > @qapi-type is T_type_info.schema_name, the masked name
>
> Right.
>
> >> If .qapi-type is non-null, you can use it to look up precise type
> >> information via QAPI introspection, i.e. query-qmp-schema.
> >>
> >> Correct?
> >
> > Yes
> >
> >>
> >> Possible problem: query-qmp-schema covers only types that are actually
> >> used in QMP.  But the above technique additionally wants QOM property
> >> types.  I haven't checked what your series does about this, if anything.
> >
> > Most types used as QOM properties are also used by QMP commands. But
> > it's true that types not used by QMP get schame_name = NULL atm.
>
> So the problem is real, and to reap the full benefit of your work, we
> need to solve it.  Not necessarily right away.
>
> > Should we have a new pragma? Even if we have conditions, we may end up
> > with unused types in the build, but that shouldn't be a big issue. Or
> > we would need a more complicated several step build.
>
> No need not worry about the how right now.
>
> >> ObjectPropertyInfo is only used with QMP command handlers.  It is
> >> computed from ObjectProperty.
> >>
> >> >   - Conversion of all PropertyInfo definitions from the old
> >> >     .type/.enum_table strings to the new .qapi_type pointer.
> >>
> >> The above is QOM, this is qdev.
> >>
> >> Like ObjectProperty, PropertyInfo gains a member @qapi_type pointing to
> >> its QAPITypeInfo.  Howver, this one cannot be null.
> >>
> >> PropertyInfo members @type and @enum_table are dropped, because they are
> >> redundant with qapi_type.type and .lookup.
> >>
> >> Correct?
> >
> > Almost. Every PropertyInfo has a non-NULL qapi_type, except the array
> > ones (created with DEFINE_PROP_ARRAY_INFO) which leave .qapi_type NULL
> > and derive it at registration time from
> > .element_info->qapi_type->list.
>
> Can you point me to the code setting it?  Would save me the digging.

qdev_prop_qapi_type(), when info->qapi_type is NULL (the array
PropertyInfos created by DEFINE_PROP_ARRAY_INFO), it follows
element_info->qapi_type->list.

For example, qdev_prop_uint32_list has .element_info =
&qdev_prop_uint32, so it resolves uint32_type_info.list ->
uint32List_type_info.

Not perfect, we can improve later. hopefully

>
> >                                 Either way, the resulting
> > ObjectProperty always ends up with a non-NULL qapi_type.
> >
> > PropertyInfo members @type and @enum_table are dropped, because they
> > are redundant with qapi_type->name and ->lookup.
>
> Yes.
>
> >> >   - Replacement of the generic qdev_prop_array with typed per-element
> >> >     array PropertyInfos, removing the arrayinfo/arrayfieldsize
> >> >     indirection from struct Property.
> >>
> >> Before the series, an array-valued Property's @info member is
> >> @qdev_prop_array.  @qdev_prop_array provides no information on the array
> >> elements.  Instead, Property member @arrayinfo points to the
> >> PropertyInfo for the elements, and @arrayfieldsize is the size of an
> >> element.
> >>
> >> Your series makes PropertyInfo array-capable: new members @element_info
> >> and @element_size are the elements' PropertyInfo and size.  It then adds
> >> a proper PropertyInfo for each such property, and drops
> >> @qdev_prop_array.
> >>
> >> Correct?
> >
> > Yes
> >
> >>
> >> This could perhaps be spun out and merged separately to reduce the size
> >> of future respins.
> >
> > I can split it out if it helps
>
> Splitting off self-contained parts of a big series can help if they can
> be merged quicker than the entire series.  We'll see.
>
> >> Not mentioned:
> >>
> >>     - New QOM property creation functions for creating properties of
> >>       QAPI type.  These take a QAPITypeInfo.
> >>
> >>     - Convert some properties to use them.
> >>
> >> >   - Removal of the deprecated PropertyInfo.type and .enum_table fields,
> >> >     and of the old object_property_add_enum/add_tm APIs.
> >> >
> >> > Along the way, a few pre-existing type mismatches in property
> >> > definitions are fixed, the "struct tm" RTC property is replaced with a
> >> > proper QAPI StructTm type etc. Introducing more specific types or a
> >> > "typedef" to QAPI could help provide better associated type informations
> >> > than plain "str" in many cases, for example.
> >>
> >> Examples?
> >>
> >
> > Properties using str_type_info that carry structured values with their
> > own validation: macaddr ("52:54:00:12:34:56"), PCI addresses
> > ("04:00.0"), netdev names, chardev names, drive names, UUID strings. A
> > QAPI "typedef" (or newtype) for e.g. MacAddr, PciDevAddr, UUID could
> > let tools know these aren't arbitrary strings (see "hw/nvdimm: convert
> > UUID property to QAPI-aware registration" patch).
>
> Two separate kinds:
>
> 1. Strings we need to parse: PCI addresses, MAC addresses, UUIDs, ...
>
>    Parsing maps the string to something else, usually some object that
>    isn't a string.
>
>    We avoid string parsing in QAPI/QMP whenever practical.  You quoted
>    several examples where we don't.
>
> 2. IDs of things we need to resolve
>
>    Resolving maps the string to the object it names.
>
> In both cases, we map strings to objects.
>
> QAPI is unaware of this.  It simply uses strings in generated C.
>
> What if QAPI was aware?  If the schema specified the object type, and
> how to map from string to it?  This is a step beyond a mere typedef.
> The mapping would move from handwritten code to generated code.
> Generated interfaces would use the object types instead of strings.
>
> Not now, of course.
>
> >> > Comments welcome!
> >> >
> >> > Signed-off-by: Marc-André Lureau <[email protected]>
> >>
>
>