Re: [Qemu-devel] [PATCH v3 03/10] qom: support arbitrary non-scalar properties with -object

[Eric Blake <eblake@redhat.com>]

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On 03/10/2016 11:59 AM, Daniel P. Berrange wrote:
> The current -object command line syntax only allows for
> creation of objects with scalar properties, or a list
> with a fixed scalar element type. Objects which have
> properties that are represented as structs in the QAPI
> schema cannot be created using -object.
> 
> This is a design limitation of the way the OptsVisitor
> is written. It simply iterates over the QemuOpts values
> as a flat list. The support for lists is enabled by
> allowing the same key to be repeated in the opts string.
> 
> It is not practical to extend the OptsVisitor to support
> more complex data structures while also maintaining
> the existing list handling behaviour that is relied upon
> by other areas of QEMU.

Zoltán Kővágó tried earlier with his GSoC patches for the audio
subsystem last year, but those got stalled waiting for qapi enhancements
to go in.  But I think your approach is indeed a bit nicer (rather than
making the warty OptsVisitor even wartier, just avoid it).

> 
> Fortunately there is no existing object that implements
> the UserCreatable interface that relies on the list
> handling behaviour, so it is possible to swap out the
> OptsVisitor for a different visitor implementation, so
> -object supports non-scalar properties, thus leaving
> other users of OptsVisitor unaffected.
> 
> The previously added qdict_crumple() method is able to
> take a qdict containing a flat set of properties and
> turn that into a arbitrarily nested set of dicts and
> lists. By combining qemu_opts_to_qdict and qdict_crumple()
> together, we can turn the opt string into a data structure
> that is practically identical to that passed over QMP
> when defining an object. The only difference is that all
> the scalar values are represented as strings, rather than
> strings, ints and bools. This is sufficient to let us
> replace the OptsVisitor with the QMPInputVisitor for
> use with -object.

Indeed, nice replacement.

> 
> Thus -object can now support non-scalar properties,
> for example the QMP object
> 
>   {
>     "execute": "object-add",
>     "arguments": {
>       "qom-type": "demo",
>       "id": "demo0",
>       "parameters": {
>         "foo": [
> 	  { "bar": "one", "wizz": "1" },
> 	  { "bar": "two", "wizz": "2" }
>         ]
>       }
>     }
>   }
> 
> Would be creatable via the CLI now using
> 
>     $QEMU \
>       -object demo,id=demo0,\
>               foo.0.bar=one,foo.0.wizz=1,\
>               foo.1.bar=two,foo.1.wizz=2
> 
> This is also wired up to work for the 'object_add' command
> in the HMP monitor with the same syntax.
> 
>   (hmp) object_add demo,id=demo0,\
>                    foo.0.bar=one,foo.0.wizz=1,\
> 		   foo.1.bar=two,foo.1.wizz=2

Maybe mention that the indentation is not actually present in the real
command lines typed.

> 
> Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
> ---
>  hmp.c                      |  18 +--
>  qom/object_interfaces.c    |  20 ++-
>  tests/check-qom-proplist.c | 295 ++++++++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 313 insertions(+), 20 deletions(-)
> 

> @@ -120,6 +120,7 @@ Object *user_creatable_add_type(const char *type, const char *id,
>      obj = object_new(type);
>      if (qdict) {
>          for (e = qdict_first(qdict); e; e = qdict_next(qdict, e)) {
> +
>              object_property_set(obj, v, e->key, &local_err);
>              if (local_err) {
>                  goto out;

Spurious hunk?


-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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