[Qemu-devel] [PATCH RFC 0/7] Making QOM introspectable

[Daniel P. Berrange]

There are many problems in QEMU related to introspection
of features, which Markus has been attacking/slaying for
a while. One of the remaining elephants in the corner of
the room which I've not seen work on is QOM.

QOM has a nice class/object framework, kind of like GLib's
GObject, but with the difference that properties are
registered against objects rather than classes. IIRC the
rationale for this is that in some cases the properties
registered are going to be dynamically decided at runtime,
so it isn't possible to statically define them against
classes. Now this may well be true of some properties,
but there are equally plenty of cases where the properties
/are/ invariant and could be registered against classes.

There are two core problems with registering properties
against object instances:

 - It is wasteful of memory to duplicate the allocation
   of ObjectProperty structs against each object
   instance. When you have N object instances, you
   have O(N) memory usage, instead of O(1). This is not
   a problem for objects which are mostly singletons,
   but there are cases in QEMU where you instantiate
   many instances of the same class and/or have many
   properties.

 - It prevents static introspection. Since the property
   is only registered in the object initializer, there
   is no way to programmatically query what properties
   an object supports without first instantiating it.
   Taking machine types as an example, if you want to
   introspect every machine type supported by a QEMU
   binary you thus have to launch QEMU many times,
   passing a different -M argument each time. As the
   number of different machine types & objects
   increases this quickly becomes impractical.

This series thus extends QOM to make it possible to register
properties against the classes, in addition to against object
instances. When looking up a property, a search will be done
starting at the base class, then each subclass in turn, and
finally against the object. Names are enforced to be unique
across the parent classes for sanity.

This only currently supports simple scalar properties where
the actual property value storage is managed by the object
instance. The object child and object link properties use
implicit storage in the ObjectProperty struct's 'opaque'
field, so we can't allow those against the class. Solving
this is doable, but more work, so is left as an exercise
for the future.

The first patch adds the neccessary QOM functionality. The
following 6 patches then illustrate the fairly trivial
conversions of a bunch of objects.

The eventual goal ought to be that everything is registered
against the class, leaving only the (hopefully few) cases
where per-instance properties are truely needed unconverted.

This series doesn't attempt to implement introspection
either - this would require a new QMP command such as
'qom-list-type-props' to query the properties against
classes.

I'm not really planning to spend much more time on this
myself. I'm just using this series to illustrate how I
believe the introspection problem in QOM can be fairly
easily addressed. Hopefully this will stimulate some
discussion & interest in doing the full job....

Daniel P. Berrange (7):
  qom: allow properties to be registered against classes
  hostmem: register properties against the class instead of object
  rng: register properties against the class instead of object
  tpm: register properties against the class instead of object
  cpu: avoid using object instance state in property getter
  x86-cpu: register properties against the class instead of object
  machine: register properties against the class instead of object

 backends/hostmem-file.c |  26 +++---
 backends/hostmem.c      |  41 ++++-----
 backends/rng-egd.c      |  12 +--
 backends/rng-random.c   |  10 +--
 backends/rng.c          |  14 ++-
 backends/tpm.c          |  12 +--
 hw/core/machine.c       | 193 +++++++++++++++++++--------------------
 include/qom/object.h    |  44 +++++++++
 qom/object.c            | 233 ++++++++++++++++++++++++++++++++++++++++++++++--
 target-i386/cpu.c       |  88 +++++++++++-------
 10 files changed, 476 insertions(+), 197 deletions(-)

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2.4.3