Re: sysbus_create_simple Vs qdev_create

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jul 28, 2020 at 07:38:27PM +0200, Paolo Bonzini wrote:
> On 28/07/20 09:19, Markus Armbruster wrote:
> >> the composition tree generally mirrors things that are born and die
> >> at the same time, and creating children is generally reserved to the
> >> object itself.
> >
> > Yes.  Notable exceptions: containers /machine/peripheral,
> > /machine/peripheral-anon, /machine/unattached.
> 
> And /objects too.  Apart from /machine/unattached, all these dynamic
> objects are created by the monitor or the command line.
> 
> >>                 Children are usually embedded directly in a struct, for
> >> example.
> > 
> > We sometimes use object_new() + object_property_add_child() instead.
> > Extra indirection.  I guess we'd be better off without the extra
> > indirection most of the time.  Implementation detail.
> > 
> > We sometimes use object_new() without object_property_add_child(), and
> > have qdev_realize() put the device in the /machine/unattached orphanage.
> > Meh.  I guess the orphanage feature exists to make conversion to QOM
> > slightly easier.  Could we ban its use for new boards at least?
> 
> Banning perhaps is too strong, but yes /machine/unattached is an
> anti-pattern.
> 
> >> 3) accessing the QOM graph is slow (it requires hash table lookups,
> >> string comparisons and all that), so the pointers that cache the
> >> parent-child links are needed for use in hot paths.
> > 
> > True, but only because QOM's design opts for generality, efficiency be
> > damned :)
> 
> Remember that QOM's essential feature is the visitors: unlike GObject,
> QOM is not targeted at programming languages but rather at CLI and RPC.

This is surprising to me.  I never thought QOM was targeted at
the CLI or RPC.  (Every single property mentioned in this message
don't seem to be related to the CLI or RPC.)

About the visitors: I always had the impression that usage of
visitors inside QOM is unnecessary and avoidable (compared to
QAPI, where the visitors are an essential feature).


> 
> > I never quite understood why we need to build properties at run time.
> 
> I think it was (for once) Anthony reasoning that good is better than
> perfect.  You do need run-time properties for /machine/unattached,
> /machine/peripheral, etc., so he decided to only have run-time
> properties.  Introspection wasn't considered a primary design goal.
> 
> Also, even though child properties are quite often the same for all
> objects after initialization (and possibly realization) is complete,
> this does not cover in two cases:
> 
> 1) before the corresponding objects are created---so static child
> properties would only be possible if creation of all children is moved
> to instance_init, which is guaranteed not to fail.
> 
> 2) there are cases in which a property (e.g. an int) governs how many of
> those children exist, so you cannot create all children in instance_init.

Do we really need need QOM children to be accessible using the QOM
property API?

Using the same code for both user-configurable properties and for
the list of children of a QOM object might have saved some time
years ago, but I'm not sure this is still a necessary or useful
abstraction.

-- 
Eduardo