Re: sysbus_create_simple Vs qdev_create

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/07/20 15:18, Markus Armbruster wrote:
>> Even code riddled by backwards-compatibility special cases, such as
>> -accel and -machine, can share code between themselves and -object to
>> some extent; this is thanks to functions such as object_property_parse,
>> whose parsing is deferred to visitors and hence to QAPI.
>
> QOM relies on QAPI visitors to access properties.  There is no
> integration with the QAPI schema.

Indeed it doesn't use _all_ of the QAPI goodies.  It does use visitors
and it's a major feature of QOM.

> Going through a visitor enables property access from QMP, HMP and CLI.
> 
> Access from C *also* goes through a visitor.  We typically go from C
> type to QObject and back.  Comically inefficient (which hardly matters),
> verbose to use and somewhat hard to understand (which does).

It's verbose in the getters/setters, but we have wrappers such as
object_property_set_str, object_property_set_bool etc. that do not make
it too hard to understand.

> Compare to what QOM replaced: qdev.  Properties are a layer on top of
> ordinary C.  From C, you can either use the C layer (struct members,
> basically), or the property layer for C (functions taking C types, no
> conversion to string and back under the hood), or the "text" layer
> (parse from text / format to text).
> 
> My point is not that qdev was great and QOM is terrible.  There are
> reasons we replaced qdev with QOM.  My point is QOM doesn't *have* to be
> the way it is.  It is the way it is because we made it so.

QOM didn't only replace qdev: it also removed the need to have a command
line option du jour for any new concept, e.g. all the TLS stuff, RNG
backends, RAM backends, etc.

It didn't succeed (at all) in deprecating chardev/netdev/device etc.,
but this is a very underappreciated part of QOM, and this is why I think
it's appropriate to say QOM is "C with classes and CLI/RPC
serialization", as opposed for example to "C with classes and multi
programming language interface" that is GObject.

> I've long had the nagging feeling that if we had special-cased
> containers, children and links, we could have made a QOM that was easier
> to reason about, and much easier to integrate with a QAPI schema.

That's at least plausible.  But I have a nagging feeling that it would
only cover 99% of what we're doing with QOM. :)

Paolo