Paolo Bonzini <pbonz...@redhat.com> writes:
> 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.
No argument.
>> 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.
qdev C layer:
frob->prop = 42;
Least cognitive load.
QOM has no C layer.
qdev property layer works even when @frob has incomplete type:
qdev_prop_set_int32(DEVICE(frob), "prop", 42);
This used to map property name to struct offset & copy the value.
Simple, stupid.
Nowadays, it is the same as
object_property_set_int(OBJECT(frob), "frob", 42, &error_abort);
which first converts the int to a QObject, then uses a QObject input
visitor with a virtual walk to convert it back to int and store it in
@frob. It's quite a sight in the debugger.
qdev "text" layer is really a QemuOpts layer (because that's what we had
back then). If we have prop=42 in a QemuOpts, it calls
set_property("prop", "42", frob, &err);
Nowadays, this is a thin wrapper around object_property_parse(),
basically
object_property_parse(frob, "prop", 42, &err);
Fine print: except set_property() does nothing for @prop "driver" and
"bus", which look just like properties in -device / device-add, but
aren't.
object_property_parse() uses the string input visitor, which I loathe.
>> 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.
Yes. There are good reasons for QOM.
> 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.
That's fair.
>> 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. :)
The question is whether that 1% really should be done the way it is done
:)
