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
