On Mon, Mar 16, 2026 at 04:43:54PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé <[email protected]> writes: > > > On Wed, Mar 11, 2026 at 02:05:24PM +0100, Markus Armbruster wrote: > >> I can't find a good spot in the existing discussion where the following > >> would fit neatly as a reply, so I'm starting at the top again. > >> > >> Fact: a huge part of our external interface is *accidental* and > >> virtually undocumented. > >> > >> The sane way to do an external interface is to layer it on top of more > >> powerful internal interfaces. The external interface exposes just the > >> functionality that's wanted there. The internal interfaces can evolve > >> without affecting the external one. > >> > >> QMP works that way. QEMU code uses internal C interfaces. QEMU doesn't > >> send QMP commands to itself. If we need something internally, we add it > >> to a suitable internal interface. There's no need to add it to the > >> external interface just for that. > >> > >> QOM does not work that way. The internal and the external object > >> configuration interface is one and the same. So, if we add a property > >> for internal use, we can't *not* add it to the external interface. > >> > >> This has led to an external interface that is frickin' huge: I count > >> ~1000 device types with ~16000 properties in qemu-system-aarch64 alone. > >> The vast majority is undocumented. > >> > >> Time and again we've found ourselves unsure whether certain properties > >> have external uses, or are even meant for external use. > >> > >> We have been unable / unwilling to isolate the external interface from > >> internal detail. This is madness. > >> > >> As long as we persist in this madness, a sane, properly documented > >> external interface will remain impossible. > >> > >> Do we care? If yes, we should discuss how to isolate external and > >> internal interfaces. > >> > >> This series attempts to create a bit of infrastructure for such > >> isolation: means to mark properties as internal. Is it the right > >> infrastructure? Is it enough to be a useful step? Maybe not, but then > >> I'd like to hear better ideas. > > > > For -object / object_add we introduced formal QAPI modelling of > > all Object subclasses which implement the UserCreatable interface. > > IIUC, that gives us the desired separation between internal and > > external views, as only properties declared in qapi/qom.json are > > publically settable. > > Correct. Kevin Wolf's work. > > > This work did not apply to the Device classes because the historical > > baggage with qdev being grafted onto qom, means we don't have that > > working via the UserCreatable inteface or -object/object_add. > > > > Can we bring Device into the same world though ? > > Kevin Wolf took a stab at it. I had a hard time understanding it back > then. Various pennies finally dropped when he patiently explained it to > me in person. I disliked certain aspects of its design, and wanted to > explore a bit more. Never found the time. Perhaps we should just take > it despite my design misgivings.
Yep, I vaguely recall that, but never had a chance to look at it at the time. > > > Adding 1000 device types to QAPI is a huge job, so it would need to > > be a long incremental job, unless perhaps we auto-generate QAPI > > descriptions for everything that already exists ? > > Interesting idea. > > QAPI is declarative: types and their properties are declared in a > schema. > > QOM is imperative: we execute C code to create types and their > properties. > > Extracting a QAPI schema from the C code is impossible in the completely > general case (halting problem), and merely impractical (I believe) in > the special cases we have. > > We could start with QOM introspection instead: qom-list-types and > qom-list-properties. These are only mostly complete, but should be good > enough. > > Mapping QOM types to QAPI types would involve guesswork, because QOM > doesn't have a type system, it has strings and bailing wire. > > Schema documentation would be placeholders at best. We could try to > extract documentation from -device T,help. Most properties have nothing > there, and the remainder likely needs to be rewritten completely to be > fit for purpose. Yep, in case it wasn't obvious, when I said "auto-generate QAPI descriptions", I meant generate the skeleton of the QAPI json structure with whatever partial data we have, and a human would then have to fill in the blanks to bring it upto par with QAPI expectations. The docs descriptions do seem like a big part of the manual work that would be involved here. Perhaps another oddity is the distinction between devices which can actually be created with -device, vs devices that are only internally created but can none the less be set via -global. I presume we would need to expose all devices in QAPI, regardless of being user creatable or not. > > > More generally anything we can do to bring qdev & qom closer together > > feels desirable. I dream of a future where -device/device_add are > > obsolete.... > > That would be lovely. > With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|
