Re: [PATCH v2 00/21] qom: introduce property flags to track external user input

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Mar 16, 2026 at 04:43:54PM +0100, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> 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
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