Re: [RFC 2/7] qom: deprecated embedding object structs within other objects

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

On Tue, Jun 16, 2026 at 05:15:22PM +0100, Peter Maydell wrote:
> On Tue, 16 Jun 2026 at 16:56, Daniel P. Berrangé <berrange@redhat.com> wrote:
> >
> > The QOM APIs currently allow objects to be either allocated directly
> > on the heap, or statically embedded inside the struct of another object.
> >
> > For the latter QOM has logic to avoid calling 'free' on the object when
> > finalizers complete, however, this is not sufficient to make the
> > practice safe.
> >
> > Users of QOM expect that if they call "object_ref" to acquire their own
> > reference, then object will never be freed as long as they hold it.
> >
> > This expectation is broken when an instance is embedded, as the "owner"
> > object's may be finalized, which frees the memory that is storing the
> > embedded QOM instance, even if its ref-count is still live.
> >
> > Worse still is that a user of a QOM object cannot easily tell if the
> > instance they're using is embedded or directly heap allocated.
> >
> > Mark the APIs for embedding objects as deprecated as the first step
> > towards removal of this flawed design concept. All objects must now
> > be directly heap allocated going forward, and existing usage must be
> > incrementally converted.
> 
> I think if you mark an API that we call in 800+ places and 150+ files
> as deprecated, mostly what will happen is nobody ever turns on the
> --enable-deprecated option because the output will be full of noise...

That is a challenge. The baseline is that developers have no visibility
of use of deprecated APIs at all, and we barely even document them.

My thought is that even if --enable-deprecated is rarely enabled, at
least having QEMU_DEPRECATED visible in the header files is a red flag
for developers to look elsewhere for their solution.

With regards,
Daniel
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