Re: [Qemu-devel] [PATCH v2 2/2] vl: Partial support for non-scalar properties with -object

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

On Fri, Aug 11, 2017 at 12:47:44PM -0500, Eric Blake wrote:
> On 08/11/2017 11:05 AM, Markus Armbruster wrote:
> > We've wanted -object to support non-scalar properties for a while.
> > Dan Berrange tried in "[PATCH v4 00/10]Provide a QOM-based
> > authorization API".  Review led to the conclusion that we need to
> > replace rather than add to QemuOpts.  Initial work towards that goal
> > has been merged to provide -blockdev (commit 8746709), but there's
> > substantial work left, mostly due to an bewildering array of
> > compatibility problems.
> > 
> > Even if a full solution is still out of reach, we can have a partial
> > solution now: accept -object argument in JSON syntax.  This should
> > unblock development work that needs non-scalar properties with
> > -object.
> > 
> > The implementation is similar to -blockdev, except we use the new
> > infrastructure only for the new JSON case, and stick to QemuOpts for
> > the existing KEY=VALUE,... case, to sidestep compatibility problems.
> > 
> > If we did this for more options, we'd have to factor out common code.
> > But for one option, this will do.
> > 
> > Signed-off-by: Markus Armbruster <armbru@redhat.com>
> > ---
> >  qapi-schema.json | 14 +++++++++++---
> >  vl.c             | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 62 insertions(+), 3 deletions(-)
> > 
> >  static void object_create(bool (*type_predicate)(const char *))
> >  {
> > +    ObjectOptionsQueueEntry *e, *next;
> > +
> > +    QSIMPLEQ_FOREACH_SAFE(e, &oo_queue, entry, next) {
> > +        if (!type_predicate(e->oo->qom_type)) {
> > +            continue;
> > +        }
> > +
> > +        loc_push_restore(&e->loc);
> > +        qmp_object_add(e->oo->qom_type, e->oo->id,
> > +                       e->oo->has_props, e->oo->props, &error_fatal);
> > +        loc_pop(&e->loc);
> > +
> > +        QSIMPLEQ_REMOVE(&oo_queue, e, ObjectOptionsQueueEntry, entry);
> > +        qapi_free_ObjectOptions(e->oo);
> > +    }
> > +
> >      if (qemu_opts_foreach(qemu_find_opts("object"),
> 
> This handles all JSON forms prior to any QemuOpt forms (within the two
> priority levels), such that a command line using:
> 
>  -object type,id=1,oldstyle... -object '{'id':2, 'type':..., newstyle...}'
> 
> processes the arguments in a different order than
> 
>  -object type,id=1,oldstyle... -object type,id=2,oldstyle
> 
> But I don't see that as too bad (ideally, someone using the {} JSON
> style will use it consistently).

I don't really like such a constraint - the ordering of object
creation is already complex with some objets created at a different
point in startup to other objects. Adding yet another constraint
feels like it is painting ourselves into a corner wrt future changes.
In particular I think it is quite possible to use the dotted
form primarily, and only use JSON for the immediate scenario
where non-JSON form won't work - I expect that's how we would
use it in libvirt - I don't see libvirt changing 100% to JSON
based objects

Regards,
Daniel
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