Hi
On Tue, Jan 21, 2020 at 7:01 PM Markus Armbruster <arm...@redhat.com> wrote:
>
> Daniel P. Berrangé <berra...@redhat.com> writes:
>
> > On Tue, Jan 21, 2020 at 02:36:17PM +0100, Markus Armbruster wrote:
> >> Marc-André Lureau <marcandre.lur...@gmail.com> writes:
> >>
> >> > Hi
> >> >
> >> > On Tue, Jan 21, 2020 at 3:32 PM Stefan Hajnoczi <stefa...@gmail.com>
> >> > wrote:
> >> >>
> >> >> On Tue, Jan 21, 2020 at 06:42:47AM +0100, Markus Armbruster wrote:
> >> >> > Stefan Hajnoczi <stefa...@gmail.com> writes:
> >> >> >
> >> >> > > On Wed, Jan 15, 2020 at 01:15:17PM +0100, Markus Armbruster wrote:
> >> >> > >> Christophe de Dinechin <dinec...@redhat.com> writes:
> >> >> > >> >> On 15 Jan 2020, at 10:20, Markus Armbruster <arm...@redhat.com>
> >> >> > >> >> wrote:
> >> >> > >> * qemuMonitorJSONSetIOThread() uses it to control iothread's
> >> >> > >> properties
> >> >> > >> poll-max-ns, poll-grow, poll-shrink. Their use with -object is
> >> >> > >> documented (in qemu-options.hx), their use with qom-set is not.
> >> >> > >
> >> >> > > I'm happy to use a different interface.
> >> >> > >
> >> >> > > Writing a boilerplate "iothread-set-poll-params" QMP command in C
> >> >> > > would
> >> >> > > be a step backwards.
> >> >> >
> >> >> > No argument.
> >> >> >
> >> >> > > Maybe the QAPI code generator could map something like this:
> >> >> > >
> >> >> > > { 'command': 'iothread-set-poll-params',
> >> >> > > 'data': {
> >> >> > > 'id': 'str',
> >> >> > > '*max-ns': 'uint64',
> >> >> > > '*grow': 'uint64',
> >> >> > > '*shrink': 'uint64'
> >> >> > > },
> >> >> > > 'map-to-qom-set': 'IOThread'
> >> >> > > }
> >> >> > >
> >> >> > > And turn it into QOM accessors on the IOThread object.
> >> >> >
> >> >> > I think a generic "set this configuration to that value" command is
> >> >> > just
> >> >> > fine. qom-set fails on several counts, though:
> >> >> >
> >> >> > * Tolerable: qom-set is not actually generic, it applies only to QOM.
> >> >> >
> >> >> > * qom-set lets you set tons of stuff that is not meant to be changed
> >> >> > at
> >> >> > run time. If it breaks your guest, you get to keep the pieces.
> >> >> >
> >> >> > * There is virtually no documentation on what can be set to what
> >> >> > values,
> >> >> > and their semantics.
> >> >> >
> >> >> > In its current state, QOM is a user interface superfund site.
> >> >>
> >> >> Thoughts about a solution:
> >> >>
> >> >> Static QOM properties should be declared via QAPI instead of
> >> >> imperatively via QOM APIs. That way they are introspectable and type
> >> >> information is present in the schema.
> >> >>
> >> >> The QAPI code generator could emit a function that is callable from
> >> >> .class_init(). This eliminates the need to manually call
> >> >> object_class_property_add().
> >>
> >> We need to make up our minds what exactly we want generated. Then we
> >> can design the QAPI language, and code up the generator.
> >>
> >> Skeleton QOM type, to help with the discussion:
> >>
> >> #define TYPE_FOO "foo"
> >>
> >> #define FOO(obj) OBJECT_CHECK(Foo, (obj), TYPE_FOO)
> >> #define FOO_CLASS(klass) \
> >> OBJECT_CLASS_CHECK(FooClass, (klass), TYPE_FOO)
> >> #define FOO_GET_CLASS(obj) \
> >> OBJECT_GET_CLASS(FooClass, (obj), TYPE_FOO)
> >>
> >> typedef FooClass {
> >> ParentClass parent_class;
> >> ... // hand-written per-class state
> >> }
> >>
> >> struct Chardev {
> >> ParentObject parent_obj;
> >> ... // hand-written instance (per-object) state
> >> };
> >>
> >> static const TypeInfo char_type_info = {
> >> .name = TYPE_FOO,
> >> .parent = TYPE_OBJECT,
> >> .instance_size = sizeof(Foo),
> >> .instance_init = ..., // methods to initialize
> >> .instance_post_init = ..., // and finalize instances,
> >> .instance_finalize = ..., // all optional
> >> .abstract = ..., // true or false (d'oh)
> >> .class_size = sizeof(FooClass),
> >> .class_init = ..., // methods to initialize
> >> .class_base_init = ..., // classes, optional
> >> .class_data = ..., // extra argument for them
> >> .interfaces = ...
> >> };
> >>
> >> There's substantial boilerplate, with plenty of hand-written code in the
> >> gaps. What of the boilerplate do we plan to generate? How do we plan
> >> to fill the gaps, if any?
> >
> > FWIW, even without a QOM generator, we can do waaaaaaay better on the
> > amount of boilerplate needed for QOM without very much work. It just
> > needs a few convenience macros writing.
> >
> > QOM is not GObject, but is heavily inspired by it and so looking at
> > GObject gives us a design pattern we can aim to match in terms of
> > amount of boilerplate.
> >
> > What we do manually with TypeInfo struct there has essentially always
> > been done by a 1 line macro in GObject:
> >
> > G_DEFINE_TYPE(virIdentity, vir_identity, G_TYPE_OBJECT)
> >
> > If implementing interfaces, there's 1 extra line needed per interface
> > to associate them.
> >
> >
> > https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DEFINE-TYPE:CAPS
> >
> >
> > And what we do in the header file to add the 4 or more FOO_XXX macros,
> > and the class struct and the object struct has recently been turned
> > into a 2-liner:
> >
> > #define VIR_TYPE_IDENTITY vir_identity_get_type()
> > G_DECLARE_FINAL_TYPE(virIdentity, vir_identity, VIR, IDENTITY, GObject);
> >
> >
> > https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DECLARE-FINAL-TYPE:CAPS
> >
> > Or
> >
> > #define VIR_TYPE_IDENTITY vir_identity_get_type()
> > G_DECLARE_DERIVABLE_TYPE(virIdentity, vir_identity, VIR, IDENTITY,
> > GObject);
> >
> >
> > https://developer.gnome.org/gobject/stable/gobject-Type-Information.html#G-DECLARE-DERIVABLE-TYPE:CAPS
> >
> >
> > It would be nice to have a QOM code generator so that we can statically
> > declare properties & parent/child/interface relationships, but for an
> > immediate low cost win, better macros would be very useful IMHO.
>
> Volunteers?
>
Actually, we are not that far off from being able to use GObject
altogether (I hacked something like that to play with), but I
disgress...
So introducing GObject-like macros? sure!
There are plenty of refactoring to do. The problem when touching the
whole code-base, imho, is review time. It may take a couple of
hours/days to come up with a cocci/spatch, and make various patches
here and there. But it takes often weeks and a lot of constant push to
various folks to get all the reviews (as seens by the qdev prop-ptr
series earlier for example). How can we better address whole code-base
changes?
--
Marc-André Lureau
