Re: [Qemu-devel] [PATCH 07/23] target-i386: convert cpuid features into properties

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 4 Oct 2012 09:43:41 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Oct 04, 2012 at 08:53:22AM +0200, Igor Mammedov wrote:
> > On Wed, 3 Oct 2012 13:54:34 -0300
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > On Wed, Oct 03, 2012 at 06:24:11PM +0200, Igor Mammedov wrote:
> > > > On Wed, 03 Oct 2012 17:20:46 +0200
> > > > Paolo Bonzini <pbonzini@redhat.com> wrote:
> > > > 
> > > > > Il 03/10/2012 17:03, Eduardo Habkost ha scritto:
> > > > > > On Tue, Oct 02, 2012 at 05:38:45PM -0300, Eduardo Habkost wrote:
> > > > > >> (Now replying on the right thread, to keep the discussion in the
> > > > > >> right place. I don't know how I ended up replying to a
> > > > > >> pre-historic version of the patch, sorry.)
> > > > > >>
> > > > > >> On Tue, Oct 02, 2012 at 05:36:59PM +0200, Igor Mammedov wrote:
> > > > > >> [...]
> > > > > >>> @@ -1938,6 +2043,12 @@ static void x86_cpu_initfn(Object *obj)
> > > > > >>>      object_property_add(obj, "tsc-frequency", "int",
> > > > > >>>                          x86_cpuid_get_tsc_freq,
> > > > > >>>                          x86_cpuid_set_tsc_freq, NULL, NULL,
> > > > > >>> NULL);
> > > > > >>> +    x86_register_cpuid_properties(obj, feature_name);
> > > > > >>> +    x86_register_cpuid_properties(obj, ext_feature_name);
> > > > > >>> +    x86_register_cpuid_properties(obj, ext2_feature_name);
> > > > > >>> +    x86_register_cpuid_properties(obj, ext3_feature_name);
> > > > > >>> +    x86_register_cpuid_properties(obj, kvm_feature_name);
> > > > > >>> +    x86_register_cpuid_properties(obj, svm_feature_name);
> > > > > >>
> > > > > >> Stupid question about qdev:
> > > > > >>
> > > > > >> - qdev_prop_set_globals() is called from device_initfn()
> > > > > >> - device_initfn() is called before the child class
> > > > > >> instance_init() function (x86_cpu_initfn())
> > > > > >> - So, qdev_prop_set_globals() gets called before the CPU class
> > > > > >>   properties are registered.
> > > > > >>
> > > > > >> So this would defeat the whole point of all the work we're doing,
> > > > > >> that is to allow compatibility bits to be set as machine-type
> > > > > >> global properties. But I don't know what's the right solution
> > > > > >> here.
> > > > > >>
> > > > > >> Should the qdev_prop_set_globals() call be moved to qdev_init()
> > > > > >> instead? Should the CPU properties be registered somewhere else?
> > > > > 
> > > > > Properties should be registered (for all objects, not just CPUs) in
> > > > > the instance_init function.  This is device_initfn.
> > > > > 
> > > > > I would add an instance_postinit function that is called at the end
> > > > > of object_initialize_with_type, that is after instance_init, and in
> > > > > the opposite order (i.e. from the leaf to the root).
> > > > 
> > > > You've meant something like that?
> > > > 
> > > 
> > > That's almost exactly the same code I wrote here. :-)
> > > 
> > > The only difference is that I added post_init to the struct Object
> > > documentation comments, and added a unit test. The unit test required
> > > the qdev-core/qdev split, so we could compile it without bringing too
> > > many dependencies. I will submit it soon.
> > > 
> > After irc discussion, Anthony suggested to use static properties instead
> > of dynamic ones that we use now. 
> > 
> > But  qdev_prop_set_globals() in device_initfn() is still causes problems
> > even with static properties.
> > 
> > For x86 CPU classes we were going dynamically generate CPU classes and
> > store pointer to appropriate cpudef from builtin_x86_defs in class field
> > for each CPU class and then init default feature words values from this
> > field int x86_cpu_initfn().
> > 
> > However with qdev_prop_set_globals() in device_initfn() that is called
> > before x86_cpu_initfn() it won't work because defaults in
> > x86_cpu_initfn() will overwrite whatever was set by
> > qdev_prop_set_globals().
> 
> We can set the default values on class_init, instead. The class_init
> function for each CPU model can get the x86_def_t struct as the data
> pointer.
> 
> I still think that the interface to build the DeviceClass.props array on
> class_init is really painful to use, but it's still doable.

You mean dynamic building of DeviceClass.props arrays for each CPU sub-class?

> 
> > 
> > IMHO from general POV it's not correct to set properties before object is
> > completely created.
> 
> If I understood it correctly, the point of all this is to allow
> properties (and their defaults) to be introspected by just looking at
> the class, without having to create any object.
> 
> IMO the problem is that we don't have any decent mechanism to implement
> machine-type compatibility behavior that doesn't involve having to deal
> with the strict rules of global properties. All this work is blocking us
> from implementing many necessary bug fixes.
> 
> It's OK if the long-term goal is to move the code to this generic,
> flexible, fully-instropectable, complex framework, but we need to have a
> way to implement bug fixes without waiting for all this work to be
> finished.
> 
> 
> > But Anthony wants to keep qdev_prop_set_globals() qdev only thing, so
> > could we move it from device_initfn() to qdev_init() or some other place
> > then?
>