Re: [Qemu-devel] [PATCH 1/2] ppc/xics: remove set_nr_irqs() handler from XICSStateClass

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Feb 13, 2017 at 03:09:16PM +0100, Cédric Le Goater wrote:
> Today, the ICS (Interrupt Controller Source) object is created and
> realized by the init and realize routines of the XICS object, but some
> of the parameters are only known at the machine level.
> 
> These parameters are passed from the sPAPR machine to the ICS object
> in a rather convoluted way using property handlers and a class handler
> of the XICS object. The number of irqs required to allocate the IRQ
> state objects in the ICS realize routine is one of them.
> 
> Let's simplify the process by creating the ICS object along with the
> XICS object at the machine level and link the ICS into the XICS list
> of ICSs at this level also. In the sPAPR machine, there is only a
> single ICS but that will change with the PowerNV machine.
> 
> Also, QOMify the creation of the objects and get rid of the
> superfluous code.
> 
> Signed-off-by: Cédric Le Goater <clg@kaod.org>

I like the basic idea here: while the ics and icp objects are pretty
straightforward, the "xics" object has always been a bit of a hack,
with logic that really belongs in the machine.

But.. I don't think the approach here really works.  Specifically..

[snip]
> -static XICSState *try_create_xics(const char *type, int nr_servers,
> -                                  int nr_irqs, Error **errp)
> -{
> -    Error *err = NULL;
> -    DeviceState *dev;
> +static XICSState *try_create_xics(const char *type, const char *type_ics,
> +                                  int nr_servers, int nr_irqs, Error **errp)
> +{
> +    Error *err = NULL, *local_err = NULL;
> +    XICSState *xics;
> +    ICSState *ics = NULL;
> +
> +    xics = XICS_COMMON(object_new(type));
> +    qdev_set_parent_bus(DEVICE(xics), sysbus_get_default());
> +    object_property_set_int(OBJECT(xics), nr_servers, "nr_servers", &err);
> +    object_property_set_bool(OBJECT(xics), true, "realized", &local_err);
> +    error_propagate(&err, local_err);
> +    if (err) {
> +        goto error;
> +    }
>  
> -    dev = qdev_create(NULL, type);
> -    qdev_prop_set_uint32(dev, "nr_servers", nr_servers);
> -    qdev_prop_set_uint32(dev, "nr_irqs", nr_irqs);
> -    object_property_set_bool(OBJECT(dev), true, "realized", &err);
> +    ics = ICS_SIMPLE(object_new(type_ics));
> +    object_property_add_child(OBJECT(xics), "ics", OBJECT(ics), NULL);
> +    object_property_set_int(OBJECT(ics), nr_irqs, "nr-irqs", &err);
> +    object_property_set_bool(OBJECT(ics), true, "realized", &local_err);
> +    error_propagate(&err, local_err);
>      if (err) {
> -        error_propagate(errp, err);
> -        object_unparent(OBJECT(dev));
> -        return NULL;
> +        goto error;
> +    }
> +
> +    ics->xics = xics;
> +    QLIST_INSERT_HEAD(&xics->ics, ics, list);

Poking into the ics and xics objects directly from the machine here
violates abstraction even worse than the existing xics device does.
In fact, avoiding that is basically why the xics device exists in the
first place.

I've thought about this a bit more, and I think I know how to solve
this better now.

I think that "xics" shouldn't be a concrete object at all, instead it
should be a QOM interface, implemented by the machine type.  Both ICS
and ICP would take a link property to find their xics.  The xics
interface would provide methods to return an ics object given an irq
number and to return an icp object given a server number. This gives
full control of the irq and server number spaces back to the machine
type, which is really where it belongs.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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