Re: [Qemu-devel] [PATCH 23/35] acpi:piix4: make plug/unlug callbacks generic

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 07, 2014 at 04:22:06PM +0200, Igor Mammedov wrote:
> On Mon, 7 Apr 2014 16:25:30 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Mon, Apr 07, 2014 at 03:12:11PM +0200, Igor Mammedov wrote:
> > > On Mon, 7 Apr 2014 15:07:15 +0300
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > 
> > > > On Mon, Apr 07, 2014 at 02:00:37PM +0200, Igor Mammedov wrote:
> > > > > On Mon, 7 Apr 2014 14:32:41 +0300
> > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > > > 
> > > > > > On Fri, Apr 04, 2014 at 03:36:48PM +0200, Igor Mammedov wrote:
> > > > > > > ... and report error if plugged in device is not supported.
> > > > > > > Later generic callbacks will be used by memory hotplug.
> > > > > > > 
> > > > > > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > > > > 
> > > > > > 
> > > > > > OK in that case, how about teaching all hotplug callbacks about this?
> > > > > > 
> > > > > > There are two ATM:
> > > > > > shpc_device_hotplug_cb
> > > > > > pcie_cap_slot_hotplug_cb
> > > > > > 
> > > > > > Teach them both to fail gracefully if they get
> > > > > > an object that is not a pci device.
> > > > > > 
> > > > > > Afterwards, simply iterate over all objects of type
> > > > > > TYPE_HOTPLUG_HANDLER
> > > > > > and look for one that will accept your object.
> > > > > Then you would never know if any hotplug handler has actually
> > > > > handled event.
> > > > 
> > > > Why not? Check the error.
> > > > If no one accepts your object, return error to user.
> > > > 
> > > > > I think hotplug handler should return error if unsupported
> > > > > device passed in rather than ignore it. It makes catching
> > > > > wiring errors easier.
> > > > 
> > > > Absolutely.
> > > > 
> > > > > Dropping error so that we could not care which hotplug handler
> > > > > should be notified, looks like a wrong direction and makes
> > > > > system more fragile.
> > > > 
> > > > That's not what I was suggesting.
> > > > 
> > > > > It shouldn't be up to consumer to determine that event should
> > > > > be routed to it, but rather by external routing that knows
> > > > > what and when should be notified.
> > > > 
> > > > Yes. So
> > > > 
> > > > 	for each hotplug handler (&err)
> > > > 		handler->plug(device, &err)
> > > > 		if (!err)
> > > > 			break;
> > > here it would break on the first handler that doesn't support device
> > > and tells so to caller.
> > 
> > This is pseudo-code, I really mean !err == no error reported.
>  * what if all handlers returned error, err might not reflect the actual
>    error returned from handler that cares about device?

We can use a special error code to mean "hotplug not supported".

>  * What if there would be more handlers that could or should handle event
>    for device?

That's actually very useful. We could scan top to bottom
so e.g. acpi can intercept bridges.

>  * What if only some of compatible handler should handle event

Each one can check whether it's applicable.

>  * What if handler should conditionally handle event and only 
>    caller knows about condition and have access to them?

Doesn't sound possible.

>  * What about ordering in which handlers should be called?
> 
> Broadcast would be useful if it were impossible to know in advance
> which hotplug handler to use. Is there use case for this?

ACPI vs SHPC would be an example. For that one, we need to order
them top to bottom.

> > 
> > > And there isn't any routing here, it just blindly broadcast to every
> > > handler, regardless whether it's right or not.
> > 
> > Yes - handlers verify what they can support.
> Sure handlers could verify, there is no harm in extra checking, but
> handlers should not decide what to handle. That's what I'm against from.
> It should be upto caller to decide if handler is the right one and call it.
> There shouldn't be a chance for random/wrong handler to be called.
> 
> > 
> > > If broadcast should be ever done than it probably should be a part of
> > > DEVICE class and part of
> > >   [Qemu-devel] [PATCH 08/35] qdev: hotplug for buss-less devices
> > > patch and be generic to all devices.
> > 
> > Not sure what's suggested here.
> above looks like generic code that should be part of Device.realize()
> and should replace 08/35 patch if it's deemed as acceptable.
> I think implementing design like that requires much more though
> if viable and out of scope of this series.
> 
> > 
> > > 
> > > Andreas,
> > >   since you care about QDEV
> > >   do you have an opinion on ^^^ discussion?
> > > 
> > > > 
> > > > 
> > > > 	if (err)
> > > > 		hotplug failed - destroy device
> > > > 
> > > > 
> > > > > > 
> > > > > > 
> > > > > > > ---
> > > > > > >  hw/acpi/piix4.c | 31 ++++++++++++++++++++++---------
> > > > > > >  1 file changed, 22 insertions(+), 9 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/hw/acpi/piix4.c b/hw/acpi/piix4.c
> > > > > > > index 67dc075..4341f82 100644
> > > > > > > --- a/hw/acpi/piix4.c
> > > > > > > +++ b/hw/acpi/piix4.c
> > > > > > > @@ -310,19 +310,32 @@ static void piix4_pm_powerdown_req(Notifier *n, void *opaque)
> > > > > > >      acpi_pm1_evt_power_down(&s->ar);
> > > > > > >  }
> > > > > > >  
> > > > > > > -static void piix4_pci_device_plug_cb(HotplugHandler *hotplug_dev,
> > > > > > > -                                     DeviceState *dev, Error **errp)
> > > > > > > +static void piix4_device_plug_cb(HotplugHandler *hotplug_dev,
> > > > > > > +                                 DeviceState *dev, Error **errp)
> > > > > > >  {
> > > > > > >      PIIX4PMState *s = PIIX4_PM(hotplug_dev);
> > > > > > > -    acpi_pcihp_device_plug_cb(&s->ar, s->irq, &s->acpi_pci_hotplug, dev, errp);
> > > > > > > +
> > > > > > > +    if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_DEVICE)) {
> > > > > > > +        acpi_pcihp_device_plug_cb(&s->ar, s->irq, &s->acpi_pci_hotplug, dev,
> > > > > > > +                                  errp);
> > > > > > > +    } else {
> > > > > > > +        error_setg(errp, "acpi: device plug request for not supported device"
> > > > > > > +                   " type: %s", object_get_typename(OBJECT(dev)));
> > > > > > > +    }
> > > > > > >  }
> > > > > > >  
> > > > > > > -static void piix4_pci_device_unplug_cb(HotplugHandler *hotplug_dev,
> > > > > > > -                                       DeviceState *dev, Error **errp)
> > > > > > > +static void piix4_device_unplug_cb(HotplugHandler *hotplug_dev,
> > > > > > > +                                   DeviceState *dev, Error **errp)
> > > > > > >  {
> > > > > > >      PIIX4PMState *s = PIIX4_PM(hotplug_dev);
> > > > > > > -    acpi_pcihp_device_unplug_cb(&s->ar, s->irq, &s->acpi_pci_hotplug, dev,
> > > > > > > -                                errp);
> > > > > > > +
> > > > > > > +    if (object_dynamic_cast(OBJECT(dev), TYPE_PCI_DEVICE)) {
> > > > > > > +        acpi_pcihp_device_unplug_cb(&s->ar, s->irq, &s->acpi_pci_hotplug, dev,
> > > > > > > +                                    errp);
> > > > > > > +    } else {
> > > > > > > +        error_setg(errp, "acpi: device unplug request for not supported device"
> > > > > > > +                   " type: %s", object_get_typename(OBJECT(dev)));
> > > > > > > +    }
> > > > > > >  }
> > > > > > >  
> > > > > > >  static void piix4_update_bus_hotplug(PCIBus *pci_bus, void *opaque)
> > > > > > > @@ -553,8 +566,8 @@ static void piix4_pm_class_init(ObjectClass *klass, void *data)
> > > > > > >       */
> > > > > > >      dc->cannot_instantiate_with_device_add_yet = true;
> > > > > > >      dc->hotpluggable = false;
> > > > > > > -    hc->plug = piix4_pci_device_plug_cb;
> > > > > > > -    hc->unplug = piix4_pci_device_unplug_cb;
> > > > > > > +    hc->plug = piix4_device_plug_cb;
> > > > > > > +    hc->unplug = piix4_device_unplug_cb;
> > > > > > >  }
> > > > > > >  
> > > > > > >  static const TypeInfo piix4_pm_info = {
> > > > > > > -- 
> > > > > > > 1.9.0
> >