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Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc.
From: |
Alistair Francis |
Subject: |
Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc. |
Date: |
Tue, 19 May 2020 21:51:27 -0700 |
On Tue, May 19, 2020 at 9:26 PM Markus Armbruster <address@hidden> wrote:
>
> Alistair Francis <address@hidden> writes:
>
> > On Tue, May 19, 2020 at 8:11 AM Markus Armbruster <address@hidden> wrote:
> >>
> >> We commonly plug devices into their bus right when we create them,
> >> like this:
> >>
> >> dev = qdev_create(bus, type_name);
> >>
> >> Note that @dev is a weak reference. The reference from @bus to @dev
> >> is the only strong one.
> >>
> >> We realize at some later time, either with
> >>
> >> object_property_set_bool(OBJECT(dev), true, "realized", errp);
> >>
> >> or its convenience wrapper
> >>
> >> qdev_init_nofail(dev);
> >>
> >> If @dev still has no QOM parent then, realizing makes the
> >> /machine/unattached/ orphanage its QOM parent.
> >>
> >> Note that the device returned by qdev_create() is plugged into a bus,
> >> but doesn't have a QOM parent, yet. Until it acquires one,
> >> unrealizing the bus will hang in bus_unparent():
> >>
> >> while ((kid = QTAILQ_FIRST(&bus->children)) != NULL) {
> >> DeviceState *dev = kid->child;
> >> object_unparent(OBJECT(dev));
> >> }
> >>
> >> object_unparent() does nothing when its argument has no QOM parent,
> >> and the loop spins forever.
> >>
> >> Device state "no QOM parent, but plugged into bus" is dangerous.
> >>
> >> Paolo suggested to delay plugging into the bus until realize. We need
> >> to plug into the parent bus before we call the device's realize
> >> method, in case it uses the parent bus. So the dangerous state still
> >> exists, but only within realization, where we can manage it safely.
> >>
> >> This commit creates infrastructure to do this:
> >>
> >> dev = qdev_new(type_name);
> >> ...
> >> qdev_realize_and_unref(dev, bus, errp)
> >>
> >> Note that @dev becomes a strong reference here.
> >> qdev_realize_and_unref() drops it. There is also plain
> >> qdev_realize(), which doesn't drop it.
> >>
> >> The remainder of this series will convert all users to this new
> >> interface.
> >>
> >> Cc: Michael S. Tsirkin <address@hidden>
> >> Cc: Marcel Apfelbaum <address@hidden>
> >> Cc: Alistair Francis <address@hidden>
> >> Cc: Gerd Hoffmann <address@hidden>
> >> Cc: Mark Cave-Ayland <address@hidden>
> >> Cc: David Gibson <address@hidden>
> >> Signed-off-by: Markus Armbruster <address@hidden>
> >> ---
> >> include/hw/qdev-core.h | 11 ++++-
> >> hw/core/bus.c | 14 +++++++
> >> hw/core/qdev.c | 94 ++++++++++++++++++++++++++++++++++++++++++
> >> 3 files changed, 118 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> >> index b870b27966..fba29308f7 100644
> >> --- a/include/hw/qdev-core.h
> >> +++ b/include/hw/qdev-core.h
> >> @@ -57,7 +57,7 @@ typedef void (*BusUnrealize)(BusState *bus);
> >> * After successful realization, setting static properties will fail.
> >> *
> >> * As an interim step, the #DeviceState:realized property can also be
> >> - * set with qdev_init_nofail().
> >> + * set with qdev_realize() or qdev_init_nofail().
> >> * In the future, devices will propagate this state change to their
> >> children
> >> * and along busses they expose.
> >> * The point in time will be deferred to machine creation, so that values
> >> @@ -322,7 +322,13 @@ compat_props_add(GPtrArray *arr,
> >>
> >> DeviceState *qdev_create(BusState *bus, const char *name);
> >> DeviceState *qdev_try_create(BusState *bus, const char *name);
> >> +DeviceState *qdev_new(const char *name);
> >> +DeviceState *qdev_try_new(const char *name);
> >> void qdev_init_nofail(DeviceState *dev);
> >> +bool qdev_realize(DeviceState *dev, BusState *bus, Error **errp);
> >> +bool qdev_realize_and_unref(DeviceState *dev, BusState *bus, Error
> >> **errp);
> >> +void qdev_unrealize(DeviceState *dev);
> >> +
> >> void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
> >> int required_for_version);
> >> HotplugHandler *qdev_get_bus_hotplug_handler(DeviceState *dev);
> >> @@ -411,6 +417,9 @@ typedef int (qdev_walkerfn)(DeviceState *dev, void
> >> *opaque);
> >> void qbus_create_inplace(void *bus, size_t size, const char *typename,
> >> DeviceState *parent, const char *name);
> >> BusState *qbus_create(const char *typename, DeviceState *parent, const
> >> char *name);
> >> +bool qbus_realize(BusState *bus, Error **errp);
> >> +void qbus_unrealize(BusState *bus);
> >> +
> >> /* Returns > 0 if either devfn or busfn skip walk somewhere in cursion,
> >> * < 0 if either devfn or busfn terminate walk somewhere in
> >> cursion,
> >> * 0 otherwise. */
> >> diff --git a/hw/core/bus.c b/hw/core/bus.c
> >> index 08c5eab24a..bf622604a3 100644
> >> --- a/hw/core/bus.c
> >> +++ b/hw/core/bus.c
> >> @@ -169,6 +169,20 @@ BusState *qbus_create(const char *typename,
> >> DeviceState *parent, const char *nam
> >> return bus;
> >> }
> >>
> >> +bool qbus_realize(BusState *bus, Error **errp)
> >> +{
> >> + Error *err = NULL;
> >> +
> >> + object_property_set_bool(OBJECT(bus), true, "realized", &err);
> >> + error_propagate(errp, err);
> >> + return !err;
> >> +}
> >> +
> >> +void qbus_unrealize(BusState *bus)
> >> +{
> >> + object_property_set_bool(OBJECT(bus), true, "realized", &error_abort);
> >
> > Not false?
> >
> > Alistair
>
> Reasons it's &error_abort:
I meant why is this not setting the bool to false instead of true?
>
> 1. PATCH 06 and 07 transform variations of
>
> object_property_set_bool(..., false, "realized", &error_abort);
>
> to
>
> qdev_unrealize(...);
>
> No untransformed unrealization remain. Thus, we always abort on
> unrealization error before this series.
>
> 2. If unrealize could fail, we'd be in deep trouble. Recent commit
> b69c3c21a5 "qdev: Unrealize must not fail" explains:
>
> Devices may have component devices and buses.
>
> Device realization may fail. Realization is recursive: a device's
> realize() method realizes its components, and device_set_realized()
> realizes its buses (which should in turn realize the devices on that
> bus, except bus_set_realized() doesn't implement that, yet).
>
> When realization of a component or bus fails, we need to roll back:
> unrealize everything we realized so far. If any of these unrealizes
> failed, the device would be left in an inconsistent state. Must not
> happen.
Makes sense. Maybe worth putting this in a comment here?
>
> device_set_realized() lets it happen: it ignores errors in the roll
> back code starting at label child_realize_fail.
>
> Since realization is recursive, unrealization must be recursive, too.
> But how could a partly failed unrealize be rolled back? We'd have to
> re-realize, which can fail. This design is fundamentally broken.
>
> device_set_realized() does not roll back at all. Instead, it keeps
> unrealizing, ignoring further errors.
>
> It can screw up even for a device with no buses: if the lone
> dc->unrealize() fails, it still unregisters vmstate, and calls
> listeners' unrealize() callback.
>
> bus_set_realized() does not roll back either. Instead, it stops
> unrealizing.
>
> Fortunately, no unrealize method can fail, as we'll see below.
>
> Clearer now?
Clear on the error_abort.
>
> With any luck, people will use the simpler qdev_unrealize() and
> qbus_unrealize(), which is the form that doesn't let them get the error
> handling wrong. I like it when interfaces make misuse hard :)
Sounds good :)
Alistair
>
- [PATCH 34/55] qom: Less verbose object_initialize_child(), (continued)
- [PATCH 34/55] qom: Less verbose object_initialize_child(), Markus Armbruster, 2020/05/19
- [PATCH 39/55] sysbus: Tidy up sysbus_init_child_obj()'s @childsize arg, part 1, Markus Armbruster, 2020/05/19
- [PATCH 40/55] hw/arm/armsse: Pass correct child size to sysbus_init_child_obj(), Markus Armbruster, 2020/05/19
- [PATCH 49/55] sysbus: sysbus_init_child_obj() is now unused, drop, Markus Armbruster, 2020/05/19
- [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/19
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Alistair Francis, 2020/05/19
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc.,
Alistair Francis <=
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Paolo Bonzini, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Paolo Bonzini, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Paolo Bonzini, 2020/05/20
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/25
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Paolo Bonzini, 2020/05/25
- Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Markus Armbruster, 2020/05/29
Re: [PATCH 03/55] qdev: New qdev_new(), qdev_realize(), etc., Gerd Hoffmann, 2020/05/20