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Re: sysbus_create_simple Vs qdev_create
From: |
Paolo Bonzini |
Subject: |
Re: sysbus_create_simple Vs qdev_create |
Date: |
Tue, 28 Jul 2020 19:38:27 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 |
On 28/07/20 09:19, Markus Armbruster wrote:
>> the composition tree generally mirrors things that are born and die
>> at the same time, and creating children is generally reserved to the
>> object itself.
>
> Yes. Notable exceptions: containers /machine/peripheral,
> /machine/peripheral-anon, /machine/unattached.
And /objects too. Apart from /machine/unattached, all these dynamic
objects are created by the monitor or the command line.
>> Children are usually embedded directly in a struct, for
>> example.
>
> We sometimes use object_new() + object_property_add_child() instead.
> Extra indirection. I guess we'd be better off without the extra
> indirection most of the time. Implementation detail.
>
> We sometimes use object_new() without object_property_add_child(), and
> have qdev_realize() put the device in the /machine/unattached orphanage.
> Meh. I guess the orphanage feature exists to make conversion to QOM
> slightly easier. Could we ban its use for new boards at least?
Banning perhaps is too strong, but yes /machine/unattached is an
anti-pattern.
>> 3) accessing the QOM graph is slow (it requires hash table lookups,
>> string comparisons and all that), so the pointers that cache the
>> parent-child links are needed for use in hot paths.
>
> True, but only because QOM's design opts for generality, efficiency be
> damned :)
Remember that QOM's essential feature is the visitors: unlike GObject,
QOM is not targeted at programming languages but rather at CLI and RPC.
> I never quite understood why we need to build properties at run time.
I think it was (for once) Anthony reasoning that good is better than
perfect. You do need run-time properties for /machine/unattached,
/machine/peripheral, etc., so he decided to only have run-time
properties. Introspection wasn't considered a primary design goal.
Also, even though child properties are quite often the same for all
objects after initialization (and possibly realization) is complete,
this does not cover in two cases:
1) before the corresponding objects are created---so static child
properties would only be possible if creation of all children is moved
to instance_init, which is guaranteed not to fail.
2) there are cases in which a property (e.g. an int) governs how many of
those children exist, so you cannot create all children in instance_init.
Paolo
> It's makes reasoning about properties harder, and introspection brittle.
- Re: sysbus_create_simple Vs qdev_create, (continued)
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/17
- Re: sysbus_create_simple Vs qdev_create, Eduardo Habkost, 2020/07/17
- Re: sysbus_create_simple Vs qdev_create, Daniel P . Berrangé, 2020/07/17
- Re: sysbus_create_simple Vs qdev_create, Peter Maydell, 2020/07/17
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/20
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/20
- Re: sysbus_create_simple Vs qdev_create, Eduardo Habkost, 2020/07/20
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/21
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/27
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/28
- Re: sysbus_create_simple Vs qdev_create,
Paolo Bonzini <=
- Re: sysbus_create_simple Vs qdev_create, Eduardo Habkost, 2020/07/28
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/29
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/29
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/29
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/30
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/30
- Re: sysbus_create_simple Vs qdev_create, Markus Armbruster, 2020/07/30
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/30
- Re: sysbus_create_simple Vs qdev_create, Eduardo Habkost, 2020/07/29
- Re: sysbus_create_simple Vs qdev_create, Paolo Bonzini, 2020/07/29