From: Markus Armbruster <armbru@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Philippe Mathieu-Daudé" <philmd@redhat.com>,
"Daniel P. Berrangé" <berrange@redhat.com>,
"Eduardo Habkost" <ehabkost@redhat.com>,
"Pratik Parvati" <pratikp@vayavyalabs.com>,
qemu-devel@nongnu.org
Subject: Re: sysbus_create_simple Vs qdev_create
Date: Thu, 30 Jul 2020 14:36:45 +0200 [thread overview]
Message-ID: <87zh7hxjqa.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <0d7a9407-1df6-0c9b-0695-2f438f0de129@redhat.com> (Paolo Bonzini's message of "Thu, 30 Jul 2020 13:09:58 +0200")
Paolo Bonzini <pbonzini@redhat.com> writes:
> On 30/07/20 12:03, Markus Armbruster wrote:
>> qdev C layer:
>>
>> frob->prop = 42;
>>
>> Least cognitive load.
>>
>> QOM has no C layer.
>
> Not really, a QOM object is totally free to do frob->prop = 42. And
> just like we didn't do that outside device implementation in qdev as our
> tithe to the Church of Information Hiding; the same applies to QOM.
I screwed up the part of my argument that actually has a hope to be
valid, let me try again.
With qdev, you can always do frob->prop = 42, because properties are
always backed by a struct member.
With QOM, properties are built around visitor-based getters and setters.
This means you can always do (but never actually would do) something
like
fortytwo = qnum_from_int(42);
v = qobject_input_visitor_new(fortytwo);
set_prop(OBJECT(frob), v, "prop", cookie, &err);
visit_free(v);
qobject_unref(fortytwo);
where set_prop() is the setter you passed to object_property_add(), and
@cookie is the opaque value you passed along with it.
*Maybe* set_prop() wraps around a simpler setter you can call directly,
or even a struct member you can set directy. QOM does not care.
And that's my point: QOM does not care for the C layer.
>> qdev property layer works even when @frob has incomplete type:
>>
>> qdev_prop_set_int32(DEVICE(frob), "prop", 42);
>>
>> This used to map property name to struct offset & copy the value.
>> Simple, stupid.
>>
>> Nowadays, it is the same as
>>
>> object_property_set_int(OBJECT(frob), "frob", 42, &error_abort);
>>
>> which first converts the int to a QObject, then uses a QObject input
>> visitor with a virtual walk to convert it back to int and store it in
>> @frob. It's quite a sight in the debugger.
>
> Yes, but thatt's just because we never bothered to create single-type
> visitors. For a good reason though: I don't think the extra QAPI code
> is worth (not even that much) nicer backtraces when we already have a
> QObject as a battle-tested variant type.
>
>> qdev "text" layer is really a QemuOpts layer (because that's what we had
>> back then). If we have prop=42 in a QemuOpts, it calls
>>
>> set_property("prop", "42", frob, &err);
>>
>> Nowadays, this is a thin wrapper around object_property_parse(),
>> basically
>>
>> object_property_parse(frob, "prop", 42, &err);
>>
>> Fine print: except set_property() does nothing for @prop "driver" and
>> "bus", which look just like properties in -device / device-add, but
>> aren't.
>
> Ugly indeed. They should be special cased up in the caller, probably,
> or use the long-discussed "remainder" feature of the QAPI schema.
qdev_device_add() is still stuck in the QemuOpts age.
>> object_property_parse() uses the string input visitor, which I loathe.
>
> Apart from the list syntax, the string input visitor is decent I think.
It's a death trap:
/*
* The string input visitor does not implement support for visiting
* QAPI structs, alternates, null, or arbitrary QTypes. Only flat lists
* of integers (except type "size") are supported.
*/
"Does not implement support for visiting" is polite language for
"crashes when you dare to visit".
>>>> I've long had the nagging feeling that if we had special-cased
>>>> containers, children and links, we could have made a QOM that was easier
>>>> to reason about, and much easier to integrate with a QAPI schema.
>>>
>>> That's at least plausible. But I have a nagging feeling that it would
>>> only cover 99% of what we're doing with QOM. :)
>>
>> The question is whether that 1% really should be done the way it is done
>> :)
>
> And that's a very fair question, but it implies non-trivial design work,
> so the smiley changes to a frown. :(
True!
next prev parent reply other threads:[~2020-07-30 12:37 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-14 16:09 sysbus_create_simple Vs qdev_create Pratik Parvati
2020-07-14 16:17 ` Pratik Parvati
2020-07-14 17:02 ` Philippe Mathieu-Daudé
2020-07-15 8:32 ` Markus Armbruster
2020-07-15 13:58 ` Pratik Parvati
2020-07-15 14:11 ` Peter Maydell
2020-07-15 14:37 ` Markus Armbruster
2020-07-16 22:21 ` Eduardo Habkost
2020-07-17 5:10 ` Markus Armbruster
2020-07-17 16:23 ` Eduardo Habkost
2020-07-17 16:30 ` Daniel P. Berrangé
2020-07-17 17:15 ` Peter Maydell
2020-07-20 7:39 ` Markus Armbruster
2020-07-20 7:38 ` Markus Armbruster
2020-07-20 15:59 ` Eduardo Habkost
2020-07-21 6:00 ` Markus Armbruster
2020-07-27 14:29 ` Paolo Bonzini
2020-07-28 7:19 ` Markus Armbruster
2020-07-28 17:38 ` Paolo Bonzini
2020-07-28 22:47 ` Eduardo Habkost
2020-07-29 9:54 ` Paolo Bonzini
2020-07-29 13:18 ` Markus Armbruster
2020-07-29 16:08 ` Paolo Bonzini
2020-07-30 10:03 ` Markus Armbruster
2020-07-30 11:09 ` Paolo Bonzini
2020-07-30 12:36 ` Markus Armbruster [this message]
2020-07-30 13:38 ` Paolo Bonzini
2020-07-29 14:32 ` Eduardo Habkost
2020-07-29 16:01 ` Paolo Bonzini
2020-07-29 16:08 ` Eduardo Habkost
2020-07-29 16:14 ` Paolo Bonzini
2020-07-29 7:46 ` Markus Armbruster
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87zh7hxjqa.fsf@dusky.pond.sub.org \
--to=armbru@redhat.com \
--cc=berrange@redhat.com \
--cc=ehabkost@redhat.com \
--cc=pbonzini@redhat.com \
--cc=philmd@redhat.com \
--cc=pratikp@vayavyalabs.com \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).