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Wed, 29 Jul 2020 13:18:40 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id E38E41132FD2; Wed, 29 Jul 2020 15:18:38 +0200 (CEST) From: Markus Armbruster To: Paolo Bonzini Subject: Re: sysbus_create_simple Vs qdev_create References: <87lfjkvo81.fsf@dusky.pond.sub.org> <20200716222130.GO1274972@habkost.net> <87tuy6k9pa.fsf@dusky.pond.sub.org> <20200717162312.GR1274972@habkost.net> <87r1t6hc0f.fsf@dusky.pond.sub.org> <20200720155955.GV1274972@habkost.net> <87v9ihbe6u.fsf@dusky.pond.sub.org> <50e31ece-215c-a632-e5a2-86ae7ab3abab@redhat.com> <87lfj4f6nz.fsf@dusky.pond.sub.org> <759959d1-f320-734a-ac5e-a60db6b1bc23@redhat.com> <20200728224733.GP225270@habkost.net> <422d7879-3fdc-d38e-259f-2477b9d3c169@redhat.com> Date: Wed, 29 Jul 2020 15:18:38 +0200 In-Reply-To: <422d7879-3fdc-d38e-259f-2477b9d3c169@redhat.com> (Paolo Bonzini's message of "Wed, 29 Jul 2020 11:54:35 +0200") Message-ID: <87zh7i5uj5.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=205.139.110.61; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/29 09:18:45 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Philippe =?utf-8?Q?Mathieu-Dau?= =?utf-8?Q?d=C3=A9?= , "Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9?=" , Eduardo Habkost , Pratik Parvati , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Paolo Bonzini writes: > On 29/07/20 00:47, Eduardo Habkost wrote: >> On Tue, Jul 28, 2020 at 07:38:27PM +0200, Paolo Bonzini wrote: >>> 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. >> >> This is surprising to me. I never thought QOM was targeted at >> the CLI or RPC. (Every single property mentioned in this message >> don't seem to be related to the CLI or RPC.) > > See https://www.mail-archive.com/qemu-devel@nongnu.org/msg674110.html > for an explanation. > >> About the visitors: I always had the impression that usage of >> visitors inside QOM is unnecessary and avoidable (compared to >> QAPI, where the visitors are an essential feature). > > But as I explained in that other message, the main difference between > QOM and something like GObject is eactly the QAPI integration, and that > is where CLI and RPC enter the game: for example the possibility to > share code between -object and HMP object_add on one side and QMP > object-add on the other side. > > Even code riddled by backwards-compatibility special cases, such as > -accel and -machine, can share code between themselves and -object to > some extent; this is thanks to functions such as object_property_parse, > whose parsing is deferred to visitors and hence to QAPI. QOM relies on QAPI visitors to access properties. There is no integration with the QAPI schema. Going through a visitor enables property access from QMP, HMP and CLI. Access from C *also* goes through a visitor. We typically go from C type to QObject and back. Comically inefficient (which hardly matters), verbose to use and somewhat hard to understand (which does). Compare to what QOM replaced: qdev. Properties are a layer on top of ordinary C. From C, you can either use the C layer (struct members, basically), or the property layer for C (functions taking C types, no conversion to string and back under the hood), or the "text" layer (parse from text / format to text). My point is not that qdev was great and QOM is terrible. There are reasons we replaced qdev with QOM. My point is QOM doesn't *have* to be the way it is. It is the way it is because we made it so. >> Do we really need need QOM children to be accessible using the QOM >> property API? >> >> Using the same code for both user-configurable properties and for >> the list of children of a QOM object might have saved some time >> years ago, but I'm not sure this is still a necessary or useful >> abstraction. > > The main thing we get from it is that the QOM paths treat children and > links the same, and links are properties. To be honest it's not a > feature that is very much developed, so perhaps we can remove it but we > need to evaluate the impact of losing it. 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.