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([2001:b07:6468:f312:101f:6e7:e073:454c]) by smtp.gmail.com with ESMTPSA id mf17sm1945228ejb.56.2020.07.29.09.08.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Jul 2020 09:08:18 -0700 (PDT) Subject: Re: sysbus_create_simple Vs qdev_create To: Markus Armbruster 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> <87zh7i5uj5.fsf@dusky.pond.sub.org> From: Paolo Bonzini Message-ID: <6ee49ad2-8b6b-cb6f-c3c9-b440631cfc75@redhat.com> Date: Wed, 29 Jul 2020 18:08:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <87zh7i5uj5.fsf@dusky.pond.sub.org> Content-Language: en-US X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=205.139.110.61; envelope-from=pbonzini@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 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: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , =?UTF-8?Q?Daniel_P=2e_Berrang=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" On 29/07/20 15:18, Markus Armbruster wrote: >> 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. Indeed it doesn't use _all_ of the QAPI goodies. It does use visitors and it's a major feature of QOM. > 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). It's verbose in the getters/setters, but we have wrappers such as object_property_set_str, object_property_set_bool etc. that do not make it too hard to understand. > 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. QOM didn't only replace qdev: it also removed the need to have a command line option du jour for any new concept, e.g. all the TLS stuff, RNG backends, RAM backends, etc. It didn't succeed (at all) in deprecating chardev/netdev/device etc., but this is a very underappreciated part of QOM, and this is why I think it's appropriate to say QOM is "C with classes and CLI/RPC serialization", as opposed for example to "C with classes and multi programming language interface" that is GObject. > 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. :) Paolo