From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 49485CD98C5 for ; Mon, 15 Jun 2026 14:02:08 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wZ7sq-0005sU-IP; Mon, 15 Jun 2026 10:01:48 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wZ7sf-0005r1-EJ for qemu-rust@nongnu.org; Mon, 15 Jun 2026 10:01:38 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wZ7sd-0003Fi-Bv for qemu-rust@nongnu.org; Mon, 15 Jun 2026 10:01:37 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1781532093; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=VCYvyD6JCgalr+O+68KnyshiG9KqipO3rBtqI6S4UPQ=; b=Br4CFGEqoRXpFdX8Hq5s7GEMiyCWKk+wQrmZt8gNk8xEjO4XrpLcZSRb+I7Zb/LK9zlpOw GwzJ487efxG1+KT8RAf8LNsuWxPGkX9Q0SIezYpivq6PL2G+9SVXJ1hY5e257qcqqg/GLc uIajgyI8AZBe7hr1snhzv6rryfkpjGY= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-685-3ilvKkUSOUCzUpaIP1Umug-1; Mon, 15 Jun 2026 10:01:25 -0400 X-MC-Unique: 3ilvKkUSOUCzUpaIP1Umug-1 X-Mimecast-MFC-AGG-ID: 3ilvKkUSOUCzUpaIP1Umug_1781532081 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E58211955E9E; Mon, 15 Jun 2026 14:01:19 +0000 (UTC) Received: from redhat.com (unknown [10.44.49.222]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 47A4C18005AE; Mon, 15 Jun 2026 14:01:15 +0000 (UTC) Date: Mon, 15 Jun 2026 15:01:11 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Mark Cave-Ayland Cc: Peter Maydell , Akihiko Odaki , qemu-devel@nongnu.org, Paolo Bonzini , Manos Pitsidianakis , qemu-rust@nongnu.org, Peter Xu , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Alberto Garcia , Kevin Wolf , Hanna Reitz , qemu-block@nongnu.org Subject: Re: [PATCH 2/2] qom: Manage references to embedded child objects Message-ID: References: <20260615-embedded-v1-0-bb0c65bf126c@rsg.ci.i.u-tokyo.ac.jp> <20260615-embedded-v1-2-bb0c65bf126c@rsg.ci.i.u-tokyo.ac.jp> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.3.2 (2026-04-26) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 X-Mimecast-MFC-PROC-ID: CAm0rS1moAMgBjzN3SyH0gkUS8IAqdChY1aYU5MiaNM_1781532081 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 8 X-Spam_score: 0.8 X-Spam_bar: / X-Spam_report: (0.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-rust@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: QEMU Rust-related patches and discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org Sender: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org On Mon, Jun 15, 2026 at 02:42:10PM +0100, Mark Cave-Ayland wrote: > On 15/06/2026 10:28, Peter Maydell wrote: > > > On Mon, 15 Jun 2026 at 09:35, Daniel P. Berrangé wrote: > > > > > > On Mon, Jun 15, 2026 at 09:31:36AM +0100, Peter Maydell wrote: > > > > On Mon, 15 Jun 2026 at 09:13, Daniel P. Berrangé wrote: > > > > > Do we know how many examples we have of embedding objects inside > > > > > another ? > > > > > > > > It is an extremely common pattern for device and SoC model > > > > implementations; we've been recommending it for years. > > > > > > > > > I would much prefer if we forbid the embedding of objects. It is > > > > > horrible design practice to have some QOM objects which can be > > > > > freed via reference count and some which cannot. > > > > > > > > That would be a very large amount of code to rewrite to the > > > > new paradigm. I don't object inherently ("you have pointers to > > > > your child objects" works better when they might be implemented > > > > in Rust and might play better with being able to create > > > > machines and wire them up on the command line); I'm just > > > > noting how much work it would be if you wanted to make > > > > embedding forbidden. > > > > > > Would it be a more tractable problem to "fix" object structure only > > > incrementally as they gain a need to be managed/reference from > > > Rust code, or does the Rust usage already implicitly extend too > > > broadly > > > > At the moment we have exactly 2 Rust devices. You can see the > > workaround we ended up with for the PL011 in commits 5b87a07e768 > > and cc3d262aa93a4, where we pad out the C device struct and > > assert on the Rust side that its version isn't any bigger. > > > > For that particular problem we could say "you need to refactor > > all users of device X to not embed it before creating the Rust > > version". There is I think only one embedded use of the PL011 > > (I was actually expecting more). This does have the awkward > > effect that anybody wanting to do a "rewrite device in rust" > > is now forced into "do a big refactor of some old C code they > > don't care about". We would also need to actually document and > > provide some good examples of "this is how we think we should > > be writing device models that have child objects now". > > I think given the legacy of the QEMU codebase then this will always be an > issue, but we've solved this in the past with the introduction of > MemoryRegions so there's no reason we couldn't do it again. > > This is a similar problem to the use of non-class object properties I was > discussing in the thread with Daniel: it seems the general consensus is that > they shouldn't exist, but this hasn't been formalised anywhere. If we can > formalise the decision not to use embedded QOM objects, then we can start by > ensuring that new submissions don't use them and go from there. > > Question: who currently should make these decisions? Is it restricted to the > QOM maintainers? Ultimately it would need a docs update on object.h to declare the object_initialize_child related methods to be deprecated which the QOM maintainers would have to queue. Since this impacts device maintainers in general though, IMHO we need broad consensus that it is a better approach & we're willing to convert existing code incrementally. We have ~150 devices using this for 824 children $ git grep -l object_initialize_child | wc -l 150 $ git grep object_initialize_child | wc -l 824 Where updates are desired, we'd be looking at tedious changes like thsi: diff --git a/hw/isa/piix.c b/hw/isa/piix.c index 31fa53e6a4..86757c56e5 100644 --- a/hw/isa/piix.c +++ b/hw/isa/piix.c @@ -355,16 +355,23 @@ static void pci_piix_realize(PCIDevice *dev, const char *uhci_type, /* USB */ if (d->has_usb) { - object_initialize_child(OBJECT(dev), "uhci", &d->uhci, uhci_type); - qdev_prop_set_int32(DEVICE(&d->uhci), "addr", dev->devfn + 2); - if (!qdev_realize(DEVICE(&d->uhci), BUS(pci_bus), errp)) { + d->uhci = UHCI(object_new_with_props( + uhci_type, OBJECT(d), "uhci", errp, NULL)); + if (!d->uhci) { + return; + } + qdev_prop_set_int32(DEVICE(d->uhci), "addr", dev->devfn + 2); + if (!qdev_realize(DEVICE(d->uhci), BUS(pci_bus), errp)) { return; } } diff --git a/hw/isa/piix.c b/hw/isa/piix.c index 31fa53e6a4..86757c56e5 100644 --- a/hw/isa/piix.c +++ b/hw/isa/piix.c @@ -355,16 +355,23 @@ static void pci_piix_realize(PCIDevice *dev, const char *uhci_type, /* USB */ if (d->has_usb) { - object_initialize_child(OBJECT(dev), "uhci", &d->uhci, uhci_type); - qdev_prop_set_int32(DEVICE(&d->uhci), "addr", dev->devfn + 2); - if (!qdev_realize(DEVICE(&d->uhci), BUS(pci_bus), errp)) { + d->uhci = UHCI(object_new_with_props( + uhci_type, OBJECT(d), "uhci", errp, NULL)); + if (!d->uhci) { + return; + } + qdev_prop_set_int32(DEVICE(d->uhci), "addr", dev->devfn + 2); + if (!qdev_realize(DEVICE(d->uhci), BUS(pci_bus), errp)) { return; } } Noting that the compiler won't complain if we forget to update any lines like - qdev_prop_set_int32(DEVICE(&d->uhci), "addr", dev->devfn + 2); + qdev_prop_set_int32(DEVICE(d->uhci), "addr", dev->devfn + 2); because the explicit DEVICE(..) cast is hiding the type error we would otherwise get from using "Device **" instead of "Device *". With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|