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 lists.gnu.org (lists.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 2EA7DD767D8 for ; Thu, 31 Oct 2024 15:58:31 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1t6XYw-0000hJ-1w; Thu, 31 Oct 2024 11:58:18 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1t6XYu-0000gm-63 for qemu-devel@nongnu.org; Thu, 31 Oct 2024 11:58:16 -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 1t6XYs-0006im-3q for qemu-devel@nongnu.org; Thu, 31 Oct 2024 11:58:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1730390292; 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=05NNA//5SrjxtfS8Q+J3PZZuqlTVC6E42OAJmltZxCE=; b=h+GVaSvso7DE3/iPRvU+2dQABNT2EzpcmePHelrnPfnpAwWw8Mu/5st7M2SmbUJct+krM5 o/UIKc6Dl0zMYtSFDlxrAHDpqrT/NjxYjg2yBwlbi8nqFgsMBPtXgNh4P6ZQV8DCug1/Zf K1tP41ZQAeXf2PXZ8ATndEiXsdqcDsQ= Received: from mx-prod-mc-05.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-259-bX8kGMBjNiyy0ZNrg0RbNQ-1; Thu, 31 Oct 2024 11:58:07 -0400 X-MC-Unique: bX8kGMBjNiyy0ZNrg0RbNQ-1 Received: from mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.4]) (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-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5A97A1955F68; Thu, 31 Oct 2024 15:58:06 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.18]) by mx-prod-int-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 14C5F300018D; Thu, 31 Oct 2024 15:58:00 +0000 (UTC) Date: Thu, 31 Oct 2024 15:57:57 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, Markus Armbruster , Peter Maydell , Eduardo Habkost , Paolo Bonzini , Mark Cave-Ayland , Igor Mammedov , "Michael S . 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David Alan Gilbert" , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , =?utf-8?Q?C=C3=A9dric?= Le Goater , Fabiano Rosas , Juraj Marcin Subject: Re: [PATCH RFC v2 0/7] QOM: Singleton interface Message-ID: References: <20241029211607.2114845-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/2.2.12 (2023-09-09) X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 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: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.366, 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=-0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Oct 30, 2024 at 03:08:08PM -0400, Peter Xu wrote: > On Wed, Oct 30, 2024 at 06:07:08PM +0000, Daniel P. Berrangé wrote: > > On Tue, Oct 29, 2024 at 05:16:00PM -0400, Peter Xu wrote: > > > v1: https://lore.kernel.org/r/20241024165627.1372621-1-peterx@redhat.com > > > > > > This patchset introduces the singleton interface for QOM. I didn't add a > > > changelog because there're quite a few changes here and there, plus new > > > patches. So it might just be easier to re-read, considering the patchset > > > isn't large. > > > > > > I switched v2 into RFC, because we have reviewer concerns (Phil and Dan so > > > far) that it could be error prone to try to trap every attempts to create > > > an object. My argument is, if we already have abstract class, meanwhile we > > > do not allow instantiation of abstract class, so the complexity is already > > > there. I prepared patch 1 this time to collect and track all similar > > > random object creations; it might be helpful as a cleanup on its own to > > > deduplicate some similar error messages. Said that, I'm still always open > > > to rejections to this proposal. > > > > > > I hope v2 looks slightly cleaner by having not only object_new_allowed() > > > but also object_new_or_fetch(). > > > > For me, that doesn't really make it much more appealing. Yes, we already have > > an abstract class, but that has narrower impact, as there are fewer places > > in which which we can trigger instantiation of an abstract class, than > > where we can trigger instantiation of arbitrary objects and devices. > > There should be exactly the same number of places that will need care for > either abstract or singleton. I tried to justify this with patch 1. > > I still think patch 1 can be seen as a cleanup too on its own (dedups the > same "The class is abstract" error message), tracking random object > creations so logically we could have the idea on whether a class can be > instantiated at all, starting with abstract class. I think patch 1 might be incomplete, as I'm not seeing what checks for abstract or singleton classes in the 'qdev_new' code paths, used by -device / device_add QMP. This is an example of the risks of adding more failure scenarios to object_add. > > NB, my view point would have been different if 'object_new' had an > > "Error *errp" parameter. That would have made handling failure a > > standard part of the design pattern for object construction, thus > > avoiding adding asserts in the 'object_new' codepath which could be > > triggered by unexpected/badly validated user input. > > Yes I also wished object_new() can take an Error** when I started working > on it. It would make this much easier, indeed. I suppose we don't need > that by not allowing instance_init() to fail at all, postponing things to > realize(). I suppose that's a "tactic" QEMU chose explicitly to make it > easy that object_new() callers keep like before with zero error handling > needed. At least for TYPE_DEVICE it looks all fine if all such operations > can be offloaded into realize(). I'm not sure user creatable has those > steps also because of this limitation. > > I was trying to do that with object_new_allowed() here instead, whenever it > could be triggered by an user input. We could have an extra layer before > reaching object_new() to guard any user input, and I think > object_new_allowed() could play that role. When / If we want to introduce > Error** to object_new() some day (or a variance of it), we could simply > move object_new_allowed() into it. Yes, having thought about this today, I came up with a way that we could introduce a object_new_dynamic() variant with "Error *errp" instead of asserts, and *crucially* force its use in the unsafe scenarios. ie any place that is not passing a const,static string. I've CC'd you on an RFC series that mocks up this idea. That would be sufficient to remove my objections wrt the singleton concept. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|