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 70A88C3ABBC for ; Tue, 6 May 2025 15:38:01 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uCKMm-0002aj-06; Tue, 06 May 2025 11:37:56 -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 1uCKMk-0002ZJ-MM for qemu-devel@nongnu.org; Tue, 06 May 2025 11:37:54 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uCKMi-0003Kg-51 for qemu-devel@nongnu.org; Tue, 06 May 2025 11:37:54 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1746545871; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=skz6KJITmDsnIkiTQSvkece8gmL9zkoBs34jG2iqABE=; b=I43qcwV3AtulM57dBmANtXEm9PIJmKmiV6onyPZYKH3Pg12cmyGPjKIMourQ4USnaG1VA4 +VKqQZ6PD6QJ/kX3TTIS7MvA0Y19FDzmCLnjGkbA1IPrKbpGx5jvIXThK4l9EcQMoBdUZi o7ZN4iPTHFwiQNhW8RswKnGZYkhSzGw= Received: from mx-prod-mc-02.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-332-JhNyM_QeNLqmDaCXmnUNyg-1; Tue, 06 May 2025 11:37:49 -0400 X-MC-Unique: JhNyM_QeNLqmDaCXmnUNyg-1 X-Mimecast-MFC-AGG-ID: JhNyM_QeNLqmDaCXmnUNyg_1746545867 Received: from mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.15]) (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-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 947C11955DB9; Tue, 6 May 2025 15:37:46 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.27]) by mx-prod-int-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 949271956094; Tue, 6 May 2025 15:37:44 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id ECCC921E66C3; Tue, 06 May 2025 17:37:41 +0200 (CEST) From: Markus Armbruster To: Akihiko Odaki Cc: Jason Wang , Dmitry Fleytman , Sriram Yagnaraman , "Michael S. Tsirkin" , Luigi Rizzo , Giuseppe Lettieri , Vincenzo Maffione , Andrew Melnychenko , Yuri Benditovich , Paolo Bonzini , Daniel P. =?utf-8?Q?Berrang=C3=A9?= , Eduardo Habkost , Michael Roth , Marcel Apfelbaum , Philippe =?utf-8?Q?Mathieu-Daud?= =?utf-8?Q?=C3=A9?= , Yanan Wang , Zhao Liu , Lei Yang , qemu-devel@nongnu.org Subject: Re: [PATCH v4 2/4] qdev-properties: Accept bool for OnOffAuto In-Reply-To: <6e6935dd-fae7-4cce-acad-69609eba9b6e@daynix.com> (Akihiko Odaki's message of "Thu, 6 Feb 2025 19:16:44 +0900") References: <20250108-virtio-v4-0-cbf0aa04c9f9@daynix.com> <20250108-virtio-v4-2-cbf0aa04c9f9@daynix.com> <87cyfwxveo.fsf@pond.sub.org> <44b21e4c-b076-41bb-9564-1e7a8cf4a450@daynix.com> <87r04bs8sj.fsf@pond.sub.org> <6e6935dd-fae7-4cce-acad-69609eba9b6e@daynix.com> Date: Tue, 06 May 2025 17:37:41 +0200 Message-ID: <87r011rbqy.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.0 on 10.30.177.15 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.414, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Akihiko Odaki writes: > On 2025/02/06 18:48, Markus Armbruster wrote: >> Akihiko Odaki writes: [...] >> I understand we have something like this: >> >> * true: on if possible, else off >> >> * false: off (always possible) >> >> Which one is the default? > > It depends. Some properties have true by default. The others have false. > >> >> There is no way to reliably configure "on", i.e. fail if it's not >> possible. I agree that's a problem. >> >>> This problem can be solved >>> using an existing mechanism, OnOffAuto, which differentiates the "auto" >>> state and explicit the "on" state. >> >> I guess you're proposing something like this: >> >> * auto: on if possible, else off >> >> * on: on if possible, else error >> >> * off: off (always possible) >> >> Which one is the default? > > I converted on to auto and off to false in a following patch. > >> >>> However, converting bool to OnOffAuto surfaces another problem: they >>> disagree how "on" and "off" should be written. Please note that the >>> disagreement already exists and so it is nice to solve anyway. >> >> Yes, converting bool to OnOffAuto is an incompatible change. > > Not just about conversion, but this inconsistency require users to know > whether a property is bool or OnOffAuto and change how the values are > written in JSON accordingly. This somewhat hurts usability. > >> >>> This patch tries to solve it by tolerating bool values for OnOffAuto. As >>> you pointed out, this approach has a downside: it makes OnOffAuto more >>> complicated by having multiple ways to express the same thing. >> >> It also affects existing uses of OnOffAuto, where such a change is >> unnecessary and undesirable. To be clear: this is pretty much a deal-breaker for me. We established above that you need certain boolean properties to take a third state. I'm willing to discuss patches that change exactly these properties. I'm going to reject patches that affect properties that do not need such a change. >>> Another approach is to have one unified way to express "on"/"off" for >>> bool and OnOffAuto. This will give three options in total: >>> >>> 1. Let OnOffAuto accept JSON bool and "on"/"off" (what this patch does) >> >> The parenthesis is inaccurate. This patch only affects qdev properties. >> It does not affect use of OnOffAuto elsewhere, e.g. QOM object >> "sev-guest" property "legacy-vm-type", or QMP command blockdev-add >> argument "locking" with driver "file". >> >>> 2. Let OnOffAuto and bool accept JSON bool and deprecate "on"/"off" >>> 3. Let OnOffAuto and bool accept "on"/"off" and deprecate JSON bool >> >> For each of these options: >> >> (a) Change exactly the uses of OnOffAuto that need to become tri-state >> >> (b) Change all qdev properties (currently a superset of (a); what this >> patch does) >> >> (c) Change all uses of OnOffAuto >> >> I dislike (c) and especially (b). >> >>> I'm fine with either of these approaches; they are at least better than >>> the current situation where users need to care if the value is OnOffAuto >>> or bool when they just want to express on/off. Please tell me what you >>> prefer. >> >> We managed to maneuver ourselves into a bit of a corner in just a few >> simple steps: >> >> * The obvious type for a flag is bool. >> >> * The obvious type for a small set of values is enum. >> >> * Thus, the obvious type for a tri-state is enum. >> >> * But this prevents growing a flag into a tri-state compatibly. Which >> is what you want to do. >> >> However, we actually have a second way to do a tri-state: optional bool, >> i.e. present and true, present and false, absent. >> >> Permit me a digression... I'm not a fan of assigning "absent" a meaning >> different from any present value. But it's a design choice QAPI made. > > It's a new insight I didn't know. Properties in qdev have a default > value instead of special "absent". But if QAPI does have special > "absent", perhaps qdev may be modified to align with. Nothing stops you from creating qdev properties with a special "absent" value. All you need is a special value that cannot be set. In fact, the humble "str" property already works that way: it's a char * where null means "absent". Code can recognize "absent" and do whatever needs doing then. For instance, consider device "ide-cd". It has three such properties: "ver", "serial", and "model". "ver" defaults to "2.5+", "serial" to some unique string, but "model" defaults to NULL. Since you cannot set such a value, it effectively means "absent". The code responsible for this is in ide_dev_initfn(): if (!dev->version) { dev->version = g_strdup(s->version); } if (!dev->serial) { dev->serial = g_strdup(s->drive_serial_str); } Note it leaves a null dev->model null. >> Using optional that way can occasionally lead to trouble. Consider >> migrate-set-parameters. Its arguments are all optional. For each >> argument present, the respective migration parameter is set to the >> argument value. You cannot use this to reset a migration parameter from >> present to absent. Matters for parameters where "absent" has a meaning >> different from any "present" value. >> >> End of digression. >> >> Start of next digression :) >> >> Note that qdev properties are generally optional. The only way to make >> them mandatory is to reject their default value in .realize(). When >> users set this default value explicitly, the error message will almost >> certainly be confusing. >> >> End of digression. >> >> Optional bool may enable a fourth solution: >> >> 4. Make "absent" mean on if possible, else off, "present and true" mean >> on if possible, else error, and "present and false" mean off (always >> possible). >> >> This changes the meaning of "present and true", but it's precisely >> the change you want, isn't it? > > We have "false by default" properties so it unfortunately does not work. Then make the code make "absent" mean what you need it to mean. Just like the code from ide_dev_initfn() I quoted above. >> Yet another solutions: >> >> 5. Alternate of bool and an enum with a single value "auto". >> >> Falls apart with the keyval visitor used for the command line. >> Fixable, I believe, but a good chunk of work and complexity. > > I may have missed something, but I think that will break JSON string > literals "on" and "off". Unbreaking it will be a good chunk of work and complexity, I believe. >> My gut feeling: explore 4. first.