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 097BFC61DA4 for ; Tue, 14 Feb 2023 11:49:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pRtoT-0004PZ-UO; Tue, 14 Feb 2023 06:49:33 -0500 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 1pRtoS-0004PG-4p for qemu-devel@nongnu.org; Tue, 14 Feb 2023 06:49:32 -0500 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 1pRtoQ-00059o-Gf for qemu-devel@nongnu.org; Tue, 14 Feb 2023 06:49:31 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1676375369; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=e4v9ZEE3vGAgeUjZ2N7LBHb7aitT7QGLD7n8ZNeVjHs=; b=hC14dh/w3ItQ2YpLyIGBuqA+p6emNGaa26P/C2RU5JtkX1/smrG11XLzlQ1ZsJJiOD1yA/ ytxTk86bNFAI6bPeVAzf3ri9rfhEu0uHKbBUAtyNIIEQQV0D/sY4K16KkO5WbzM/9125b+ xRI7DfSxPrDmBDs468aFKAl8xPiGLxc= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-531-2rBl5wDuNFuz6QNdakKJlw-1; Tue, 14 Feb 2023 06:49:26 -0500 X-MC-Unique: 2rBl5wDuNFuz6QNdakKJlw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 12F04101A55E; Tue, 14 Feb 2023 11:49:26 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.192.13]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BD24718EC7; Tue, 14 Feb 2023 11:49:25 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id AB9F121E6A1F; Tue, 14 Feb 2023 12:49:24 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Peter Krempa , eduardo@habkost.net, antonkuchin@yandex-team.ru, philmd@linaro.org, mst@redhat.com, "reviewer:Incompatible changes" , qemu-devel@nongnu.org, Vladimir Sementsov-Ogievskiy , den-plotnikov@yandex-team.ru, marcel.apfelbaum@gmail.com, pbonzini@redhat.com, eblake@redhat.com Subject: Re: [PATCH v4 14/16] qapi: deprecate "device" field of DEVICE_* events References: <20230213140103.1518173-1-vsementsov@yandex-team.ru> <20230213140103.1518173-15-vsementsov@yandex-team.ru> <87bklwoce9.fsf@pond.sub.org> Date: Tue, 14 Feb 2023 12:49:24 +0100 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Tue, 14 Feb 2023 11:14:14 +0000") Message-ID: <87ttzojwl7.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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, 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 Daniel P. Berrang=C3=A9 writes: > On Tue, Feb 14, 2023 at 10:25:22AM +0100, Peter Krempa wrote: >> On Tue, Feb 14, 2023 at 09:54:22 +0100, Markus Armbruster wrote: >> > Daniel P. Berrang=C3=A9 writes: >> >=20 >> > > On Mon, Feb 13, 2023 at 05:01:01PM +0300, Vladimir Sementsov-Ogievsk= iy wrote: >> > >> The device field is redundant, because QOM path always include devi= ce >> > >> ID when this ID exist. >> > > >> > > The flipside to that view is that applications configuring QEMU are >> > > specifying the device ID for -device (CLI) / device_add (QMP) and >> > > not the QOM path. IOW, the device ID is the more interesting field >> > > than QOM path, so feels like the wrong one to be dropping. >> >=20 >> > QOM path is a reliable way to identify a device. Device ID isn't: >> > devices need not have one. Therefore, dropping the QOM path would be >> > wrong. >> >=20 >> > > Is there any real benefit to dropping this ?=20 >> >=20 >> > The device ID is a trap for the unwary: relying on it is fine until you >> > run into a scenario where you have to deal with devices lacking IDs. >>=20 >> Note that libvirt's code is still using the 'device' bit rather than QOM >> path and the fix might not be entirely trivial although should not be >> too hard. > > What's the documented way to construct a QOM path, given only an ID as > input ? QOM paths a gap in our documentation, even though the composition tree structure has been stable since day one, and is de facto ABI. Short answer: "/machine/peripheral/ID". Long answer follows. We have three "containers" under /machine that serve as parents for devices: * /machine/peripheral/ Parent of user-created devices with ID. Children are named "ID". Put there by qdev_set_id(), called from qdev_device_add_from_qdict(). On "user-created": Nothing stops board code to abuse qdev_set_id() for onboard devices, directly or indirectly, but it really, really shouldn't. * /machine/peripheral-anon/ Parent of user-created devices without ID. Children are named "device[N]", where N counts up from zero. Put there by qdev_set_id(), called from qdev_device_add_from_qdict(). Again, abuse by board code is possible, but would be wrong. Beware: a particular device's N changes when the set of devices created before it grows or shrinks. Messing with the machine type can change it (different onboard devices). * /machine/unattached/ Surrogate parent of onboard devices created without a parent. Put there by device_set_realized() (general case), qdev_connect_gpio_out_named() (input pins) , memory_region_do_init() (memory regions), qemu_create_machine() (the main sysbus). I believe this container was created as a convenience, so we don't have to retrofit parents to existing code. Probably abused ever since.