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 9852AC43458 for ; Wed, 1 Jul 2026 13:37:50 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wev7s-00037T-RW; Wed, 01 Jul 2026 09:37:16 -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 1wev7r-00037H-Dk for qemu-devel@nongnu.org; Wed, 01 Jul 2026 09:37:15 -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 1wev7p-00042S-KZ for qemu-devel@nongnu.org; Wed, 01 Jul 2026 09:37:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1782913031; 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=0PmtpMO/nMpYv77e5X23T8MgyYWNfiBq1aNlzuRUAe0=; b=AHutz8BRkW5smrK/7HFQ7LYmBoOBbLgCDNZY3TQ8oc35zMpZ5sjPXlCUfY84vCFjVujgRR BHrJAgFG3macvMyP7g099/kZ8pztsbwaJUqacKcPu5ssq2/hGrl1Wt7TERR0IEL4VGn2/0 eHKx7iqpELsB9x2Ro0dj6t8TiRJ8O80= 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-691-_9Dkiz9iO6yEtOZtl1iKVQ-1; Wed, 01 Jul 2026 09:37:08 -0400 X-MC-Unique: _9Dkiz9iO6yEtOZtl1iKVQ-1 X-Mimecast-MFC-AGG-ID: _9Dkiz9iO6yEtOZtl1iKVQ_1782913026 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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 551DD1944EAE; Wed, 1 Jul 2026 13:37:06 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.44.22.4]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 346B01800612; Wed, 1 Jul 2026 13:37:05 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id B986021E6920; Wed, 01 Jul 2026 15:37:02 +0200 (CEST) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: marcandre.lureau@redhat.com, qemu-devel@nongnu.org, devel@lists.libvirt.org, Paolo Bonzini , "Dr. David Alan Gilbert" , Alex =?utf-8?Q?Benn=C3=A9e?= , Christian Brauner , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Peter Krempa Subject: Re: [PATCH v5 32/35] monitor: add support for auto-deleting monitors upon close In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Wed, 1 Jul 2026 09:10:34 +0100") References: <20260624173752.2928717-1-berrange@redhat.com> <20260624173752.2928717-33-berrange@redhat.com> <178233405051.3955748.10990724544083287885.b4-review@b4> <87zf0busnt.fsf@pond.sub.org> Date: Wed, 01 Jul 2026 15:37:02 +0200 Message-ID: <87mrwa4yrl.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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: 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-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development 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 Wed, Jul 01, 2026 at 08:32:06AM +0200, Markus Armbruster wrote: >> marcandre.lureau@redhat.com writes: >>=20 >> >> The default monitor is usually a long lived object that will exist for >> >> the entire lifetime of the VM. A monitor can only service a single >> >> client at a time though, and so it might be desirable to hotplug >> >> additional monitors at runtime for specific tasks. If doing that, >> >> however, there is a need to remove the monitor when it is no longer >> >> needed. >>=20 >> Whatever adds the additional monitor can also delete it. The fact that >> you propose other means suggests you believe this would be cumbersome in >> practice. Why? > > The use is is that someone/something wants to spawn a script that > does some job with the QEMU monitor. The thing that spawns the > script adds the new monitor and launches the script. Having > auto-delete means that you do not need to then keep track of that > script to perform cleanup of the dynamically added monitor. It > gives you "do the right thing" behaviour automatically when the > script exits, closing its monitor connection. > > The initial series proposed by Christian supported the ability > to run "object-del' on the monitor itself - a "self delete" > essentially. That is very awkward from the code POV, as it > required special case hanlding to ensure the QMP response to > the delete action got sent on the socket before the delete > action took place. It also made it impossible to then delete > the character device. > > Auto-delete gives us a better solution with less code complexity. I agree "self delete" is problematic. I wonder how important the ability to fire and forget a script with a dedicated monitor is. I'd (naively?) expect whatever spawns the script to reap its exit status. If it is important, what about fire and forget a script with a dedicated character device? Can't come up with a use case for that within ten seconds. However, we use character devices for all kinds of crap. Food for thought, not a demand. >> >> Allowing a client to run "object-del" against its own monitor adds >> >> complex edge cases, as it would be desirable to send the QMP response >> >> despite the monitor sending it being deleted. Doing "object-del" alone >> >> will also result in orphaning a character device backend instance, as >> >> there is no opportunity to run the companion "chardev-del" command. >> >>=20 >> >> A simpler way to ensure cleanup is to add the concept of auto-deleting >> >> monitor objects. Specifically when the "CHR_EVENT_CLOSED" event is >> >> emitted, the equivalent of "object-del" + "chardev-del" can be run >> >> internally. Since the transient client has already droppped its >> >> monitor connection, there is no synchronization to be concerned about. >>=20 >> If object-del or chardev-del fail, there's no way to report the error. >> Can they fail? > > object-del can fail if > > * An object with the specified "id" does not exist. That shouldn't > happen in this case but harmless if it odes. > * object_del command tries to delete the monitor that > is servicing the object_del command. Cannot happen with > auto-delete > * the monitor has not finished initializing it BH with > chardev handlers. Cannot happen if we know we have a > live connection already. > > chardev-del can fail if > > * The chardev with "id" does not exist. SHouldn't happen > but is harmless if it does > * The chardev reports it is "busy" - aka the frontend > is still connected - we just deleted it so cannot happen > * Record/replay is in use - a niche use case > > So I don't think errors are a problem. > >> Do we always want to delete both monitor and character device? > > IMHO yes they are a pair whose lifetime should be tied together > for normal use. Would we make monitor auto-delete delete its character device if character devices also had an auto-delete feature? [...]