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 484AEC43327 for ; Wed, 1 Jul 2026 14:19:56 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wevmw-0007uj-Cz; Wed, 01 Jul 2026 10:19:42 -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 1wevmu-0007ub-Kq for qemu-devel@nongnu.org; Wed, 01 Jul 2026 10:19:40 -0400 Received: from mx.treblig.org ([2a00:1098:5b::1]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wevms-0002OO-Dv for qemu-devel@nongnu.org; Wed, 01 Jul 2026 10:19:40 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=treblig.org ; s=bytemarkmx; h=Content-Type:MIME-Version:Message-ID:Subject:From:Date:From :Subject; bh=m4bQvNHDs6XwQLXSCduuBqKVav7YlWO7rkKAwP8IZBc=; b=Fo8EDsSI1hytdL/N SIQwF0VH/fU7FKAT84JZ6UmX0ZU3tlAEHYjrdhEUyOx/oEseqB1A14yhUfBk9lQ/f56ZW7eCN7e3U 148CZtRAyUwa86uN2sE4/h/gzyEOreWZQ+ngqb6DE9+7bJomWRdwd9v1zqIN0nlQ96zpaG6xKfJyE /lF+q6wiaRlGou+7iIbEFETiBjae72m9ACLn3LiPzMR7ZzI3qm3e+199iLJsLOZzkTJgC/pz3eop8 C5zJ6l92sjgkexoLWtCeNqm7wuTRD6if5rjg5mSRCAN9EgCUtRaFbu7SiM4vh4iqkBDkA5AbKA4v6 bJ+TUf25+aZxLlD8/A==; Received: from dg by mx.treblig.org with local (Exim 4.98.2) (envelope-from ) id 1wevmm-0000000BSVX-3Kau; Wed, 01 Jul 2026 14:19:32 +0000 Date: Wed, 1 Jul 2026 14:19:32 +0000 From: "Dr. David Alan Gilbert" To: Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= Cc: Markus Armbruster , marcandre.lureau@redhat.com, qemu-devel@nongnu.org, devel@lists.libvirt.org, Paolo Bonzini , Alex =?iso-8859-1?Q?Benn=E9e?= , Christian Brauner , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Peter Krempa Subject: Re: [PATCH v5 32/35] monitor: add support for auto-deleting monitors upon close Message-ID: References: <20260624173752.2928717-1-berrange@redhat.com> <20260624173752.2928717-33-berrange@redhat.com> <178233405051.3955748.10990724544083287885.b4-review@b4> <87zf0busnt.fsf@pond.sub.org> <87mrwa4yrl.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Chocolate: 70 percent or better cocoa solids preferably X-Operating-System: Linux/6.12.88+deb13-amd64 (x86_64) X-Uptime: 14:17:57 up 46 days, 17:30, 3 users, load average: 0.05, 0.04, 0.00 User-Agent: Mutt/2.2.13 (2024-03-09) Received-SPF: pass client-ip=2a00:1098:5b::1; envelope-from=dg@treblig.org; helo=mx.treblig.org 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, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: 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. Berrangé (berrange@redhat.com) wrote: > On Wed, Jul 01, 2026 at 03:37:02PM +0200, Markus Armbruster wrote: > > Daniel P. Berrangé writes: > > > > > On Wed, Jul 01, 2026 at 08:32:06AM +0200, Markus Armbruster wrote: > > >> marcandre.lureau@redhat.com writes: > > >> > > >> >> 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. > > >> > > >> 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. > > In a hypothetical libvirt usage of this functionality, libvirt would > not be spawning the script or even know what its PID is. Libvirt would > just create a auto-delete monitor, open an FD to it, and pass the FD > back to the libvirt client & forget about it. > > > >> >> 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. > > >> >> > > >> >> 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. > > >> > > >> 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? > > That's a possibility. > > We would have to accept that it allows users to create a scenario in > which auto-delete will frequenty fail. > > ie configuring the chardev for auto-delete, but not having the > frontend auto-delete, which would trigger the "busy" check. This feels like it has some overlap with 'yank'? Where you can force a channel to get out of the way even if you don't delete it at that point. Dave > Not convinced that's nicer ? > > 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 :| > -- -----Open up your eyes, open up your mind, open up your code ------- / Dr. David Alan Gilbert | Running GNU/Linux | Happy \ \ dave @ treblig.org | | In Hex / \ _________________________|_____ http://www.treblig.org |_______/