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 3C19BC43458 for ; Wed, 1 Jul 2026 13:46:36 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wevGZ-000655-Te; Wed, 01 Jul 2026 09:46:16 -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 1wevGR-00064S-7o for qemu-devel@nongnu.org; Wed, 01 Jul 2026 09:46:07 -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 1wevGO-0007RO-H9 for qemu-devel@nongnu.org; Wed, 01 Jul 2026 09:46:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1782913563; 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=HbB998jaC+hWnrxKMzzt8H82XlG8CHfZRQnJXOKRDTQ=; b=hRc07zPLEgzqW7bBVzM/9qGFH9aZPhonSeGWloYeHIuJElFmsqWJha5ybee0nL33qCAqSt AgkUPdKdRcgcc53lsbR/GOvAUll8qjBdObMXQsFBjBFVFfNahchUJylmiZREiJXENrdBmM H48qB4tu3Td8YSxc1K0uCe15fd7Vh1k= 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-634-zuLVNv-bNO-0_JpNbMpoxQ-1; Wed, 01 Jul 2026 09:46:00 -0400 X-MC-Unique: zuLVNv-bNO-0_JpNbMpoxQ-1 X-Mimecast-MFC-AGG-ID: zuLVNv-bNO-0_JpNbMpoxQ_1782913558 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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 642511944E45; Wed, 1 Jul 2026 13:45:58 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.35]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1B8211955D53; Wed, 1 Jul 2026 13:45:53 +0000 (UTC) Date: Wed, 1 Jul 2026 14:45:50 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster 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 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87mrwa4yrl.fsf@pond.sub.org> User-Agent: Mutt/2.3.2 (2026-04-26) X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@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_H4=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: , 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, 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. 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 :|