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Fri, 7 Nov 2025 13:10:34 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.18]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7CBF71800361; Fri, 7 Nov 2025 13:10:30 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 6B91421E6A27; Fri, 07 Nov 2025 14:10:27 +0100 (CET) From: Markus Armbruster To: Igor Mammedov Cc: salil.mehta@opnsrc.net, qemu-devel@nongnu.org, qemu-arm@nongnu.org, mst@redhat.com, salil.mehta@huawei.com, maz@kernel.org, jean-philippe@linaro.org, jonathan.cameron@huawei.com, lpieralisi@kernel.org, peter.maydell@linaro.org, richard.henderson@linaro.org, andrew.jones@linux.dev, david@redhat.com, philmd@linaro.org, eric.auger@redhat.com, will@kernel.org, ardb@kernel.org, oliver.upton@linux.dev, pbonzini@redhat.com, gshan@redhat.com, rafael@kernel.org, borntraeger@linux.ibm.com, alex.bennee@linaro.org, gustavo.romero@linaro.org, npiggin@gmail.com, harshpb@linux.ibm.com, linux@armlinux.org.uk, darren@os.amperecomputing.com, ilkka@os.amperecomputing.com, vishnu@os.amperecomputing.com, gankulkarni@os.amperecomputing.com, karl.heubaum@oracle.com, miguel.luis@oracle.com, zhukeqian1@huawei.com, wangxiongfeng2@huawei.com, wangyanan55@huawei.com, wangzhou1@hisilicon.com, linuxarm@huawei.com, jiakernel2@gmail.com, maobibo@loongson.cn, lixianglai@loongson.cn, shahuang@redhat.com, zhao1.liu@intel.com, devel@lists.libvirt.org Subject: Re: [PATCH RFC V6 22/24] monitor,qdev: Introduce 'device_set' to change admin state of existing devices In-Reply-To: <20251103092754.6dda4aa0@fedora> (Igor Mammedov's message of "Mon, 3 Nov 2025 09:27:54 +0100") References: <20251001010127.3092631-1-salil.mehta@opnsrc.net> <20251001010127.3092631-23-salil.mehta@opnsrc.net> <87plawh2sz.fsf@pond.sub.org> <20251009145125.6583a24a@fedora> <87wm54nmyt.fsf@pond.sub.org> <20251017165044.76b39f5c@fedora> <87v7k96cnz.fsf@pond.sub.org> <20251029110802.2ffa51e4@fedora> <878qguos4l.fsf@pond.sub.org> <20251103092754.6dda4aa0@fedora> Date: Fri, 07 Nov 2025 14:10:27 +0100 Message-ID: <87a50y9ef0.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 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: -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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-arm@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-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org Sender: qemu-arm-bounces+qemu-arm=archiver.kernel.org@nongnu.org Igor Mammedov writes: > On Wed, 29 Oct 2025 12:38:02 +0100 > Markus Armbruster wrote: > >> Igor Mammedov writes: >> >> > On Mon, 20 Oct 2025 13:22:08 +0200 >> > Markus Armbruster wrote: >> > >> >> Igor Mammedov writes: >> >> >> >> > On Thu, 09 Oct 2025 16:55:54 +0200 >> >> > Markus Armbruster wrote: >> >> [...] >> >> >> >> I feel it's best to start the design process with ensvisaged uses. Can >> >> >> you tell me a bit more about the uses you have in mind? >> >> > >> >> > We have nic failover 'feature' >> >> > https://www.qemu.org/docs/master/system/virtio-net-failover.html >> >> > to make it work we do abuse hotplug and that poses problem >> >> > during migration, since: >> >> > - unplugging primary device releases resources (which might not be >> >> > possible to claim back in case migration failure) >> >> >> >> Serious reliability issue with no work-around. >> >> >> >> > - it's similar on destination side, where attempt to hotplug >> >> > primary might fail die to insufficient resources leaving guest >> >> > on 'degraded' virtio-net link. >> >> >> >> Obvious work-around is failing the migration. Same as we do when we >> >> can't create devices. >> >> >> >> > Idea was that instead of hotplug we can power off primary device, >> >> > (it will still exist and keep resources), initiate migration, >> >> > and then on target do the same starting with primary fully realized >> >> > but powered of (and failing migration early if it can't claim resources, >> >> > safely resuming QEMU on source incl. primary link), and then guest >> >> > failover driver on destination would power primary on as part of >> >> > switching to primary link. >> >> >> >> I can see how power on / off makes more sense than hot plug / unplug. >> >> >> >> > Above would require -device/device_add support for specifying device's >> >> > power state as minimum. >> >> >> >> The obvious way to control a device's power state with -device / >> >> device_add is a qdev property. Easy enough. >> >> >> >> Do we need to control a device's power state after it's created? If I >> >> understand your use case correctly, the answer is yes. -device / >> >> device_add can't do that. >> > >> > Could you elaborate why why -device/device_add can't do that? >> >> -device / device_add create, configure, and realize a new device. >> >> They can't reconfigure an existing device. In particular, they can't be >> used to control an existing device's power state. > > Sorry, I've misread as we can't use both for creating device in powered off state. > > Perhaps we should consider a new specialized QMP command to > manipulate runtime power state. (Like it was suggested by Daniel) I prefer few generic commands to many specialized commands whenever practical. However, designing a generic interface can be harder, sometimes much harder, than designing a specialized one. The generic command Salil proposed has serious flaws, as discussed upthread. None of us has promising ideas on how to do a generic command that isn't flawed by design. A more specialized one seems to be the only visible path forward. >> >> qom-set could, but friends don't let friends use it in production. >> >> >> >> Any other prior art for controlling device state at run time via QMP? >> >> >> >> [...]