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Mon, 20 Oct 2025 11:22:14 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.19]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id AAB571800451; Mon, 20 Oct 2025 11:22:11 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id CB84A21E6A27; Mon, 20 Oct 2025 13:22:08 +0200 (CEST) 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: <20251017165044.76b39f5c@fedora> (Igor Mammedov's message of "Fri, 17 Oct 2025 16:50:44 +0200") 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> Date: Mon, 20 Oct 2025 13:22:08 +0200 Message-ID: <87v7k96cnz.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=170.10.129.124; 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_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=unavailable 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 Igor Mammedov writes: > On Thu, 09 Oct 2025 16:55:54 +0200 > Markus Armbruster wrote: > >> Igor Mammedov writes: [...] >> > It's likely /me who to blame for asking to invent generic >> > device-set QMP command. >> > I see another application (beside ARM CPU power-on/off) for it, >> > PCI devices to simulate powering on/off them at runtime without >> > actually removing device. >> >> I prefer generic commands over collecting ad hoc single-purpose >> commands, too. Getting the design right can be difficult. >> >> > wrt command, >> > I'd use only 'id' with it to identify target device >> > (i.e. no template matching nor QMP path either). >> > To enforce rule, what user hasn't named explicitly by providing 'id' >> > isn't meant to be accessed/manged by user later on. >> >> Works well, except when we need to access / manage onboard devices. >> That's still an unsolved problem. >> >> > potentially we can invent specialized power_set/get command as >> > an alternative if it makes design easier. >> > But then we would be spawning similar commands for other things, >> > where as device-set would cover it all. But then I might be >> > over-complicating things by suggesting a generic approach. >> >> Unclear. >> >> 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. 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? [...]