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Tsirkin" To: Igor Mammedov Cc: Dongli Zhang , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , qemu-devel@nongnu.org, qemu-s390x@nongnu.org, xen-devel@lists.xenproject.org, dave@treblig.org, anisinha@redhat.com, philmd@mailo.com, aurelien@aurel32.net, mjrosato@linux.ibm.com, alifm@linux.ibm.com, farman@linux.ibm.com, richard.henderson@linaro.org, iii@linux.ibm.com, david@kernel.org, pasic@linux.ibm.com, borntraeger@linux.ibm.com, cohuck@redhat.com, alex@shazbot.org, clg@redhat.com, akrowiak@linux.ibm.com, jjherne@linux.ibm.com, sstabellini@kernel.org, anthony@xenproject.org, edgar.iglesias@gmail.com, pbonzini@redhat.com, eblake@redhat.com, armbru@redhat.com, joe.jin@oracle.com Subject: Re: [PATCH 0/8] Force detach PCI devices for ACPI-based and PCIe native hot-unplug Message-ID: <20260903160401-mutt-send-email-mst@kernel.org> References: <20260824011420.752806-1-dongli.zhang@oracle.com> <01ee4841-76d8-4749-a80e-eb945ab8799a@oracle.com> <20260903172835.0e253a25@imammedo> MIME-Version: 1.0 In-Reply-To: <20260903172835.0e253a25@imammedo> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: EAh02STIwY8mYAYNjkdh-89iPgDAiRh1oc3sXZI54ZU_1788466646 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-purgate-ID: tlsNG-ebf023/1788466650-585C7B50-598268B2/0/0 X-purgate-type: clean X-purgate-size: 6661 On Thu, Sep 03, 2026 at 05:28:35PM +0200, Igor Mammedov wrote: > On Wed, 26 Aug 2026 09:15:47 -0700 > Dongli Zhang wrote: > > > On Mon, Aug 24, 2026 7:44:49AM -0700, Daniel P. Berrangé wrote: > > > On Sun, Aug 23, 2026 at 06:13:30PM -0700, Dongli Zhang wrote: > > >> Hot-unplugging a PCI device can require cooperation from the guest. For > > >> ACPI PCI hotplug, QEMU notifies the guest through ACPI and the guest > > >> eventually writes the ACPI PCI eject register. For PCIe native hotplug, > > >> QEMU notifies the guest through the PCIe hotplug mechanism and waits for > > >> the slot unplug flow to complete. Only after that completion does QEMU > > >> unrealize the device and emit DEVICE_DELETED. > > >> > > >> This can leave a device stuck in the unplug pending state when the guest > > >> does not cooperate. Examples include: > > >> > > >> 1. The guest has panicked, or the relevant ACPI/PCI hotplug driver is > > >> unavailable. > > >> > > >> 2. The guest is stalled and cannot handle the hot-unplug event. For > > >> example, stalling the Linux [irq/9-acpi] kernel thread can reproduce this > > >> for ACPI-based hot-unplug. > > >> > > >> 3. The device was attached to a slot that the guest cannot use. For > > >> example, a pcie-root-port only supports slot 0. If a device is added to a > > >> non-zero slot below a pcie-root-port, the guest may never discover the > > >> device and therefore may never complete the unplug request. > > all of above is actually expected, no (functioning) driver => no hotplug/unplug. > it's the guest problem. Once device it exposed to guest its life-cycle > not longer owned by QEMU. > > That's what one would see in real hw as well, you press eject button > but it will not do anything if OS doesn't process it. > also see comment at the end. > > > >> > > >> The non-zero slot case has also been discussed in: > > >> > > >> hw/pci: warn when PCIe device is plugged into non-zero slot of downstream port > > >> https://gitlab.com/qemu-project/qemu/-/commit/ > > > ca92eb5defcf9d1c2106341744a73a03cf26e824 > > >> > > >> hw/pci: add comment to explain checking for available function 0 in pci hotplug > > >> https://gitlab.com/qemu-project/qemu/-/ > > > commit/67d045a0ef5b9c5f871c3a1d87325a8a42d2b9d5 > > >> > > >> pci: don't skip function 0 occupancy verification for devfn auto assign > > >> https://gitlab.com/qemu-project/qemu/-/commit/ > > > e228d62b4af29bca698ec57efdceb46f392f5444 > > >> > > >> For example, if root-port.1 is a pcie-root-port, the following command adds > > >> a vhost-scsi-pci device to an invalid slot: > > >> > > >> (qemu) device_add vhost-scsi-pci,id=scsi01,wwpn=naa.5001405324af0985,bus=root-port.1,addr=01.0 > > >> warning: PCI: slot 1 is not valid for vhost-scsi-pci, parent device only allows plugging into slot 0. > > > > > > This rather looks like it should be a fatal error, not a mere warning. > > > > > > If I follow the commit ca92eb5def it links to https://bugzilla.redhat.com/show_bug.cgi?id=2128929 > > > which states that this configuration is going to lead to a crash in > > > QEMU on guest OS shutdown. IMHO that crash is sufficient to justify > > > making this a fatal error. > > > > > > If we actually wanted this to remain a warning, then that shutdown > > > crash would need to be fixed. > > > > > > > Thank you very much! > > > > I see that the issue has been fixed. The ticket mentions the following. > > > > "What I am observing is that it seems when the slot ID != 0, the guest OS seems > > to ignore this and we never seem to hit ich9_pm_device_unplug_cb()." > > > > Based on my experience and evaluation, ACPI-based hotplug is more likely to > > encounter an issue where the guest VM does not respond to an unplug operation. > > I'm not sure it's a good idea to delete device when guest still thinks it's there > (you can make guesses on QEMU side if it's in use, how useful those are is questionable). > > as far as I know, ACPI hotplug has no notion of surprise removal (pls educate me if it's not the case), why would it not? how do you think you can pull a laptop out of a dock? I expect bus check + _STA and config space saying it is gone will do exactly that. Here's linux code: static void acpiphp_check_bridge(struct acpiphp_bridge *bridge) { struct acpiphp_slot *slot; /* Bail out if the bridge is going away. */ if (bridge->is_going_away) return; if (bridge->pci_dev) pm_runtime_get_sync(&bridge->pci_dev->dev); list_for_each_entry(slot, &bridge->slots, node) { struct pci_bus *bus = slot->bus; struct pci_dev *dev, *tmp; if (slot_no_hotplug(slot)) { ; /* do nothing */ } else if (device_status_valid(get_slot_status(slot))) { /* remove stale devices if any */ list_for_each_entry_safe_reverse(dev, tmp, &bus->devices, bus_list) if (PCI_SLOT(dev->devfn) == slot->device) trim_stale_devices(dev); /* configure all functions */ enable_slot(slot, true); } else { disable_slot(slot); } } if (bridge->pci_dev) pm_runtime_put(&bridge->pci_dev->dev); } so weirdly it wants bus check on a parent bus, otherwise it will not trim devices? probably a bug, but easy to work around. > so I wouldn't do what you are proposing here at all, it's basically asking for disaster to happen. > And all this is basically for dealing with abused qemu flexibility. > > Please (re)formulate usecase and make it more clear as what is eludes me > no matter how many times i've read this cover letter. > > On positive note: > > What you can try to implement is native PCI-E support for surprise removal. > How hard that would be I don't know. And I would well expect if one deviates from > real hw expectations/configs (such as not 0 slot/partial func removal), > one would quickly stumble upon issues as that's not what what vendors write/test > drivers for. > > Even if it's not likely to be used in practice (guest still might not support it), > it may serve as test-bed for guest drivers. > > > Thank you very much! > > > > Dongli Zhang > >