Re: [PATCH 0/8] Force detach PCI devices for ACPI-based and PCIe native hot-unplug

[Dongli Zhang <dongli.zhang@xxxxxxxxxx>]

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.
>> 
>> 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.

Thank you very much!

Dongli Zhang