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

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

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.

> 
> In this situation, the device may be impossible to remove through normal
> guest-cooperative hot-unplug. Production environments also need a host side
> recovery option when the guest kernel is the reason that unplug does not
> complete.
> 
> This series adds a force option to QMP device_del and HMP device_del. When
> requested, QEMU asks the selected hotplug controller to complete the unplug
> through a new force_unplug callback.
> 
> This series implements forced unplug for ACPI PCI hotplug and PCIe native
> hotplug, which cover common pc/q35/virt cases. SHPC is not implemented by
> this series.
> 
> 
> Dongli Zhang (8):
>   qdev: Add force argument to qdev_unplug
>   qdev: hotplug: Add force_unplug handler callback
>   qdev: Support forced device_del in QMP and HMP
>   hw/acpi/pcihp: acpi/pcihp: Add forced slot unplug helper
>   hw/acpi/piix4: Support forced PCI unplug
>   hw/acpi/ich9: Support forced PCI unplug
>   hw/acpi/ged: Support forced PCI unplug
>   hw/pci/pcie: Support forced PCIe native unplug
> 
>  hmp-commands.hx                 | 11 ++++++-----
>  hw/acpi/acpi-pci-hotplug-stub.c |  6 ++++++
>  hw/acpi/generic_event_device.c  | 15 +++++++++++++++
>  hw/acpi/ich9.c                  | 15 +++++++++++++++
>  hw/acpi/pcihp.c                 | 17 +++++++++++++++++
>  hw/acpi/piix4.c                 | 15 +++++++++++++++
>  hw/core/hotplug.c               | 11 +++++++++++
>  hw/isa/lpc_ich9.c               |  1 +
>  hw/pci/pcie.c                   | 20 ++++++++++++++++++++
>  hw/pci/pcie_port.c              |  1 +
>  hw/s390x/s390-pci-bus.c         |  4 ++--
>  hw/vfio/ap.c                    |  2 +-
>  hw/vfio/ccw.c                   |  2 +-
>  hw/vfio/pci.c                   |  2 +-
>  hw/xen/xen-legacy-backend.c     |  2 +-
>  hw/xen/xen_pvdev.c              |  2 +-
>  include/hw/acpi/ich9.h          |  2 ++
>  include/hw/acpi/pcihp.h         |  3 +++
>  include/hw/core/hotplug.h       | 12 ++++++++++++
>  include/hw/core/qdev.h          |  2 +-
>  include/hw/pci/pcie.h           |  2 ++
>  qapi/qdev.json                  | 11 +++++++++--
>  system/qdev-monitor.c           | 23 +++++++++++++++++------
>  23 files changed, 160 insertions(+), 21 deletions(-)
> 
> base-commit: eea8fe61b8be8f3016e522e6af24924a0266ca95
> 
> Thank you very much!
> 
> Dongli Zhang
> 

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 :|