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Revision as of 03:23, 4 February 2010 by Poelstra (talk | contribs) (accepted by FESCo on 2010-02-02)
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KVM Stable PCI Addresses


Allow devices in KVM guest virtual machines to retain the same PCI address allocations as other devices are added or removed from the guest configuration.

This is particularily important for Windows guests in order to prevent warnings or reactivation when device addresses change.


Current status

  • Targeted release: Fedora 13
  • Last updated: 2010-01-26
  • Percentage of completion: 100%


  • New QEMU option -device permits specification of PCI device * address. It doesn't work for VGA devices, yet, but QEMU always assigns PCI device address 2 to the VGA.
  • libvirt uses this to keep PCI device addresses stable.

Detailed Description

QEMU allocates PCI addresses to devices (mostly) in the order it creates the devices, which depends in part on the order they appear on the command line. Built-in PCI devices - like the IDE, USB and VGA controllers - are allocated first.

Windows will warn users when a device's PCI address is changed and may even require the Windows install to be reactivated. In order to prevent this, we should do what we can to ensure that the device does not move PCI between slots as devices are added or removed to/from the guest configuration.

A related problem is that of guest ABI changes between versions of QEMU. That is, updating to a newer version of QEMU may cause devices to change subtly (e.g. PCI class of a device or additional capabilities) which again may require Windows to be reactivated. See also Features/KVM Stable Guest ABI.

A number of solutions were discussed upstream. In the end, it was decided to let QEMU allocate device addresses for new devices, then store them in the guest's XML configuration, and specify them on the command line when recreating the guest. This relies on an an improved info pci for querying the address, and the new -device for specifying it.

Benefit to Fedora

This feature would remove a significant issue with Fedora's virtualization support of Windows.


Requires work on both QEMU and libvirt. No other Fedora packages would be affected.

How To Test

  1. Start a guest with two PCI NICs
  2. Note the PCI slot numbers of each NIC
  3. Remove the first NIC and re-start the guest
  4. Check that the slot number of the second NIC hasn't changed

User Experience

It should be possible to add/remove devices without causing Windows guests to require reactivation.


The main dependency is a mature -device in upstream QEMU.

Contingency Plan

No contingency plan required; if one of the solutions is not implemented, the feature will not be available and nothing else will be affected.


Release Notes

KVM guests in Fedora now have stable PCI addresses, reducing the chance that Windows guests will require reactivation as guest configuration is modified.

Comments and Discussion