From Fedora Project Wiki

< Features

Revision as of 17:29, 6 July 2009 by Markmc (talk | contribs) (pci_add -> host_net_add)

KVM NIC Hotplug


Allow the addition of a guest network interface (NIC) a guest virtual machine without needing to restart the guest.


Current status

  • Targeted release: Fedora 12
  • Last updated: 2009-06-10
  • Percentage of completion: 0%


  • Allow the QEMU host_net_add monitor command to be passed a file descriptor via SCM_RIGHTS
  • Make libvirt use a unix domain socket for QEMU monitor interaction
  • Implement NIC hotplug in the libvirt QEMU driver by passing the tap file descriptor to QEMU over the monitor socket
  • Check that it works well in virt-manager


  • None

Detailed Description

NIC hotplug is a relatively common virtualization feature. The idea is that one should be able to add and remove NICs from a guest while it is running. For example, you might want to connect the guest to an additional network, without having to restart the guest.

This is currently supported by QEMU, but only if you allow QEMU to create and configure the tap device itself.

libvirt prefers to create the tap device and pass the file descriptor to QEMU via the command line. In order to support hotplug, we need QEMU to allow us to pass a file descriptor over the QEMU monitor socket.

The only way this could conceivably be supported is to have QEMU support file descriptor passing via SCM_RIGHTS on a unix socket.

The alternative is to use persistent tap devices, but this makes error/cleanup handling far more difficult and also requires that the QEMU process has permissions to /dev/net/tun.

Benefit to Fedora

NIC hotplug is a commonly requested virtualization feature.


As described above, changes to QEMU and libvirt are required.

How To Test

  1. Run virt-manager and start a guest
  2. Go to Details, click "Add Hardware", Network, add to Virtual Network, Forward, Finish
  3. virt-manager should not display a "The device could not be attached to the running machine" warning
  4. Log into the guest and use "ifconfig" to confirm that the NIC is visible in the guest
  5. Bring the interface up and test that it's working correctly

User Experience

See the previous section.


None, outside of what's describe above.

Contingency Plan

We may consider persistent tap devices if upstream QEMU does not accept the need for SCM_RIGHTS support.

If neither gets implemented on time, the status quo will remain - virt-manager warns users that they have to restart the guest in order for the NIC to be visible in the guest.



Release Notes

Fedora 11 adds the ability to add a network interface to a running KVM guest using libvirt/virt-manager.

Comments and Discussion