[Bug 2033681] Re: Calico still uses vif type tap and it causes failures with libvirt 9.5.0

Fri May 22 04:35:12 UTC 2026

Uploaded to noble, thanks.

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https://bugs.launchpad.net/bugs/2033681

Title:
  Calico still uses vif type tap and it causes failures with libvirt
  9.5.0

Status in Ubuntu Cloud Archive:
  In Progress
Status in Ubuntu Cloud Archive caracal series:
  In Progress
Status in Ubuntu Cloud Archive dalmatian series:
  Won't Fix
Status in Ubuntu Cloud Archive epoxy series:
  In Progress
Status in Ubuntu Cloud Archive flamingo series:
  In Progress
Status in Ubuntu Cloud Archive gazpacho series:
  Fix Committed
Status in OpenStack Compute (nova):
  Fix Released
Status in nova package in Ubuntu:
  Fix Released
Status in nova source package in Noble:
  In Progress
Status in nova source package in Questing:
  In Progress
Status in nova source package in Resolute:
  Fix Released

Bug description:
  [ Impact ]

  Starting with libvirt 9.5.0, the default behavior for TAP devices changed; it now expects to manage the creation and lifecycle of the TAP device itself.
  In the case of using OpenStack with Calico networking, Calico is designed to pre-create the TAP device before handing it off to libvirt. Because libvirt 10.0.0 tries to "own" the device Nova already created, it fails to launch the instance.

  The patch explicitly adds managed="no" to the interface configuration
  in the libvirt domain XML. This tells libvirt to skip its management
  attempt and simply use the device provided by Nova, restoring the
  intended workflow.

  [ Test Plan ]

  Ubuntu does not have support for Neutron Calico (it isn't packaged)
  but to test and verify this issue we don't need it because it is just
  a matter of adding a tap device to an existing vm which we can do
  manually.

  To test this we can do:

   * Deploy Openstack with Neutron OVN

   * Create a guest vm with one port (the tap device will be created by
  libvirt)

  # openstack server create --flavor m1.tiny --image cirros --network
  test-net test-vm

   * Stop the vm

  # openstack server stop test-vm
  or
  # virsh shutdown instance-00000001

   * Manually create a new tap device and add it to the vm libvirt xml

  # sudo ip tuntap add dev tap1 mode tap
  # sudo ip link set tap1 up
  # virsh edit instance-00000001

  On the <interface> section replace with  <target dev='tap1'/>

   * Start the vm

  # virsh start instance-00000001
  error: Failed to start domain 'instance-00000001'
  error: Requested operation is not valid: The tap1 interface already exists

  * Without the patch this will cause an error but with the patch it
  should work.

  * With the patch, the VM can boot successfully.

  This patch explicitly adds managed="no" to the interface configuration
  in the libvirt domain XML. This tells libvirt to skip its management
  attempt and simply use the device provided by Nova, restoring the
  intended workflow.

  We will do this manually and edit the XML again and add managed = no.

  Now try to start the VM again;
  # virsh start instance-00000001
  Domain 'instance-00000001' started

  [Where problems could occur]

  This change specifically targets the XML generation for TAP interfaces.
  Since Noble requires libvirt >= 10.0.0, we are not worried about backwards compatibility with extremely old libvirt versions that might not recognize the attribute.

  [ Other Info ]

  The bug has been reported upstream:
  https://bugs.launchpad.net/nova/+bug/2033681

  This fix is already merged upstream in Nova (see:
  https://review.opendev.org/c/openstack/nova/+/967570) and is required
  for Nova to function correctly on any distribution using libvirt 9.5.0
  or newer, which includes Ubuntu Noble.

  [ Old description ]
  Description
  ===========
  Calico (out of tree) uses vif type tap. But libvirt doesn't like pre-existing tap devices https://github.com/libvirt/libvirt/commit/a2ae3d299cf from libvirt 9.5.0. This causes openstack clusters that run calico networking backend to fail during instance creation.

  Steps to reproduce
  ==================

  Expected result
  ===============
  The VM is able to boot without any problems

  Actual result

  Other information
  =================

  13:34:38 < sean-k-mooney> calico is apparently still using vif type
  tap
  https://github.com/projectcalico/calico/blob/cf7fa35475eba84f5afcd7f53ac7d07dcb403202/networking-
  calico/networking_calico/plugins/ml2/drivers/calico/test/lib.py#L66C31-L66C34

  13:35:06 < sean-k-mooney> vif type tap is not supported by our os-vif code so its usign the legacy fallback
  13:35:51 < sean-k-mooney> https://github.com/openstack/nova/blob/master/nova/virt/libvirt/vif.py#L595-L596
  13:36:15 < sean-k-mooney> https://github.com/openstack/nova/blob/master/nova/virt/libvirt/vif.py#L420-L430
  13:36:48 < sean-k-mooney> https://github.com/openstack/nova/blob/master/nova/virt/libvirt/designer.py#L44-L55

  13:37:40 < sean-k-mooney> zer0c00l: with that said the tap was always ment to be created by libvirt so it sound like calico might have been doing things it shoudl not have been
  13:38:03 < zer0c00l> sean-k-mooney: Thanks for looking into this. :(
  13:38:36 < sean-k-mooney> we could proably correct this with a bug fix
  13:38:52 < sean-k-mooney> jsut setting managed='no'
  13:39:13 < sean-k-mooney> here https://github.com/openstack/nova/blob/master/nova/virt/libvirt/vif.py#L427
  13:39:54 < sean-k-mooney> the problem is that the there is no way to test this really upstream
  13:40:06 < sean-k-mooney> well beyond unit/fucntional tests
  13:40:12 < sean-k-mooney> but we dont have any calico ci
  13:40:37 < sean-k-mooney> calico should be the only backend using vif_type=tap
  13:40:52 < sean-k-mooney> but im not sure if we woudl need a config option in the workarounds section for this or not

  Potential patch
  ===============
  diff --git a/nova/virt/libvirt/config.py b/nova/virt/libvirt/config.py
  index 47e92e3..5af3ce4 100644
  --- a/nova/virt/libvirt/config.py
  +++ b/nova/virt/libvirt/config.py
  @@ -1749,6 +1749,7 @@
           self.device_addr = None
           self.mtu = None
           self.alias = None
  +        self.managed = 'no'

       def __eq__(self, other):
           if not isinstance(other, LibvirtConfigGuestInterface):
  @@ -1851,7 +1852,7 @@
               dev.append(vlan_elem)

           if self.target_dev is not None:
  -            dev.append(etree.Element("target", dev=self.target_dev))
  +            dev.append(etree.Element("target", dev=self.target_dev, managed=self.managed))

           if self.vporttype is not None:
               vport = etree.Element("virtualport", type=self.vporttype)

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