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There have been some reports that, due to libvirt always trying to assign the lowest numbered macvtap / tap device name possible, a new guest would sometimes be started using the same tap device name as previously used by another guest that is in the process of being destroyed *as the new guest is starting. In some cases this has led to, for example, the old guest's qemuProcessStop() code deleting a port from an OVS switch that had just been re-added by the new guest (because the port name is based on only the device name using the port). Similar problems can happen (and I believe have) with nwfilter rules and bandwidth rules (which are both instantiated based on the name of the tap device). A couple patches have been previously proposed to change the ordering of startup and shutdown processing, or to put a mutex around everything related to the tap/macvtap device name usage, but in the end no matter what you do there will still be possible holes, because the device could be deleted outside libvirt's control (for example, regular tap devices are automatically deleted when the qemu process terminates, and that isn't always initiated by libvirt but could instead happen completely asynchronously - libvirt then has no control over the ordering of shutdown operations, and no opportunity to protect it with a mutex.) But this only happens if a new device is created at the same time as one is being deleted. We can effectively eliminate the chance of this happening if we end the practice of always looking for the lowest numbered available device name, and instead just keep an integer that is incremented each time we need a new device name. At some point it will need to wrap back around to 0 (in order to avoid the IFNAMSIZ 15 character limit if nothing else), and we can't guarantee that the new name really will be the *least* recently used name, but "math" suggests that it will be *much* less common that we'll try to re-use the *most* recently used name. This patch implements such a counter for macvtap/macvlan, replacing the existing, and much more complicated, "ID reservation" system. The counter is set according to whatever macvtap/macvlan devices are already in use by guests when libvirtd is started, incremented each time a new device name is needed, and wraps back to 0 when either INT_MAX is reached, or when the resulting device name would be longer than IFNAMSIZ-1 characters (which actually is what happens when the template for the device name is "maccvtap%d"). The result is that no macvtap name will be re-used until the host has created (and possibly destroyed) 99,999,999 devices. Signed-off-by: Laine Stump <laine@redhat.com> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
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==============================
Libvirt API for virtualization
==============================
Libvirt provides a portable, long term stable C API for managing the
virtualization technologies provided by many operating systems. It
includes support for QEMU, KVM, Xen, LXC, bhyve, Virtuozzo, VMware
vCenter and ESX, VMware Desktop, Hyper-V, VirtualBox and the POWER
Hypervisor.
For some of these hypervisors, it provides a stateful management
daemon which runs on the virtualization host allowing access to the
API both by non-privileged local users and remote users.
Layered packages provide bindings of the libvirt C API into other
languages including Python, Perl, PHP, Go, Java, OCaml, as well as
mappings into object systems such as GObject, CIM and SNMP.
Further information about the libvirt project can be found on the
website:
https://libvirt.org
License
=======
The libvirt C API is distributed under the terms of GNU Lesser General
Public License, version 2.1 (or later). Some parts of the code that are
not part of the C library may have the more restrictive GNU General
Public License, version 2.0 (or later). See the files ``COPYING.LESSER``
and ``COPYING`` for full license terms & conditions.
Installation
============
Instructions on building and installing libvirt can be found on the website:
https://libvirt.org/compiling.html
Contributing
============
The libvirt project welcomes contributions in many ways. For most components
the best way to contribute is to send patches to the primary development
mailing list. Further guidance on this can be found on the website:
https://libvirt.org/contribute.html
Contact
=======
The libvirt project has two primary mailing lists:
* libvirt-users@redhat.com (**for user discussions**)
* libvir-list@redhat.com (**for development only**)
Further details on contacting the project are available on the website:
https://libvirt.org/contact.html