Installation guide

Chapter 4. Virtualization limitations

Para-virtualized devices

Para-virtualized devices, which use the virtio drivers, are PCI devices. Presently, guests are

limited to a maximum of 32 PCI devices. Some PCI devices are critical for the guest to run and

these devices cannot be removed. The default, required devices are:

• the host bridge,

• the ISA bridge and usb bridge (The usb and isa bridges are the same device),

• the graphics card (using either the Cirrus or qxl driver), and

• the memory balloon device.

Out of the 32 available PCI devices for a guest 4 are not removable. This means there are only

28 PCI slots available for additional devices per guest. Every para-virtualized network or block

device uses one slot. Each guest can use up to 28 additional devices made up of any combination

of para-virtualized network, para-virtualized disk devices, or other PCI devices using VT-d.

Migration limitations

Live migration is only possible with CPUs from the same vendor (that is, Intel to Intel or AMD to

AMD only).

The No eXecution (NX) bit must be set to on or off for both CPUs for live migration.

Storage limitations

The host should not use disk labels to identify file systems in the fstab file, the initrd file or

used by the kernel command line. If less privileged users, especially virtualized guests, have write

access to whole partitions or LVM volumes the host system could be compromised.

Guest should not be given write access to whole disks or block devices (for example, /dev/sdb).

Virtualized guests with access to block devices may be able to access other block devices on the

system or modify volume labels which can be used to compromise the host system. Use partitions

(for example, /dev/sdb1) or LVM volumes to prevent this issue.

SR-IOV limitations

SR-IOV is only supported with the following devices:

Intel® 82576NS Gigabit Ethernet Controller (igb driver)

Intel® 82576EB Gigabit Ethernet Controller (igb driver)

Neterion X3100 Series 10GbE PCIe (vxge driver)

Intel® 82599ES 10 Gigabit Ethernet Controller (ixgbe driver)

Intel® 82599EB 10 Gigabit Ethernet Controller (ixgbe driver)

PCI passthrough limitations

PCI passthrough (attaching PCI devices to guests) should work on systems with the AMD IOMMU

or Intel VT-d technologies.

4.3. Application limitations

There are aspects of virtualization which make virtualization unsuitable for certain types of

applications.