Installation guide

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.
Hence, of the 32 available PCI devices for a guest, 4 are not removable. This means there
are 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.
Mig rat io n limit at io n s
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.
See Chapter 21, Xen live migration and Chapter 22, KVM live migration for more details on live
migration.
St o rag e limit at io n s
The host should not use disk labels to identify file systems in the /etc/fstab file, the
initrd file or in the kernel command line. A security weakness exists if less privileged
users or guests have write access to entire partitions or LVM volumes.
Guests should not be given write access to whole disks or block devices (for example,
/dev/sdb). 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.
Instead, you should use partitions (for example, /dev/sdb1) or LVM volumes.
5.3. Xen limit at ions
Note
All limitations in this chapter are for Red Hat Enterprise Linux 5.11 except where noted.
Xen h o st ( d o m0) limit at ion s
A limit of 254 para-virtualized block devices per host exists. The total number of block devices
attached to guests cannot exceed 254.
Red Hat En t erp rise Lin ux 5 Virt ualizat ion Guid e
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