Chapter 4. Virtualization Restrictions

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This chapter covers additional support and product restrictions of the virtualization packages in Red Hat Enterprise Linux 6.

4.1. KVM Restrictions

The following restrictions apply to the KVM hypervisor:
Maximum vCPUs per guest
The maximum amount of virtual CPUs that is supported per guest varies depending on which minor version of Red Hat Enterprise Linux 6 you are using as a host machine. The release of 6.0 introduced a maximum of 64, while 6.3 introduced a maximum of 160. As of version 6.7, a maximum of 240 virtual CPUs per guest is supported.
Constant TSC bit
Systems without a Constant Time Stamp Counter require additional configuration. Refer to Chapter 14, KVM Guest Timing Management for details on determining whether you have a Constant Time Stamp Counter and configuration steps for fixing any related issues.
Virtualized SCSI devices
SCSI emulation is not supported with KVM in Red Hat Enterprise Linux.
Virtualized IDE devices
KVM is limited to a maximum of four virtualized (emulated) IDE devices per guest virtual machine.
Migration restrictions
Device assignment refers to physical devices that have been exposed to a virtual machine, for the exclusive use of that virtual machine. Because device assignment uses hardware on the specific host where the virtual machine runs, migration and save/restore are not supported when device assignment is in use. If the guest operating system supports hot plugging, assigned devices can be removed prior to the migration or save/restore operation to enable this feature.
Live migration is only possible between hosts with the same CPU type (that is, Intel to Intel or AMD to AMD only).
For live migration, both hosts must have the same value set for the No eXecution (NX) bit, either on or off.
For migration to work, cache=none must be specified for all block devices opened in write mode.


Failing to include the cache=none option can result in disk corruption.
Storage restrictions
There are risks associated with giving guest virtual machines write access to entire disks or block devices (such as /dev/sdb). If a guest virtual machine has access to an entire block device, it can share any volume label or partition table with the host machine. If bugs exist in the host system's partition recognition code, this can create a security risk. Avoid this risk by configuring the host machine to ignore devices assigned to a guest virtual machine.


Failing to adhere to storage restrictions can result in risks to security.
Core dumping restrictions
Core dumping uses the same infrastructure as migration and requires more device knowledge and control than device assignement can provide. Therefore, core dumping is not supported when device assignment is in use.
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