Chapter 21. Feature support and limitations in RHEL 8 virtualization

A set of support limitations applies to virtualization in Red Hat Enterprise Linux 8 (RHEL 8). This means that when you use certain features or exceed a certain amount of allocated resources when using virtual machines in RHEL 8, Red Hat will not support these guests unless you have a specific subscription plan.

Features listed in Recommended features in RHEL 8 virtualization have been tested and certified by Red Hat to work with the KVM hypervisor on a RHEL 8 system. Therefore, they are fully supported and recommended for use in virtualization in RHEL 8.

Features listed in Unsupported features in RHEL 8 virtualization may work, but are not supported and not intended for use in RHEL 8. Therefore, Red Hat strongly recommends not using these features in RHEL 8 with KVM.

Resource allocation limits in RHEL 8 virtualization lists the maximum amount of specific resources supported on a KVM guest in RHEL 8. Guests that exceed these limits are not supported by Red Hat.

In addition, unless stated otherwise, all features and solutions used by the documentation for RHEL 8 virtualization are supported. However, some of these have not been completely tested and therefore may not be fully optimized.

The following features are recommended for use with the KVM hypervisor included with Red Hat Enterprise Linux 8 (RHEL 8):

Any other hardware architectures are not supported for using RHEL 8 as a KVM virtualization host, and Red Hat highly discourages doing so. Notably, this includes the 64-bit ARM architecture (ARM 64).

Note, however, that by default, your guest OS does not use the same subscription as your host. Therefore, you must activate a separate licence or subscription for the guest OS to work properly.

In addition, the pass-through devices that you attach to the VM must be supported by both the host OS and the guest OS.

Similarly, for optimal function of your deployment, Red Hat recommends that the CPU model and features that you define in the XML configuration of a VM are supported by both the host OS and the guest OS.

To view the certified CPUs and other hardware for various versions of RHEL, see the Red Hat Ecosystem Catalog.

When Creating a VM by using the command line, the virt-install utility provides multiple methods of setting the machine type.

The recommended machine types for KVM virtual machines on supported architectures, and the corresponding values for the --machine option, are as follows. Y stands for the latest minor version of RHEL 8.

To obtain the machine type of an existing VM:

# virsh dumpxml VM-name | grep machine=

To view the full list of machine types supported on your host:

# /usr/libexec/qemu-kvm -M help

The following features are not supported by the KVM hypervisor included with Red Hat Enterprise Linux 8 (RHEL 8):

Notably, Red Hat does not support using systems with the 64-bit ARM architecture (ARM 64) for KVM virtualization on RHEL 8.

For a list of guest OSs supported on RHEL hosts, Red Hat Virtualization (RHV), or other virtualization solutions, see Certified Guest Operating Systems in Red Hat OpenStack Platform, Red Hat Virtualization, OpenShift Virtualization and Red Hat Enterprise Linux with KVM.

To create VMs in containers, Red Hat recommends using the OpenShift Virtualization offering.

Note, however, that the qemu-img utility is supported for management of virtual disk images.

To set up I/O throttling in RHEL 8, use virsh blkiotune. This is also known as libvirt-side I/O throttling. For instructions, see Disk I/O throttling in virtual machines.

QEMU-side I/O throttling is also supported in RHOSP. For details, see Setting Resource Limitation on Disk and the Use Quality-of-Service Specifications section in the RHOSP Storage Guide.

In addition, OpenShift Virtualizaton supports QEMU-side I/O throttling as well.

Other solutions:

In addition, note that non-live VM snapshots are deprecated in RHEL 8. Therefore, creating or loading a snapshot of a shut-down VM is supported, but Red Hat recommends not using it.

Other solutions:

Note that vhost-user is supported in RHOSP, but only for virtio-net interfaces. For details, see virtio-net implementation and vhost user ports.

Note that the S3 and S4 states are also currently not supported in any other virtualization solution provided by Red Hat.

Note that virtio-crypto devices are also not supported in any other virtualization solution provided by Red Hat.

Note that net_failover is also currently not supported in any other virtualization solution provided by Red Hat.

Note, however, that using virtiofs is supported by RHEL 9. For more information, see Configuring and managing virtualization in RHEL 9.

TCG-based guests can be recognized by examining its XML configuration, for example using the "virsh dumpxml" command.

The following limits apply to virtualized resources that can be allocated to a single KVM virtual machine (VM) on a Red Hat Enterprise Linux 8 (RHEL 8) host.

Each PCI bridge adds a new bus, potentially enabling another 512 device addresses. However, some buses do not make all 512 device addresses available for the user; for example, the root bus has several built-in devices occupying slots.

To run a virtual machine (VM) on RHEL, you must use a disk image with a supported format. You can also convert certain unsupported disk images to a supported format.

The following tables provide comparative information about the support state of selected virtualization features in RHEL 8 across the supported system architectures.

Note that some of the unsupported features are supported on other Red Hat products, such as Red Hat Virtualization and Red Hat OpenStack platform. For more information, see Unsupported features in RHEL 8 virtualization.