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Chapter 21. Feature support and limitations in RHEL 8 virtualization

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This document provides information about feature support and restrictions in Red Hat Enterprise Linux 8 (RHEL 8) virtualization.

21.1. How RHEL virtualization support works

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.

Important

Many of these limitations do not apply to other virtualization solutions provided by Red Hat, such as Red Hat Virtualization (RHV), OpenShift Virtualization or Red Hat OpenStack Platform (RHOSP).

21.3. Unsupported features in RHEL 8 virtualization

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

Important

Many of these limitations may not apply to other virtualization solutions provided by Red Hat, such as OpenShift Virtualization or Red Hat OpenStack Platform (RHOSP).

Features supported by RHV 4.2 and later, or RHOSP 13 and later, are described as such in the following paragraphs.

Host system architectures

RHEL 8 with KVM is not supported on any host architectures that are not listed in Recommended features in RHEL 8 virtualization.

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

Guest operating systems

KVM virtual machines (VMs) that use the following guest operating systems (OSs) are not supported on a RHEL 8 host:

  • Microsoft Windows 8.1 and earlier
  • Microsoft Windows Server 2008 R2 and earlier
  • macOS
  • Solaris for x86 systems
  • Any OS released before 2009

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.

Creating VMs in containers

Red Hat does not support creating KVM virtual machines in any type of container that includes the elements of the RHEL 8 hypervisor (such as the QEMU emulator or the libvirt package).

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

The QEMU command line

QEMU is an essential component of the virtualization architecture in RHEL 8, but it is difficult to manage manually, and improper QEMU configurations may cause security vulnerabilities. Therefore, using qemu-* command-line utilities, such as qemu-kvm is not supported by Red Hat. Instead, use libvirt utilities, such as virsh, virt-install, and virt-xml, as these orchestrate QEMU according to the best practices.

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

vCPU hot unplug

Removing a virtual CPU (vCPU) from a running VM, also referred to as a vCPU hot unplug, is not supported in RHEL 8.

Memory hot unplug

Removing a memory device attached to a running VM, also referred to as a memory hot unplug, is unsupported in RHEL 8.

QEMU-side I/O throttling

By using the virsh blkdeviotune utility to configure maximum input and output levels for operations on virtual disk, also known as QEMU-side I/O throttling, is not supported in RHEL 8.

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.

Storage live migration

Migrating a disk image of a running VM between hosts is not supported in RHEL 8.

Other solutions:

  • Storage live migration is supported in RHOSP, but with some limitations. For details, see Migrate a Volume.

Live snapshots

Creating or loading a snapshot of a running VM, also referred to as a live snapshot, is not supported in RHEL 8.

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:

vhost-user

RHEL 8 does not support the implementation of a user-space vHost interface.

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

S3 and S4 system power states

Suspending a VM to the Suspend to RAM (S3) or Suspend to disk (S4) system power states is not supported. Note that these features are disabled by default, and enabling them will make your VM not supportable by Red Hat.

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

S3-PR on a multipathed vDisk

SCSI3 persistent reservation (S3-PR) on a multipathed vDisk is not supported in RHEL 8. As a consequence, Windows Cluster is not supported in RHEL 8.

virtio-crypto

Using the virtio-crypto device in RHEL 8 is not supported and its use is therefore highly discouraged.

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

Incremental live backup

Configuring a VM backup that only saves VM changes since the last backup, also known as incremental live backup, is not supported in RHEL 8, and Red Hat highly discourages its use.

net_failover

Using the net_failover driver to set up an automated network device failover mechanism is not supported in RHEL 8.

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

TPM passthrough

Assigning a physical Trusted Platform Module (TPM) device by using the passthrough back end to a VM is unsupported on RHEL 8 hosts. Instead, use the vTPM functionality, which uses the emulator back end and is fully supported.

virtiofs

Sharing files between the host and its VMs by using the virtiofs file system is unsupported in RHEL 8.

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

TCG

QEMU and libvirt include a dynamic translation mode by using the QEMU Tiny Code Generator (TCG). This mode does not require hardware virtualization support. However, TCG is not supported by Red Hat.

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

  • The configuration file of a TCG guest contains the following line:

    <domain type='qemu'>
  • The configuration file of a KVM guest contains the following line:

    <domain type='kvm'>

SR-IOV InfiniBand networking devices

Attaching InfiniBand networking devices to VMs by using Single-root I/O virtualization (SR-IOV) is not supported.

21.4. Resource allocation limits in RHEL 8 virtualization

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.

Important

Many of these limitations do not apply to other virtualization solutions provided by Red Hat, such as Red Hat Virtualization (RHV), OpenShift Virtualization, or Red Hat OpenStack Platform (RHOSP).

Maximum vCPUs per VM

For the maximum amount of vCPUs and memory that is supported on a single VM running on a RHEL 8 host, see: Virtualization limits for Red Hat Enterprise Linux with KVM

PCI devices per VM

RHEL 8 supports 64 PCI device slots per VM bus, and 8 PCI functions per device slot. This gives a theoretical maximum of 512 PCI functions per bus when multi-function capabilities are enabled in the VM, and no PCI bridges are used.

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.

Virtualized IDE devices

KVM is limited to a maximum of 4 virtualized IDE devices per VM.

21.5. Supported disk image formats

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.

Supported disk image formats for VMs

You can use disk images that use the following formats to run VMs in RHEL:

  • qcow2 - Provides certain additional features, such as compression.
  • raw - Might provide better performance.
  • luks - Disk images encrypted by using the Linux Unified Key Setup (LUKS) specification.

Supported disk image formats for conversion

  • If required, you can convert your disk images between the raw and qcow2 formats by using the qemu-img convert command.
  • If you require converting a vmdk disk image to a raw or qcow2 format, convert the VM that uses the disk to KVM by using the virt-v2v utility.
  • To convert other disk image formats to raw or qcow2, you can use the qemu-img convert command. For a list of formats that work with this command, see the QEMU documentation.

    Note that in most cases, converting the disk image format of a non-KVM virtual machine to qcow2 or raw is not sufficient for the VM to correctly run on RHEL KVM. In addition to converting the disk image, corresponding drivers must be installed and configured in the guest operating system of the VM. For supported hypervisor conversion, use the virt-v2v utility.

21.6. An overview of virtualization features support in RHEL 8

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

Table 21.1. Device hot plug and hot unplug
 Intel 64 and AMD64IBM ZIBM POWER

CPU hot plug

Supported

Supported

Supported

CPU hot unplug

UNSUPPORTED

UNSUPPORTED

UNSUPPORTED

Memory hot plug

Supported

UNSUPPORTED

Supported

Memory hot unplug

UNSUPPORTED

UNSUPPORTED

UNSUPPORTED

PCI hot plug

Supported

Supported [a]

Supported

PCI hot unplug

Supported

Supported [b]

Supported

[a] Requires using virtio-*-ccw devices instead of virtio-*-pci
[b] Requires using virtio-*-ccw devices instead of virtio-*-pci
Table 21.2. Other selected features
 Intel 64 and AMD64IBM ZIBM POWER

NUMA tuning

Supported

UNSUPPORTED

Supported

SR-IOV devices

Supported

UNSUPPORTED

Supported

virt-v2v and p2v

Supported

UNSUPPORTED

UNSUPPORTED

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.

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