Chapter 21. Feature support and limitations in RHEL 8 virtualization
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.
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).
21.2. Recommended features in RHEL 8 virtualization
The following features are recommended for use with the KVM hypervisor included with Red Hat Enterprise Linux 8 (RHEL 8):
Host system architectures
RHEL 8 with KVM is only supported on the following host architectures:
- AMD64 and Intel 64
- IBM Z - IBM z13 systems and later
- IBM POWER8
- IBM POWER9
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).
RHEL 8 documentation primarily describes AMD64 and Intel 64 features and usage. For information about the specific of using RHEL 8 virtualization on different architectures, see:
Guest operating systems
Red Hat provides support with KVM virtual machines that use specific guest operating systems (OSs). For a detailed list of supported guest OSs, see the Certified Guest Operating Systems in the Red Hat KnowledgeBase.
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.
Machine types
To ensure that your VM is compatible with your host architecture and that the guest OS runs optimally, the VM must use an appropriate machine type.
When Creating a VM by using the command line, the virt-install
utility provides multiple methods of setting the machine type.
-
When you use the
--os-variant
option,virt-install
automatically selects the machine type recommended for your host CPU and supported by the guest OS. -
If you do not use
--os-variant
or require a different machine type, use the--machine
option to specify the machine type explicitly. -
If you specify a
--machine
value that is unsupported or not compatible with your host,virt-install
fails and displays an error message.
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.
-
On Intel 64 and AMD64 (x86_64):
pc-q35-rhel8.Y.0
--machine=q35
-
On IBM Z (s390x):
s390-ccw-virtio-rhel8.Y.0
--machine=s390-ccw-virtio
-
On IBM POWER (PPC),
pseries-rhel8.Y.0
--machine=pseries
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
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):
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:
- Live snapshots are supported in RHOSP. For details, see Importing virtual machines into the overcloud.
- Live snapshots are also supported on OpenShift Virtualization.
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.
SGIO
Attaching SCSI devices to VMs by using SCSI generic I/O (SGIO) is not supported on RHEL 9. To detect whether your VM has an attached SGIO device, check the VM configuration for the following lines:
<disk type="block" device="lun">
<hostdev mode='subsystem' type='scsi'>
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.
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
andqcow2
formats by using theqemu-img convert
command. -
If you require converting a vmdk disk image to a
raw
orqcow2
format, convert the VM that uses the disk to KVM by using thevirt-v2v
utility. To convert other disk image formats to
raw
orqcow2
, you can use theqemu-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
orraw
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 thevirt-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.
Intel 64 and AMD64 | IBM Z | IBM 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 |
Intel 64 and AMD64 | IBM Z | IBM 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.
Additional sources
- For a complete list of unsupported features of virtual machines in RHEL 8, see Unsupported features in RHEL 8 virtualization.
- For details on the specifics for virtualization on the IBM Z architecture, see How virtualization on IBM Z differs from AMD64 and Intel 64.
- For details on the specifics for virtualization on the IBM POWER architecture, see How virtualization on IBM POWER differs from AMD64 and Intel 64.