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. 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). 21.2. 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). Note 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: Getting started with virtualization on IBM POWER Getting started with virtualization on IBM Z 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: To view the full list of machine types supported on your host: Additional resources Unsupported features in RHEL 8 virtualization Resource allocation limits in RHEL 8 virtualization Supported hosts for virtual machine migration 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 more information, see the Red Hat Knowledgebase solution 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 more information, see Red Hat Knowledgebase solutions 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 8. 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'> Additional resources Recommended features in RHEL 8 virtualization Resource allocation limits in RHEL 8 virtualization 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. Additional resources Converting virtual machines from other hypervisors to KVM with virt-v2v in RHEL 7, RHEL 8, and RHEL 9 Converting between virtual disk image formats 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 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 [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 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 .

Commands:
virsh dumpxml VM-name | grep machine=
/usr/libexec/qemu-kvm -M help
<domain type='qemu'>
<domain type='kvm'>
<disk type="block" device="lun">
<hostdev mode='subsystem' type='scsi'>