Before creating and running virtual machines with KVM, it’s essential to validate that the host meets the required hardware and software capabilities to ensure optimal performance. This guide describes how to verify system capabilities using tools such as virt-host-validate and virsh.

1. Validating CPU virtualization support

KVM requires processors with hardware virtualization extensions:

  • Intel: VT-x
  • AMD: AMD-V

To check if your CPU is compatible, run:

grep --color -Ew 'svm|vmx|lm' /proc/cpuinfo
  • vmx → Intel CPU with VT-x
  • svm → AMD CPU with AMD-V
  • lm → 64-bit support

If your CPU lacks these extensions, only CPU emulation will be available and performance will be significantly reduced.

2. Checking kernel modules

The kernel modules required for virtualization should be loaded:

lsmod | grep kvm
  • Intel systems: kvm_intel
  • AMD systems: kvm_amd

If they are not present, load them manually:

sudo modprobe kvm
sudo modprobe kvm_intel  # For Intel
sudo modprobe kvm_amd    # For AMD

3. Validating the host with virt-host-validate

The virt-host-validate tool runs a set of checks to ensure the system is ready for KVM:

sudo virt-host-validate

A successful result will show PASS for all tests, including:

  • Hardware virtualization
  • Presence of devices /dev/kvm, /dev/vhost-net and /dev/net/tun
  • Support for CPU and memory controllers

If a test fails it will be marked FAIL, which can affect VM performance.

Example of a successful output

QEMU: Checking for hardware virtualization              : PASS
QEMU: Checking if device /dev/vhost-net exists          : PASS
QEMU: Checking if device /dev/net/tun exists            : PASS
QEMU: Checking for cgroup 'memory' controller support   : PASS
QEMU: Checking for cgroup 'cpu' controller support      : PASS

Example of a failure due to CPU without VT-x/AMD-V

QEMU: Checking for hardware virtualization              : FAIL (Only emulated CPUs are available, performance will be significantly limited)

4. Validating host information with virsh

virsh allows querying host details and hypervisor capabilities:

virsh nodeinfo

Sample output:

CPU model:           x86_64
CPU(s):              24
CPU frequency:       4252 MHz
CPU socket(s):       1
Core(s) per socket:  12
Thread(s) per core:  2
NUMA cell(s):        1
Memory size:         62324512 KiB

To see maximum capabilities for guests:

virsh domcapabilities | grep -i max

Example snippet:

  <vcpu max='255'/>
    <mode name='maximum' supported='yes'>
      <enum name='maximumMigratable'>
      <maxphysaddr mode='passthrough' limit='48'/>

You can also list the types of devices supported by the hypervisor:

virsh domcapabilities | grep diskDevice -A 5

      <enum name='diskDevice'>
        <value>disk</value>
        <value>cdrom</value>
        <value>floppy</value>
        <value>lun</value>
      </enum>

virsh is the command-line tool to manage virtual machines and the hypervisor. It allows administering VMs, monitoring resources, managing storage, snapshots and virtual networks.

References