Modern hypervisors rely on CPU extensions called hardware-assisted virtualization: Intel VT-x and AMD-V. They let the processor run guest code directly and trap only sensitive operations, which makes virtual machines fast and simple to implement. If they are disabled, KVM, Hyper-V and Proxmox will refuse to start VMs.
Before you start
- Physical or IPMI access to the machine's firmware setup if you need to change settings.
- On a VPS you cannot change these; ask the provider about nested virtualization instead.
What the extensions provide
| Feature | Intel | AMD | Purpose |
|---|---|---|---|
| CPU virtualization | VT-x (vmx flag) | AMD-V (svm flag) | New CPU modes for guests and hypervisor |
| Second-level page tables | EPT | NPT / RVI | Hardware memory translation, far less overhead |
| I/O virtualization (IOMMU) | VT-d | AMD-Vi | Safe PCI passthrough of GPUs, NICs and disks |
Check support on Linux
grep -E -o 'vmx|svm' /proc/cpuinfo | sort -uvmx means Intel VT-x, svm means AMD-V. Also check:
lscpu | grep -i virtualizationOn Ubuntu, kvm-ok from the cpu-checker package says whether acceleration is usable. If the flag is present but /dev/kvm is missing, the feature is disabled in firmware or the kvm modules are not loaded.
Check support on Windows
Open Task Manager → Performance → CPU and look for Virtualization: Enabled. Or run systeminfo and read the Hyper-V requirements section.
Enable VT-x or AMD-V in firmware
- Reboot and enter setup (often Del, F2 or F10; on servers through the IPMI console).
- Look under Advanced → CPU Configuration or Processor.
- Enable Intel Virtualization Technology or SVM Mode. Enable VT-d / IOMMU too if you plan PCI passthrough.
- Save, then fully power off and on; some boards apply the change only after a cold boot.
Troubleshooting
- Option missing: update the BIOS; some vendors hide it on consumer boards until a newer release.
- Enabled but still not detected on Windows: another hypervisor (Hyper-V, WSL2, Memory Integrity) may own VT-x, blocking VirtualBox or VMware from using it directly.
- IOMMU groups empty: add
intel_iommu=onoramd_iommu=onto the kernel command line.
Frequently asked questions
Is enabling VT-x a security risk?
No meaningful risk for normal use. It is required by many security features, including Windows virtualization-based security.
Can a VM run without VT-x?
Only with slow software emulation such as QEMU TCG. Production hypervisors require it.
Do all server CPUs support it?
Practically all Intel Xeon and AMD EPYC CPUs do, including EPT/NPT.