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I know I can check is VT-X is supported by a processor by egrep '(vmx|svm)' /proc/cpuinfo. But even if it's supported, it can be turned off in the BIOS. How can I detect if it's turned on or off?

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VT-X information is stored in model-specific registers (MSR) 0x34 which can be accessed via msr-tools. Install it by running sudo apt-get install msr-tools then run the following commands

sudo modprobe msr
sudo rdmsr 0x3A

If the return value is 3, 5 or 7 then VT-X is enabled

See Reliably detecting if Intel VT is disabled in the BIOS

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There are two ways to verify:

  1. On Debian-based distros (such as Ubuntu), you can check support for VT-x by installing cpu-checker and run kvm-ok with sudo rights.

    If the CPU is enabled, you should see something like:

     INFO: /dev/kvm exists
     KVM acceleration can be used
    

    Otherwise, you'll get:

     INFO: /dev/kvm does not exist
     HINT:   sudo modprobe kvm_intel
     INFO: Your CPU supports KVM extensions
     INFO: KVM (vmx) is disabled by your BIOS
     HINT: Enter your BIOS setup and enable Virtualization Technology (VT),
        and then hard poweroff/poweron your system
     KVM acceleration can NOT be used
    
  2. On Debian-based and RHEL-based systems, you can use virt-host-validate which is provided by the libvirt-client package. Documentation on usage can be found here.

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A very unenviable way to check (without going into the BIOS) would be to install/open Virtualbox hypervisor, and try installing a 64-bit Linux VM and see if the settings allow you to enable VT-X. Installation of VM in virtualbox is the only context where I've encountered the need to enable VT-X/AMD-V.

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