To resolve the `VERR_VMX_IN_VMX_ROOT_MODE` error in VirtualBox on Debian Linux,

To resolve the VERR_VMX_IN_VMX_ROOT_MODE error in VirtualBox on Debian Linux, follow these steps:

Cause

The error occurs because the KVM hypervisor is active and monopolizes hardware virtualization (VMX), preventing VirtualBox from accessing it.

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Solutions

Choose one of the methods below:

Method 1: Unload KVM Modules Temporarily (Test First)

1. Unload KVM modules (reverts after reboot):

   sudo rmmod kvm_intel kvm_amd kvm

2. Verify modules are unloaded:

   lsmod | grep kvm  # Should return nothing

3. Launch your VirtualBox VM. If successful, make the change permanent with Method 2.

Method 2: Permanently Disable KVM via Blacklisting

1. Create a blacklist file:

   sudo nano /etc/modprobe.d/blacklist-kvm.conf

2. Add these lines (based on your CPU):
   • For Intel CPUs:

     blacklist kvm_intel
     blacklist kvm

   • For AMD CPUs:

     blacklist kvm_amd
     blacklist kvm

3. Save (Ctrl+O → Enter → Ctrl+X) and update:

   sudo update-initramfs -u

4. Reboot:

   sudo reboot

Method 3: Kernel Boot Parameter (If KVM is Built-In)

1. Edit GRUB configuration:

   sudo nano /etc/default/grub

2. Append to GRUB_CMDLINE_LINUX:
   • For Intel:

     modprobe.blacklist=kvm_intel,kvm

   • For AMD:

     modprobe.blacklist=kvm_amd,kvm

   Example line:

   GRUB_CMDLINE_LINUX="...existing options... modprobe.blacklist=kvm_intel,kvm"

By following these steps, VirtualBox will regain access to hardware virtualization or fall back to software emulation.