VirtualBox cannot boot windows machine anymore: cannot operate in VMX mode, please disable KVM kernel extension

[larryr]

This has been discussed - 6.12 kernel has it turned on by default. Linus Benedict Torvalds said it has to be on from now on and he owns the kernel code.

See this for fix:

Warning for VirtualBox users and Kernel 6.12

VirtualBox 7.1.4 has support for Kernel 6.12 but you have to include a parameter to the kernel command line in the bootup or VirtualBox will not start. kvm.enable_virt_at_load=0 I added it to /etc/default/grub I got this from Linux Magazine January 2025 issue.