CPU Problem in multi-level virtualization
And why only 64 bit kernel?
OK. Its KVM problem.
And where get Whonix 64 bit version?
There are no downloadable (official) Non-Qubes-Whonix 13 64 bit builds. (And I am not aware of any nonofficial builds.) Builds from source code may be possible.
State of offical 64 bit builds - #16 by Patrick
sudo apt-get update
sudo apt-get install linux-image-amd64
$ cat /sys/module/kvm_intel/parameters/nested
You still need to chose that kernel during boot. And once that works perhaps remove the old kernel so you do not need to manually choose it every time.
Now nested VM starting but it crash Whonix. Need force reset.