[NikolaNovak]

Pardon a simple question - this implies nested virtualization, or is the second step emulation?

The download is a Linux VM, gotcha.

Are other OS-s nested virtual machines inside that Linux VM, or emulators (in which case, holly mackerel, that is even more impressive :O... and also why??).

Readme seems to imply it's emulators, but it also uses the words "virtual/virtualization" or "VM images" liberally sprinkled.

[gwynforthewyn]

I imagine the author's using OpenSIMH (https://opensimh.org) or something similar, so it'd be an emulated CPU running the userlands.

I have a container that runs a 4.3 BSD userland using opensimh; it's not super hard to set up, just takes a bit of patience and willingness to learn how opensimh works.

[andreww591]

Several different SIMH forks are included, along with a lot of other emulators; there are well over 150 different emulators, with some having multiple versions and variants present to handle things like regressions related to specific OSes.

Nested virtualization for certain x86 OSes running in QEMU is supported, although you will have to enable it manually (VirtualBox has a checkbox for this in its settings). For VMs that support it, the QEMU launch scripts will automatically use KVM if available and fall back to TCG if nested virtualization isn't enabled.

[NikolaNovak]

Thx! You got a new, albeit small scale, patreon - this is awesome :)