[dgl]

The second generation was "skas" for Separate Kernel Address Space, some more background here: https://user-mode-linux.sourceforge.net/old/skas.html

The host kernel patch for skas was never merged, probably for good reason, but that and Xen/VM hardware support meant UML stopped making sense.

[g-mork]

Stopped making sense for the mass hosting use case, but it is still wildly useful for setting up and tearing down during dev work, especially when hacking on the kernel itself, which you can trivially attach GDB to