[Gllug] running Xen on another VM

[Nix]

On 22 Aug 2008, damion yates spake thusly:
> UML in UML might be, which means Xen could run Xen with effort?

UML in UML works, although until 2.6.26 you couldn't use the same
binary for each (it used to have its 'kernel space' be located at
a fixed address: now it's probed for dynamically).

However, Xen is quite different in design and as far as I know nobody
has even tried to make the hypervisor run under itself (it would be
extremely difficult: it's probably easier just to run UML under Xen).

> If I understand correctly, in UML the kernel is implemented entirely
> in normal system calls, rather than CPU code itself, so this could
> nest.

Mostly. It's better to say it's implemented entirely in unprivileged
code. The fiddling it does with ptrace(), signals, relocatable assembler
stubs, and address spaces (using one of a number of methods of varying
efficiency and host-kernel intrusiveness) is decidedly tricky, and it's
not going to run atop anything but Linux anytime soon.
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