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There is no real connection to UML here. Hardware virtualization (Intel VT, AMD-V) are much faster in practice and also don't require the guest operating system to be heavily modified. So besides as curiosity or test vehicle, approaches like UML are pretty dead.

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Thanks.

So, slow as it may be, the win for UML (which seems to still have a heartbeat) is that it can run on uP without any specific virtualisation capabilities, right? If I could run Linux on a Z80/6502 then in theory I could run a virtualised Linux on a Z80/6502.

Yes - plus the original win of UML was also being able to run virtual instances on a kernel without proper virtualization capabilities.

In the early 2000s people used to use UMLs as a hosting platform - they didn't have the same security isolation as a proper VM (or even, necessarily, of a container) though.

How do containers have better security isolation than UML?

When I tinkered with UML I think it was prior to cgroups (2007) [0] so my guess is that escaping the UML instance was easier.

[0] https://en.wikipedia.org/wiki/Cgroups

The “original” UML is/was, I believe, NetBSD running as a “rump kernel” and something that virtualization of the actual kernel does not, directly and on its own, fill the shoes of.

UML is older than NetBSD rump kernels.

I stand corrected.

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