[unwind]

To run x86 code on non-x86 machines would be my first guess. But I know nothing.

[Sintendo]

I thought so too at first, but I'd expect non-x86 host systems running x86 guests to be vulnerable if that were the case. The advisory seems to indicate otherwise.

Anyway, I've done some digging and found an explanation here: https://insinuator.net/2015/02/the-dangers-of-x86-emulation-...

tl;dr: it falls back to emulation for very specific cases that cannot be handled by hardware-assisted virtualization