QUOTE(x-fox @ Jun 8 2006, 01:26 PM)
windows can be run efficiently under qemu with a lot of tweaking.
The emulation overhead for normal userspace x86 instructions (the best, fastest case) under QEMU is at least three or four times, even being really generous. Getting a full x86 emulator with no virtualisation support to run any guest OS at more than a quarter or so of the speed of the host processor would be a tremendous feat of emulation, and QEMU certainly doesn't.
So, it depends how you define efficiently, and I wouldn't call 25% performance (or less) 'efficient'. Virtualisation allows normal userspace operations to run with zero overhead (same speed as native code) and only select kernel mode operations require lengthier emulation sequences.
It's quite possible to get it to run fast enough to play many old dos/windows games, of course.