Hi,

This is actually tested code; but still missing the SMP wake-to-idle machinery.
I still need to think about that.

I'll post my test-hack as a reply, but basically it does co-operative and
preemptive UP-like user scheduling.

Patches go on top of tip/master as they rely on the .fixup removal
recently merged in tip/x86/core.

Also, I still need to audit a bunch of mm code, because I'm not sure things are
actually as well behaved as this code supposes they are.