Short waits with umwait

I believe recent enough parts allow a small delta in the VMEXIT to be controlled by the Hypervisor. No idea if this is respected for the userspace mwait instructions, but it could be. In any case, you're coming out of the VM and when you go back in you're going to have to restart the pause. Similar for kernel/userspace. Interrupts will preempt anything like this. Otherwise you'd have a DoS opportunity.