From: Christian Borntraeger <redacted>
Date: Fri, 21 Oct 2016 13:58:53 +0200

For spinning loops people did often use barrier() or cpu_relax().
For most architectures cpu_relax and barrier are the same, but on
some architectures cpu_relax can add some latency. For example on s390
cpu_relax gives up the time slice to the hypervisor. On power cpu_relax
tries to give some of the CPU to the neighbor threads. To reduce the
latency another variant cpu_relax_lowlatency was introduced. Before this
is used in more and more places, lets revert the logic of provide a new
function cpu_relax_yield that can spend some time and for s390 yields
the guest CPU.

Sparc64, fwiw, behaves similarly to powerpc.