On Tue, Feb 24, 2015 at 11:07:37PM +0000, Peter Crosthwaite wrote:

ARM64 has the yield nop hint which has the intended semantics of
cpu_relax. Implement.

The immediate application is ARM CPU emulators. An emulator can take
advantage of the yield hint to de-prioritise an emulated CPU in favor
of other emulation tasks. QEMU A64 SMP emulation has yield awareness,
and sees a significant boot time performance increase with this change.

Could you elaborate on the QEMU SMP boot case please? Usually SMP pens
for booting make use of wfe/sev to minimise the spinning overhead.

Will