cpu_relax(), rep: nop, and PAUSE

[Valdis.Kletnieks at vt.edu]

On Wed, 20 Feb 2013 01:58:17 +0700, Mulyadi Santosa said:
> On Tue, Feb 19, 2013 at 7:20 PM, David Shwatrz <dshwatrz at gmail.com> wrote:
> > Hi, kernel newbies,
> >
> > We have:
> > #define cpu_relax()     asm volatile("rep; nop")
> > in arch/x86/boot/boot.h.
> >
> > Why don't we use the PAUSE assembler instruction here ?
>
> Just guessing, maybe rep+nop could do better power saving because
> processor is considered as idle.

The 'rep; nop' is actually a placeholder - for some CPUs, a different opcode
gets filled in during boot time.  See arch/x86/kernel/alternative.c and
arch/x86/include/asm/alternative.h for the gory details.
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