Re: [PATCH tip] x86/percpu: Rewrite arch_raw_cpu_ptr()

[Ingo Molnar]

* Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> Implement arch_raw_cpu_ptr() as a load from this_cpu_off and then
> add the ptr value to the base. This way, the compiler can propagate
> addend to the following instruction and simplify address calculation.
> 
> E.g.: address calcuation in amd_pmu_enable_virt() improves from:
> 
>     48 c7 c0 00 00 00 00 	mov    $0x0,%rax
> 	87b7: R_X86_64_32S	cpu_hw_events
> 
>     65 48 03 05 00 00 00 	add    %gs:0x0(%rip),%rax
>     00
> 	87bf: R_X86_64_PC32	this_cpu_off-0x4
> 
>     48 c7 80 28 13 00 00 	movq   $0x0,0x1328(%rax)
>     00 00 00 00
> 
> to:
> 
>     65 48 8b 05 00 00 00 	mov    %gs:0x0(%rip),%rax
>     00
> 	8798: R_X86_64_PC32	this_cpu_off-0x4
>     48 c7 80 00 00 00 00 	movq   $0x0,0x0(%rax)
>     00 00 00 00
> 	87a6: R_X86_64_32S	cpu_hw_events+0x1328
> 
> The compiler can also eliminate redundant loads from this_cpu_off,
> reducing the number of percpu offset reads (either from this_cpu_off
> or with rdgsbase) from 1663 to 1571.
> 
> Additionaly, the patch introduces 'rdgsbase' alternative for CPUs with
> X86_FEATURE_FSGSBASE. The rdgsbase instruction *probably* will end up
> only decoding in the first decoder etc. But we're talking single-cycle
> kind of effects, and the rdgsbase case should be much better from
> a cache perspective and might use fewer memory pipeline resources to
> offset the fact that it uses an unusual front end decoder resource...

So the 'additionally' wording in the changelog should have been a big hint 
already that the introduction of RDGSBASE usage needs to be a separate 
patch. ;-)

Thanks,

	Ingo