Re: [Xen-devel] [PATCH] x86: optimize this_cpu()

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 13/07/2010 14:35, "Jan Beulich" <JBeulich@xxxxxxxxxx> wrote:

> Besides the .text space savings of over 2.5k on x86-64 (1.5k for
> x86-32) this removes a load (plus a lea on x86-64) from various
> frequently executed code paths, and finally provides a reason (other
> than legibility) to prefer this_cpu() over per_cpu() in all places
> where smp_processor_id() isn't being called anyway.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
> 
> --- 2010-06-15.orig/xen/include/asm-x86/current.h 2010-07-13
> 14:38:21.000000000 +0200
> +++ 2010-06-15/xen/include/asm-x86/current.h 2010-07-13 15:12:37.000000000
> +0200
> @@ -17,6 +17,10 @@ struct vcpu;
>  struct cpu_info {
>      struct cpu_user_regs guest_cpu_user_regs;
>      unsigned int         processor_id;
> +    unsigned long        per_cpu_offset;
> +#ifdef __x86_64__
> +    unsigned long        __pad_for_stack_bottom;
> +#endif

That's just nasty. If we need the structure to be 16-byte aligned then we
should achieve it via __attribute__((__aligned__(16))). And if we add that
we may as well not ifdef it, I'm sure the up to 12 bytes padding on i386
won't cause stack overflow.

 -- Keir



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