Re: [Xen-devel] [PATCH v2 4/6] xen/arm: cpufeature: Add helper to check constant caps

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 25 Sep 2018, Julien Grall wrote:
> Some capababilities are set right during boot and will never change
> afterwards. At the moment, the function cpu_have_caps will check whether
> the cap is enabled from the memory.
> 
> It is possible to avoid the load from the memory by using an
> ALTERNATIVE. With that the check is just reduced to 1 instruction.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

I enjoyed reading the patch :-)  But I don't think it is worth going
into this extreme level of optimization. test_bit is efficient enough,
right? What do you think we need to use alternatives just to check one
bit?


> ---
> 
> This is the static key for the poor. At some point we might want to
> introduce something similar to static key in Xen.
> 
>     Changes in v2:
>         - Use unlikely
> ---
>  xen/include/asm-arm/cpufeature.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/xen/include/asm-arm/cpufeature.h 
> b/xen/include/asm-arm/cpufeature.h
> index 3de6b54301..c6cbc2ec84 100644
> --- a/xen/include/asm-arm/cpufeature.h
> +++ b/xen/include/asm-arm/cpufeature.h
> @@ -63,6 +63,18 @@ static inline bool cpus_have_cap(unsigned int num)
>      return test_bit(num, cpu_hwcaps);
>  }
>  
> +/* System capability check for constant cap */
> +#define cpus_have_const_cap(num) ({                 \
> +        bool __ret;                                 \
> +                                                    \
> +        asm volatile (ALTERNATIVE("mov %0, #0",     \
> +                                  "mov %0, #1",     \
> +                                  num)              \
> +                      : "=r" (__ret));              \
> +                                                    \
> +        unlikely(__ret);                            \
> +        })
> +
>  static inline void cpus_set_cap(unsigned int num)
>  {
>      if (num >= ARM_NCAPS)
> -- 
> 2.11.0
> 

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