[7/7] aarch64: Allow overriding HWCAP_CPUID feature check using HWCAP_MASK

  Now that LD_HWCAP_MASK (or glibc.tune.hwcap_mask) is read early enough
to influence cpu feature check in aarch64, use it to influence
multiarch selection.  Setting LD_HWCAP_MASK such that it clears
HWCAP_CPUID will now disable multiarch for the binary.

	* sysdeps/unix/sysv/linux/aarch64/cpu-features.c
	(init_cpu_features): Use glibc.tune.hwcap_mask.

Change-Id: Ia801ed6b8166b79a0cae184ee7417272f2f60593
---
 sysdeps/unix/sysv/linux/aarch64/cpu-features.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)
  

On 11/05/2017 11:51, Siddhesh Poyarekar wrote:
> Now that LD_HWCAP_MASK (or glibc.tune.hwcap_mask) is read early enough
> to influence cpu feature check in aarch64, use it to influence
> multiarch selection.  Setting LD_HWCAP_MASK such that it clears
> HWCAP_CPUID will now disable multiarch for the binary.
> 
> 	* sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> 	(init_cpu_features): Use glibc.tune.hwcap_mask.

LGTM.

> 
> Change-Id: Ia801ed6b8166b79a0cae184ee7417272f2f60593
> ---
>  sysdeps/unix/sysv/linux/aarch64/cpu-features.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index 7025062..0478fcc 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -18,18 +18,25 @@
>  
>  #include <cpu-features.h>
>  #include <sys/auxv.h>
> +#include <elf/dl-tunables.h>
>  
>  static inline void
>  init_cpu_features (struct cpu_features *cpu_features)
>  {
> -  if (GLRO(dl_hwcap) & HWCAP_CPUID)
> +#if HAVE_TUNABLES
> +  uint64_t hwcap_mask = TUNABLE_GET (glibc, tune, hwcap_mask, uint64_t);
> +#else
> +  uint64_t hwcap_mask = GLRO (dl_hwcap_mask);
> +#endif

I noted the hwcap_mask get logic is duplicated over various places.  Should we
define a helper macro for it?

> +
> +  uint64_t hwcap = GLRO (dl_hwcap) & hwcap_mask;
> +
> +  if (hwcap & HWCAP_CPUID)
>      {
>        register uint64_t id = 0;
>        asm volatile ("mrs %0, midr_el1" : "=r"(id));
>        cpu_features->midr_el1 = id;
>      }
>    else
> -    {
> -      cpu_features->midr_el1 = 0;
> -    }
> +    cpu_features->midr_el1 = 0;
>  }
>
  

On Thursday 18 May 2017 12:52 AM, Adhemerval Zanella wrote:
> I noted the hwcap_mask get logic is duplicated over various places.  Should we
> define a helper macro for it?

Sorry I missed this suggestion, I have posted a patch for it now that
applies on top of my patchset.

https://sourceware.org/ml/libc-alpha/2017-05/msg00578.html

Siddhesh