Re: [PATCH 2/5] tunables: Add LD_HWCAP_MASK to tunables

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 18/05/2017 17:07, Siddhesh Poyarekar wrote:
> Add LD_HWCAP_MASK to tunables in preparation of it being removed from
> rtld.c.  This allows us to read LD_HWCAP_MASK much earlier so that it
> can influence IFUNC resolution in aarch64.
> 
> This patch does not actually do anything other than read the
> LD_HWCAP_MASK variable and add the tunables way to set the
> LD_HWCAP_MASK, i.e. via the glibc.tune.hwcap_mask tunable.  In a
> follow-up patch, the _dl_hwcap_mask will be replaced with
> glibc.tune.hwcap_mask to complete the transition.
> 
> 	* elf/dl-tunables.list: Add glibc.tune.hwcap_mask.
> 	* scripts/gen-tunables.awk: Include dl-procinfo.h.
> 	* manual/tunables.texi: Document glibc.tune.hwcap_mask.

LGTM with a small fix below.

> ---
>  elf/dl-tunables.list     |  7 +++++++
>  manual/tunables.texi     | 22 ++++++++++++++++++++++
>  scripts/gen-tunables.awk |  1 +
>  3 files changed, 30 insertions(+)
> 
> diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
> index b9f1488..41ce9af 100644
> --- a/elf/dl-tunables.list
> +++ b/elf/dl-tunables.list
> @@ -77,4 +77,11 @@ glibc {
>        security_level: SXID_IGNORE
>      }
>    }
> +  tune {
> +    hwcap_mask {
> +      type: UINT_64
> +      env_alias: LD_HWCAP_MASK
> +      default: HWCAP_IMPORTANT
> +    }
> +  }
>  }
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index ac8c38f..37aefca 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -31,6 +31,8 @@ their own namespace.
>  @menu
>  * Tunable names::  The structure of a tunable name
>  * Memory Allocation Tunables::  Tunables in the memory allocation subsystem
> +* Hardware Capability Tunables::  Tunables that modify the hardware
> +				  capabilities seen by @theglibc{}
>  @end menu
>  
>  @node Tunable names
> @@ -190,3 +192,23 @@ number of arenas is determined by the number of CPU cores online.  For 32-bit
>  systems the limit is twice the number of cores online and on 64-bit systems, it
>  is 8 times the number of cores online.
>  @end deftp
> +
> +@node Hardware Capability Tunables
> +@section Hardware Capability Tunables
> +@cindex hardware capability tunables
> +@cindex hwcap tunables
> +@cindex tunables, hwcap
> +
> +@deftp {Tunable namespace} glibc.tune
> +Behavior of @theglibc{} can be tuned to assume specific hardware capabilities
> +by setting the following tunables in the @code{tune} namespace:
> +@end deftp
> +
> +@deftp Tunable glibc.tune.hwcap_mask
> +This tunable supersedes the @env{LD_HWCAP_MASK} environment variable and is
> +identical in features.
> +
> +The @code{AT_HWCAP} key in the Auxilliary Vector specifies instruction set
> +extensions available in the processor at runtime for some architectures.  The
> +@code{glibc.tune.hwcap_mask} tunable allows the user to mask out those
> +capabilities at runtime, thus disabling use of those extensions.

Missing @end deftp here.

> diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
> index b10b00e..93e5aff 100644
> --- a/scripts/gen-tunables.awk
> +++ b/scripts/gen-tunables.awk
> @@ -134,6 +134,7 @@ END {
>    print "# error \"Do not include this file directly.\""
>    print "# error \"Include tunables.h instead.\""
>    print "#endif"
> +  print "#include <dl-procinfo.h>\n"
>  
>    # Now, the enum names
>    print "\ntypedef enum"
>