[PATCH] uapi/auxvec: Define AT_HWCAP3 and AT_HWCAP4 aux vector, entries

[Adhemerval Zanella Netto]

On 26/09/23 19:02, Peter Bergner wrote:
> The powerpc toolchain keeps a copy of the HWCAP bit masks in our TCB for fast
> access by our __builtin_cpu_supports built-in function.  The TCB space for
> the HWCAP entries - which are created in pairs - is an ABI extension, so
> waiting to create the space for HWCAP3 and HWCAP4 until we need them is
> problematical, given distro unwillingness to apply ABI modifying patches
> to distro point releases.  Define AT_HWCAP3 and AT_HWCAP4 in the generic
> uapi header so they can be used in GLIBC to reserve space in the powerpc
> TCB for their future use.

This is different than previously exported auxv, where each AT_* constant
would have a auxv entry. On glibc it would require changing _dl_parse_auxv
to iterate over the auxv_values to find AT_HWCAP3/AT_HWCAP4 (not ideal, 
but doable).

Wouldn't be better to always export it on fs/binfmt_elf.c, along with all 
the machinery to setup it (ELF_HWCAP3, etc), along with proper documentation?

> 
> I scanned both the Linux and GLIBC source codes looking for unused AT_*
> values and 29 and 30 did not seem to be used, so they are what I went
> with.  If anyone sees a problem with using those specific values, I'm
> amenable to using other values, just let me know what would be better.
> 
> Peter
> 
> 
> Signed-off-by: Peter Bergner <bergner at linux.ibm.com>
> ---
>  include/uapi/linux/auxvec.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/include/uapi/linux/auxvec.h b/include/uapi/linux/auxvec.h
> index 6991c4b8ab18..cc61cb9b3e9a 100644
> --- a/include/uapi/linux/auxvec.h
> +++ b/include/uapi/linux/auxvec.h
> @@ -32,6 +32,8 @@
>  #define AT_HWCAP2 26	/* extension of AT_HWCAP */
>  #define AT_RSEQ_FEATURE_SIZE	27	/* rseq supported feature size */
>  #define AT_RSEQ_ALIGN		28	/* rseq allocation alignment */
> +#define AT_HWCAP3 29	/* extension of AT_HWCAP */
> +#define AT_HWCAP4 30	/* extension of AT_HWCAP */
>  
>  #define AT_EXECFN  31	/* filename of program */
>