RFC: Check IFUNC_XXX to enable/disable ifunc feature

[H.J. Lu]

On Sun, Jun 26, 2016 at 11:55 AM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 06/26/2016 10:50 AM, H.J. Lu wrote:
>> Hi,
>>
>> I'd like to check IFUNC_XXX environmental variable to enable/disable
>> the ifunc feature, XXX.  That is
>>
>> 1.  IFUNC_XXX=1 to enable XXX.
>> 2.  IFUNC_XXX=0 to disable XXX.
>>
>> Any comments?
>
> How would it work? Does it just mean that the IFUNC selector bypasses
> the disabled choices?

It will override bits in dl_x86_cpu_features.

> Also it does not meet Tunables consensus:
> "Tunable namespace should be clearly defined"
> https://sourceware.org/glibc/wiki/TuningLibraryRuntimeBehavior?highlight=%28Tunables%29
>
> Should perhaps be "GLIBC_<VER>_IFUNC_<ARCH>_<PUBLIC_IFUNC_NAME>"?

They won't be version-dependent.  How about

GLIBC_IFUNC_X86=sse2=0:....

or

GLIBC_IFUNC=sse2=0:....

since IFUNC is target dependent.

-- 
H.J.