Comment 0 for bug 2007599

Feature Description:

In glibc there are a lot of IFUNC-optimized functions, where we have different implementations for e.g. z13, z14, z15, ... . At runtime the best implementation is chosen depending on the available HWCAP-/STFLE-bits.

We need to have a mechanism to influence this decision. There already exists such a mechanism for x86 which is documented here:
GLIBC_TUNABLE: glibc.cpu.hwcaps (x86 variant)

https://www.gnu.org/software/libc/manual/html_node/Hardware-Capability-Tunables.html

Therefore we've developed the commit
"S390: Influence hwcaps/stfle via GLIBC_TUNABLES." (https://sourceware.org/git/?p=glibc.git;a=commit;h=41f67ccbe92b4fd09e1062b383e55e407ae5bfa1)
as part of our maintenance work. The s390-variant of the mentioned tunable is able to add/remove specific HWCAP-/STFLE-bits used by the IFUNC-resolvers inside glibc. Furthermore we can set the architecture-level like z13 to influence multiple bits at once.

One can query if the feature is available with (useful to check if this feature is also backported to current distro or not):
$ /lib/ld64.so.1 --list-tunables