[Bug 2007599] Re: [23.04 FEAT] [GNU2223] glibc - Influence hwcaps/stfle via GLIBC_TUNABLES glibc.cpu.hwcaps

[Frank Heimes]

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Title:
  [23.04 FEAT] [GNU2223] glibc - Influence hwcaps/stfle via
  GLIBC_TUNABLES glibc.cpu.hwcaps

Status in Ubuntu on IBM z Systems:
  Confirmed
Status in glibc package in Ubuntu:
  In Progress
Status in glibc source package in Jammy:
  Confirmed

Bug description:
  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

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