Comment 3 for bug 2007599

------- Comment From <email address hidden> 2023-02-21 10:17 EDT-------
@Simon:
please ignore the SRU justification in my last comment - that was a copy/paste error on my end. Sorry about the confusion.
(You might have already wondered why the SRU justification - except the business case - does not fit at all to the glibc fix we are talking about in this bug / LP item.)

Please find below the CORRECTED VERSION of my comment:

SRU Justification:
==================
[Business Case]
The Eclipse OpenJ9 project is developing an "InstantOn" technology that leverages Linux's Checkpoint/Restore In Userspace (CRIU) feature. By restoring the JVM process from an existing CRIU snapshot, application can start-up up to 18x faster, without any compromises to throughput or JVM capabilities. This technology is ideal for serverless computing and scaling micro services in containerized deployments.
IBM WebSphere Liberty is one of the first exploiters of OpenJ9 InstantOn technology and have measured 10x faster start-up. In a 2021 survey with Java customers on IBM Z, 80% of users ranked initial start-up / ramp-up performance as an important metric. Other Java-based middleware and libraries can also take advantage of this technology for the significant start-up improvements. This GLIBC patch addresses a functional portability issue that is encountered when users create a container image with a CRIU snapshot on a newer hardware level and deploy this image against older hardware. GLIBC caches hardware facilities, which may not be available on the older deployment target, resulting in potential illegal instructions. With GLIBC_TUNABLES fixed to influence hwcaps and stfle bits on s390x, it allows the snapshot environment to mimic older hardware levels for the snapshot.

[Impact]
* For IFUNC'ed functions within glibc, the decision can now be influenced with GLIBC_TUNABLE: glibc.cpu.hwcaps
* This can be used as test vehicle
* This is a prerequisite for Eclipse OpenJ9 projects "InstantOn" technology which speeds up start-up times by factors.

[Test Plan]
* Prepare an IBM z15 LPAR with Ubuntu (incl. this patch)
* Check if patch is applied by running command "/usr/lib/s390x-linux-gnu/ld64.so.1 --list-tunables" and check if the new tunable "glibc.cpu.hwcaps" is listed
* Prepare a small test-program calling IFUNC-optimized functions: memmem, strstr and memmove.
* Start a gdb session and break at __memmem_vx, __memmem_arch13, __strstr_vx, __strstr_arch13, __memmove_z13 and __memmove_arch13. Run the program and you should always stop in the arch13=z15 functions. Now use gdb-command "set environment GLIBC_TUNABLES glibc.cpu.hwcaps=z13" and run again. Now you should always stop in the vx=z13 functions.