XPost: linux.debian.devel.release   
   From: waldi@debian.org   
      
   Hi   
      
   We never did a real discussion about architecture baselines before, but I think   
   we should do that.  We also don't have any guidelines what we as Debian want to   
   actually support.  But given that we are a general purpose distribution, we   
   have to find a balance.   
      
   As a general guidance I would like to aim for a ten to 15 years support range   
   at release time.  The cutoff in respect to the expected 2027 release date of   
   Forky would therefor be 2012 to 2017.  More time is given for widely used   
   architectures, less for more specialized ones.   
      
   Bastian   
      
   # Current architecture baseline   
      
   Only three architectures set an explicit baseline in gcc-12 (Bookworm) and   
   gcc-15 (Forky):   
   * armhf: armv7-a+fp   
   * i386: i686   
   * s390: z196   
      
   All other architectures use the default values.   
      
   # Proposal for architecture baseline   
      
   ## amd64 (and i386)   
      
   * x86-64-v2: Supported since around 2008[^x86]. Used in RedHat 9[^redhat9x86].   
   * x86-64-v3: Supported since around 2013-2015[^x86]. Used in RedHat   
     10[^redhat10].   
      
   I propose to use the x86-64-v2 baseline in Forky.  It gives us more then the 15   
   years and everyone else already switched to it or even a newer variant.   
   Factoring in a possible LTS for Trixie, this means that x86-64-v1 will stop   
   being supported by Debian in 2030.   
      
   ## ppc64el   
      
   * POWER8: Launched in 2014[^power8].   
   * POWER9: Launched in 2017[^power9]. Used in RedHat 9[^redhat9] and   
     10[^redhat10].   
      
   I propose to use the POWER9 baseline in Forky.  The Debian infrastructure seems   
   to be comprised of all POWER9s.  This means POWER8 will stop being supported in   
   2028.   
      
   ## s390x   
      
   * z14: Launched in 2017[^z14]. Used by RedHat 10[^redhat10].   
   * z15: Launched in 2019[^z15]. Used by next Ubuntu.   
      
   I propose to use the z15 baseline in Forky.  Yes, this is less then ten years,   
   but s390 is such a niche architecture and all we do is best (often not even   
   this) effort support.  The Debian infrastructure is documented to be at least   
   z15, all accesible system are actually z16.  This means that z14 and older will   
   stop being support in 2028.   
      
   # How to see supported baseline   
      
   The dynamic linker of glibc can show what microarchiture level it considers   
   supported on the current CPU.  The "supported" marker is relevant in this case.   
      
   ## amd64   
   ```   
   % /usr/lib64/ld-linux-x86-64.so.2 --help | sed -n '/^Subdirectories of   
   glibc-hwcaps/,$p'   
   Subdirectories of glibc-hwcaps directories, in priority order:   
     x86-64-v4   
     x86-64-v3 (supported, searched)   
     x86-64-v2 (supported, searched)   
   ```   
      
   ## ppc64el   
   ```   
   % /usr/lib64/ld64.so.2 --help | sed -n '/^Subdirectories of glibc-hwcaps/,$p'   
   Subdirectories of glibc-hwcaps directories, in priority order:   
     power10   
     power9 (supported, searched)   
   ```   
      
   ## s390x   
   ```   
   % /usr/lib/ld64.so.1 --help | sed -n '/^Subdirectories of glibc-hwcaps/,/^    
   z15/p'   
   Subdirectories of glibc-hwcaps directories, in priority order:   
     z16 (supported, searched)   
     z15 (supported, searched)   
   ```   
      
   [^power8]: https://en.wikipedia.org/wiki/POWER8   
   [^power9]: https://en.wikipedia.org/wiki/POWER9   
   [^x86]: https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels   
   [^z14]: https://en.wikipedia.org/wiki/IBM_z14_(microprocessor)   
   [^z15]: https://en.wikipedia.org/wiki/IBM_z15_(microprocessor)   
   [^redhat9]: https://docs.redhat.com/en/documentation/red_hat_ent   
   rprise_linux/9/html/9.0_release_notes/architectures   
   [^redhat9x86]: https://developers.redhat.com/blog/2021/01/05/bui   
   ding-red-hat-enterprise-linux-9-for-the-x86-64-v2-microarchitecture-level   
   [^redhat10]: https://docs.redhat.com/en/documentation/red_hat_en   
   erprise_linux/10/html/considerations_in_adopting_rhel_10/architectures   
      
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