[Bug 2116086] Re: Build for RVA23U64 extension profile on RISC-V

[Heinrich Schuchardt]

Usage of the fence.i instruction on Linux is strongly discouraged. On
systems with SMP it will not work correctly without additional syscalls.
See https://man7.org/linux/man-
pages/man2/PR_RISCV_SET_ICACHE_FLUSH_CTX.2const.html.

The recommended way to flush the instruction cache is to use
__riscv_flush_icache() (https://www.man7.org/linux//man-
pages/man2/riscv_flush_icache.2.html).

Not adding Zicfencei to the compiler default would allow packages not
following this to stick out via FTBFS so that we can decide if code
changes are needed or the instruction should be allowed via explicit
compiler arguments.

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https://bugs.launchpad.net/bugs/2116086

Title:
  Build for RVA23U64 extension profile on RISC-V

Status in dpkg package in Ubuntu:
  Won't Fix
Status in gcc-14 package in Ubuntu:
  Invalid
Status in gcc-15 package in Ubuntu:
  New
Status in llvm-toolchain-20 package in Ubuntu:
  New

Bug description:
  Up to now we have been building RISC-V packages for the RV64GC
  profile. With Ubuntu 25.10 we want to move to the RVA23U64 ISA
  extension profile.

  In scripts/Dpkg/Vendor/Ubuntu.pm we have to update CPPFLAGS, CFLAGS,
  OBJCFLAGS, CXXFLAGS, OBJCXXFLAGS and add a -march= value specifying
  the extensions.

  The mandatory RVA23U64 extensions are specified in

  RVA23 Profiles, Version 1.0, 2024-10-17:
  https://drive.google.com/file/d/12QKRm92cLcEk8-5J9NI91m0fAQOxqNAq/view

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