Building a multi-arch CI pipeline for 13 targets. What could possibly go wrong?

The ci-multiplatform project is a generic, OCI-based multi-architecture CI system designed to make cross-platform testing practical for open-source projects using GitLab CI. Originally created while enabling RISC-V support for Pixman, it has since grown into an independent project under the RISE (RISC-V Software Ecosystem) umbrella: https://gitlab.com/riseproject/CI/ci-multiplatform, with a mirror on freedesktop.org: https://gitlab.freedesktop.org/pixman/ci-multiplatform The project provides multi-arch layered OCI images (Base GNU, LLVM, Meson) based on Debian, GitLab Component-style templates, and fully automated downstream test pipelines for the included examples. It supports creating customized OCI images, building and testing across 16 Linux and Windows targets – including x86, ARM, RISC-V, MIPS, and PowerPC – using unprivileged GitLab runners with QEMU user-mode emulation. Architecture-specific options (e.g., RISC-V VLEN configuration) allow developers to exercise multiple virtual hardware profiles without any physical hardware, all within a convenient job-matrix workflow. The talk covers how the system is engineered, tested, and validated across multiple GitLab instances, and what happens when unprivileged runners, QEMU quirks, toolchain differences, and architecture-specific behaviours all converge in a single pipeline. I will show how projects can adopt ci-multiplatform with minimal effort and turn multi-arch CI from a maintenance burden into a routine part of upstream development.