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Technical notes

This document preserves the deeper troubleshooting record and lessons learned without making the project landing page carry the full narrative.

Build and runtime problems

Problem Root cause Resolution
Boot decompressor failed under GCC 15 Linux 6.10 predates GCC 15's C23-flavored default; the kernel's pre-C23 bool, false, and true definitions then conflicted with reserved keywords Pin -std=gnu11 in arch/x86/boot/compressed/Makefile; an equivalent fix later landed upstream
resolve_btfids host tool failed The bundled libbpf triggered a discarded-const warning promoted to an error by the newer host compiler Disable CONFIG_DEBUG_INFO_BTF for this scoped build
radeon and i915 failed to compile Linux 6.10 GPU code and the newer compiler disagreed about counted_by handling Prune unused GPU drivers with localmodconfig for the QEMU-only target
Out-of-tree module would not build for the distribution kernel The target kernel used Clang/LTO while the initial module build used GCC Rebuild the module with LLVM=1 to match the target kernel toolchain
/lib/modules/$(uname -r)/build was missing Installed headers belonged to a newer kernel than the one still running Reboot into the kernel matching the installed headers
PID 1 hit an illegal instruction in QEMU The conservative qemu64 model omitted instructions assumed by locally built code Run this lab with -cpu max

These are observations from Linux 6.10 with the documented host toolchains, not universal remedies. In particular, -cpu max favors faithful local reproduction over migration compatibility and is not a portable deployment recommendation.

Kernel timer configuration

This lab changed the kernel timer frequency from the baseline configuration's CONFIG_HZ=1000 to CONFIG_HZ=300.

Configuration Nominal periodic tick interval Timer tradeoff
CONFIG_HZ=1000 1 ms Finer periodic-tick granularity, with up to 1000 timer interrupts per second per tick-driven CPU
CONFIG_HZ=300 Approximately 3.33 ms Coarser periodic-tick granularity, with up to 300 timer interrupts per second per tick-driven CPU

This is a timer-tick comparison, not a schedule for every task switch or scheduling decision. Tickless-kernel options can suppress ticks while CPUs are idle—and, with full dynticks, in additional contexts. Modern high-resolution timers are also not limited to one CONFIG_HZ interval.

Lessons retained

  • Toolchain drift can look like a kernel source defect. Kernel version, compiler, C dialect, module build, and target kernel must be considered as a single compatibility set.
  • Early-boot symptoms can be far removed from their cause. Here an instruction set mismatch surfaced as "Attempted to kill init" rather than a CPU-feature diagnostic.
  • bzImage and vmlinux are complementary artifacts: the former boots; the latter supplies symbols and, when configured, DWARF source information.
  • KASLR must be disabled for this simple static-symbol GDB workflow so runtime addresses match vmlinux.
  • Loading an unsigned out-of-tree module taints the kernel. That is expected in a local lab and should still be recorded when interpreting bug reports.

Boundaries

  • The 16-slot driver queue is global, non-blocking, and mutex-protected. Its partial-read cursor is shared across readers; it offers neither per-file cursors nor message fan-out.
  • Empty reads return EOF and full writes return ENOSPC. There is no poll/select, wait queue, ioctl, mmap, persistence, DMA, or interrupt handling.
  • The scripts report missing dependencies but never install them.
  • Full kernel builds require substantial time and disk space and are outside the intentionally lightweight hosted CI checks.