Boot failure triage

The boot log is a checklist

The kernel prints one line per subsystem IN BRING-UP ORDER (the order is arch/boot-flow.md). Diagnosis:

1. Nothing at all on serial - GRUB didn't run or the ISO entry is wrong. Boot the ISO's other slot entry; try BIOS vs UEFI.
2. Stops before long mode entry OK - boot.S stage: the multiboot2 handshake failed (bad magic/mbi) - capture with -serial file: and check the mbi tag parse.
3. Stops at a driver line - that family's probe failed on your hardware; the message above it names the device. Remove the device (or the QEMU flag) to confirm.
4. Reaches boot complete but no prompt - shell init; extremely rare (the 233-command registration line would be the tell).
5. Reboots after the prompt - panic or watchdog: check /etc crashlog after the second boot (dumpinfo, crashlog), and the serial dump during the event.

The self-tests are your friends

The boot sequence already proves: exception path (3/3), shell registration (233), crash recovery armed. If those lines printed, the machine underneath is sound - look at the specific feature, not the platform.

Second-boot analysis

The A/B flag + crashlog survive reboots on /etc. After any unexpected reset: dmesg (current boot), crashlog/dumpinfo (previous boot), abcfg (what GRUB tried to boot).