bare-metal-bringup

Bare-Metal Bring-Up

Overview

Bringing up code on new silicon or a new architecture is a sequence of "does the most basic thing work yet" checkpoints. Each layer has a small number of mistakes that produce total silence or a single cryptic fault, and they are almost always init ordering, trap setup, address translation, or ABI mismatches.

Core principle: Get one character out the door first, then build up one checkpoint at a time. Until you have output, you are debugging blind, so the first job is always a working console, not the feature you wanted.

When to Use

• First boot on a new arch/board, or a port to a new target
• "Boots but hangs," silent boot, or a fault before main
• Traps/interrupts not firing, or firing into garbage
• Syscalls returning wrong values or corrupting registers across the boundary
• A driver works under one emulator/accelerator but not another (TCG vs KVM)

The Checkpoint Ladder

Climb in order. Don't debug a higher rung until the one below it is solid.

1. Earliest output. Poke the UART directly (no driver, no allocator). One known byte. If you can't get a byte, nothing else is debuggable.
2. Stack and BSS. A valid, correctly-placed stack and a zeroed BSS before any C/Zig/Rust runs. On some boots the stack must live inside a specific LOAD segment or the loader drops your initrd on top of it.
3. Trap/exception vectors. Install the vector table, prove it by taking a deliberate trap and returning. Get this working before timers or interrupts.
4. Timer. Architectural timer init MUST run before any kernel timer that divides by its frequency, or you divide by zero and panic with no output.
5. Address translation. MMU/PMP/page tables. Identity-map what you need, then enable. Wrong here means a fault the instant translation turns on.
6. Interrupts. Controller (PLIC/GIC/APIC), enable bits, the right per-IRQ vs global mask model for the part.
7. Userspace / syscalls. Drop to user mode, take a syscall, return cleanly without clobbering caller-saved-by-the-ABI registers.

See bringup-gotchas.md in this skill directory for the concrete, hard-won failures at each rung.

Trap And Syscall Discipline

• Save and restore exactly what the ABI promises. The single most common syscall bug is the entry stub clobbering a register the user-side ABI expects preserved (often the arg/temp registers, or the user stack pointer saved to a per-CPU slot that a sibling thread overwrites). Build the return frame from a safe location, not from a register you're about to reuse.
• Vector alignment is part-specific. Some cores require the trap base aligned to 256 bytes and force vectored mode; some need relaxation/compression disabled so each slot stays the expected width. Read the manual, don't assume 4-byte slots.
• 64-bit values across a 32-bit syscall boundary split across two registers and sentinel checks must use pointer-width max, not u64 max, or they fall through to a null deref.

Emulator vs Accelerator

A bug that only appears under KVM (and not plain TCG) is usually a real hardware-ordering or state-save bug that TCG's looser model hides: FP/SIMD state not saved on the trap path, per-CPU pointer not set before the first IRQ, a sleeper list reusing a runqueue link. Treat "works in TCG, dies in KVM" as a genuine bug in your save/restore or ordering, not an emulator quirk.

Debugging When It's Silent

1. Bisect by checkpoint: which rung's "hello" still prints?
2. Add a raw byte at the suspect transition (before/after enabling translation, before/after the first trap).
3. Suspect ordering first (init A before B), then ABI (who clobbered what), then translation (what's mapped).
4. Fix the root cause. Do not add a retry loop or a save-on-failure hatch to paper over a corruption; find what corrupts. See silicon-grade-discipline.

Midstall House Style

• Ferrite is the reference: multi-arch RT microkernel (aarch64/riscv64/i386/x86_64) plus ESP32-C6. The gotcha file distills its bring-up bugs.
• No design docs; keep the bring-up reasoning in-conversation and go straight to the fix.
• No em dashes, no emoji.