Daniel Samson
15b70856c9
Two driver-model milestones plus a tree-wide naming pass. Suite 35/35 (QEMU) + host tests green. M11 — IRQ-as-IPC. A ring-3 driver now sleeps until its device interrupts it. New src/kernel/irq.zig: per-GSI endpoint bindings, comptime per-vector trampolines, dispatch = mask GSI -> LAPIC EOI -> notifyLocked, all under one lock region. irq_bind/irq_ack syscalls, gated by the device claim like mmio_map. interruptDispatch no longer EOIs — each handler owns its EOI, because a level line must be masked before it is acknowledged (irq_ack is the unmask). Bindings are keyed on the owning task and released on exit (a shared endpoint's siblings survive). hpetd rewritten interrupt-driven. Tests: hpet (rewritten, reads back the I/O APIC routing) and irqfree. M12 — bus drivers. DeviceDesc gains a parent, making the device table a tree. dev_register (device_register) lets a process publish children below a device it claimed; the kernel enforces resource containment (a child's resources must nest in its parent's), so a descriptor can't fabricate a window over kernel RAM. Descriptor copied in via copyFromUser (physmap walk — an unmapped user pointer fails the call instead of faulting the kernel). Per-parent child cap bounds table exhaustion. sbin/busd.zig is a worked bus driver. Test: bus. Naming — per docs/coding-standards.md: non-acronym abbreviations spelled out (message, descriptor, device_service, scheduler, runtime, physical, interpreter, ...); acronyms kept (IPC, MMIO, DMA, HCD, ...); files are kebab-case (ipc-synchronous.zig, device-service.zig, vfs-protocol.zig, ...). Exceptions: POSIX/C ABI names and Zig idioms (init/len/ptr) kept. Module collisions resolved by specific naming (config -> parameters, device.zig alias -> device_model). AML op/Op disambiguated: op = opcode, Op = operation; per-opcode parse handlers renamed opX -> parseX. New driver docs: drivers.md, driver-model.md (bus/class/HCD shapes + the proposed M13–M16 ABI), coding-standards.md.
143 lines
9.5 KiB
Markdown
143 lines
9.5 KiB
Markdown
# danos documentation
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Notes on how danos boots and draws, written to explain the *why* behind the code
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rather than restate it. Roughly in the order things happen at runtime:
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1. **[efi.md](efi.md) — EFI / the boot process.** How UEFI firmware finds and
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runs the bootloader, what the loader gathers before `ExitBootServices`, how it
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loads the kernel ELF, and the ABI contract for the jump into the kernel. Start
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here.
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2. **[gop.md](gop.md) — the Graphics Output Protocol.** How UEFI exposes graphics
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modes (unlike fixed VGA modes), how we detect the monitor's native resolution
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from EDID and switch to it, and the pixel formats we accept or reject.
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3. **[framebuffer.md](framebuffer.md) — the framebuffer.** What the linear
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framebuffer the loader hands over actually is, and what **pitch** (stride)
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means versus width — the detail you have to get right to avoid a skewed image.
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4. **[memory-map.md](memory-map.md) — the memory map.** How the loader learns what
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physical RAM exists and hands it to the kernel in danos's own neutral format,
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rather than leaking UEFI's memory descriptors across the boundary.
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5. **[frame-allocator.md](frame-allocator.md) — the physical frame allocator.** The
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bitmap allocator that hands out and reclaims 4 KiB physical frames from that
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map — the primitive page tables and the heap are built on.
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6. **[interrupts.md](interrupts.md) — interrupts and exceptions.** The GDT, IDT and
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TSS, the exception stubs, and the handler that reports a CPU fault in red instead
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of letting it triple-fault into a silent reset.
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7. **[paging.md](paging.md) — the kernel's page tables.** Building our own 4-level
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page tables, identity-mapping the low 4 GiB, and switching CR3 off the firmware's
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tables onto ours.
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8. **[device-interrupts.md](device-interrupts.md) — device interrupts.** The Local
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APIC and its timer — the kernel's first interrupt that is *handled and returned
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from*, giving it a heartbeat.
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9. **[heap.md](heap.md) — the kernel heap.** A growable free-list allocator built on
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the VMM, exposed as a `std.mem.Allocator` so std containers work — dynamic
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allocation for the kernel.
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10. **[scheduling.md](scheduling.md) — the scheduler.** Fixed-priority preemptive
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multitasking: kernel threads, the context switch, O(1) priority selection, and
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blocking (sleep, wait queues) — the leap to a running system.
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11. **[ipc.md](ipc.md) — inter-process communication.** Bounded blocking
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message-passing channels, then synchronous call/reply between *processes* over
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endpoints — the backbone the microkernel's isolated servers talk over.
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12. **[syscall.md](syscall.md) — system calls.** How ring 3 asks the kernel for
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something: the `syscall`/`sysret` fast path, the trap frame, and why the table is
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deliberately tiny.
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13. **[drivers.md](drivers.md) — writing a driver.** The payoff: a driver is an
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ordinary ring-3 process that claims a device, maps its registers, and **sleeps
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until its hardware interrupts it**. The claim is the capability; `irq_ack` is the
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unmask.
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14. **[driver-model.md](driver-model.md) — buses, classes and host controllers.** How
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real driver stacks factor into three shapes, how families share code, and the
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proposed ABI for the three primitives still missing (capability passing, DMA +
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memory barriers, MSI).
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15. **[halting.md](halting.md) — halting.** Why a kernel can't just "exit", and
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how `while (true) hlt` parks the CPU safely once there's nothing left to do.
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Start with the north star:
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- **[vision.md](vision.md) — the vision.** danos is a **learning-by-doing**
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microkernel: minimal kernel, drivers/services isolated in user space, chosen for
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**resilience** (restartable components). Win condition: runs on the author's PC and
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both Raspberry Pis, ideally with a GUI. Real-time is an option to explore, not a
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requirement. The *why* that shapes everything below.
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- **[resilience.md](resilience.md) — resilience.** A design note (not built yet) on
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fault isolation + live restart — the reincarnation-server + capability model that
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makes "if I break it, I can restart it" real. danos's core motivation.
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Cutting across all of these:
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- **[arch.md](arch.md) — the architecture split.** How CPU-specific code is kept
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behind a build-time `arch` module so the generic kernel never names x86_64,
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leaving room for other systems (e.g. an AArch64 Raspberry Pi) later.
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- **[arm.md](arm.md) — ARM targets.** The Raspberry Pi landscape the arch split is
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aiming at: `arm` (32-bit, Pi Zero W) vs `aarch64` (64-bit, Pi 3-5), UEFI vs
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device-tree boot, and what each layer needs.
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- **[discovery.md](discovery.md) — device discovery.** A design note on learning what
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hardware exists via ACPI (x86) or device tree (ARM) behind one neutral device model —
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when to build it, and how to keep it architecture-agnostic.
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- **[acpi.md](acpi.md) — finding the ACPI tables.** The concrete x86 locator chain:
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how the loader captures the **RSDP**, hands its physical address across in `BootInfo`,
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and how the platform derives the **RSDT/XSDT** from it and walks the SDTs.
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- **[smp.md](smp.md) — multiple cores.** A design/research note on how microkernels
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(L4, seL4) handle SMP — big kernel lock vs per-CPU vs multikernel — and how the
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right choice depends on whether danos is chasing real-time or resilience.
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- **[coding-standards.md](coding-standards.md) — coding standards.** The naming rule the
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tree follows: non-acronyms are spelled out in full (`message`, not `msg`), files are
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`kebab-case`, code follows Zig's case conventions, and the handful of exceptions
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(POSIX/C ABI names, `init`/`len`/`ptr`, acronyms).
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- **[sysv.md](sysv.md) — the calling convention.** What "the kernel is SysV" means,
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and why the loader→kernel boundary has to pin it (the RDI-vs-RCX handoff).
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- **[testing.md](testing.md) — testing.** How the kernel is tested by booting it in
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QEMU and asserting on its serial output — reproducibly, and structured so the
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same tests run across architectures.
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- **[logging.md](logging.md) — logging.** The multi-sink diagnostic log (serial,
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0xE9 debugcon, file later) kept separate from the framebuffer display, plus the
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robustness path: optional framebuffer, POST-code checkpoints, and a persistent
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panic breadcrumb so the kernel survives — and can be diagnosed — with no output.
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## How the pieces relate
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The boot flow ties them together: UEFI runs the loader ([efi.md](efi.md)), which
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queries the **GOP** to pick a graphics mode ([gop.md](gop.md)), hands the kernel a
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**framebuffer** to draw into ([framebuffer.md](framebuffer.md)) and a **memory
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map** of physical RAM ([memory-map.md](memory-map.md)); the kernel turns that map
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into a **frame allocator** ([frame-allocator.md](frame-allocator.md)), installs
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its **descriptor tables** so CPU faults are caught ([interrupts.md](interrupts.md)),
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builds its own **page tables** and switches onto them ([paging.md](paging.md)),
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brings up the **heap** for dynamic allocation ([heap.md](heap.md)), starts the
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**scheduler** ([scheduling.md](scheduling.md)) and the **timer** that preempts it
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([device-interrupts.md](device-interrupts.md)) — with tasks blocking, sleeping and
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passing messages over **[IPC](ipc.md)** channels — runs, its CPU-specific bits
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behind the [arch](arch.md) boundary, and when idle, or on a panic, it **halts**
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([halting.md](halting.md)).
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Above that line the microkernel proper begins: **discovery** ([discovery.md](discovery.md),
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[acpi.md](acpi.md)) learns what hardware exists, ring-3 processes ask the kernel for
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things through the small **[syscall](syscall.md)** table, isolated servers reach each
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other over IPC **endpoints** ([ipc.md](ipc.md)), and a **[driver](drivers.md)** claims
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a device, maps its registers, and sleeps until the hardware interrupts it — which is
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the whole reason for the arrangement ([vision.md](vision.md)).
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## Source map
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| Area | Code |
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|------|------|
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| Boot methods (one per way of booting the kernel) | `src/boot/` — `efi.zig` (UEFI) → `BOOTX64.efi` |
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| Kernel entry, panic, bring-up | `src/kernel/main.zig` |
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| Shared loader↔kernel contract (`BootInfo`, `Framebuffer`, `MemoryMap`, `Syscall`, ABI) | `src/root.zig` |
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| Physical frame allocator | `src/kernel/pmm.zig` |
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| Kernel heap (`std.mem.Allocator`) | `src/kernel/heap.zig` |
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| Scheduler (fixed-priority preemptive; blocking, wait queues) | `src/kernel/scheduler.zig` |
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| Big kernel lock + interrupt-safe critical sections | `src/kernel/sync.zig` |
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| IPC channels between kernel threads (message passing) | `src/kernel/ipc.zig` |
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| IPC endpoints: cross-address-space call/reply, handles, notifications | `src/kernel/ipc-synchronous.zig` |
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| User processes: ELF loading, address spaces, the syscall table | `src/kernel/process.zig` |
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| Device tree + claim capability + `device_register` containment | `src/kernel/device-service.zig` |
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| IRQ-as-IPC: routing a device interrupt to a driver's endpoint | `src/kernel/irq.zig` |
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| Hardware discovery (ACPI/device tree) behind one neutral device model | `src/device/` |
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| Framebuffer text console (mirrors to serial) | `src/kernel/console.zig` |
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| In-kernel test cases | `src/kernel/tests.zig` |
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| Arch-specific kernel code (`halt`, GDT/IDT/TSS, exception + interrupt stubs, page tables, APIC/IO-APIC/timer, serial, linker script) | `src/kernel/arch/x86_64/` |
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| User runtime library (`rt`): syscalls, heap, stdio, IPC, device access | `lib/` |
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| User-space programs shipped in the initrd (`init`, `vfs`, `hpetd` leaf driver, `busd` bus driver) | `sbin/` |
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| Build + `run-x86-64` (QEMU/OVMF) | `build.zig` |
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| QEMU integration test harness | `test/qemu_test.py` |
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