# Developer Resources

## Design Goals & Philosophy

General design goals are outlined in [DESIGN.md](DESIGN.md). Security-relevant
design details can be found in [SECURITY.md](SECURITY.md).

## Navigating

- [kernel/](/src/kernel/): Kernel-specific code.
  - [arch/](/src/kernel/arch/): Architecture specific features like the
  display, serial, and interrupts. Each architecture is a subfolder,
  containing a file or module for each feature.
  - [boot/](/src/kernel/boot/mod.rs): Handles bootloader-managed data
  structures. Gila uses Limine. Other bootloaders are NOT supported.
    - [params.rs](/src/kernel/boot/params.rs): Command line parameter parsing.
    - [modules.rs](/src/kernel/boot/modules.rs): Kernel module handling.
  - [constants.rs](/src/kernel/constants.rs): Constants referenced elsewhere
  in the kernel.
  - [device/](/src/kernel/device/mod.rs): Functions for discovering hardware
  and assigning drivers.
    - [acpi.rs](/src/kernel/device/acpi.rs): ACPI handling functions and
    structures.
  - [log.rs](/src/kernel/log.rs): Logging structures, macros, and singletons
  for logging to serial or the display.
  - [interrupt/](/src/kernel/interrupt/mod.rs): Interrupt handlers with
  platform-agnostic APIs.
  - [main.rs](/src/kernel/main.rs): The entry point that gets called by the
  bootloader.
  - [memory.rs](/src/kernel/memory.rs): Types relating to memory regions and
  allocation.
  - [panic.rs](/src/kernel/panic.rs): The panic handler and associated
  functionality.
  - [process.rs](/src/kernel/process.rs): Process types and functions.
  - [syscall\_runner.rs](/src/kernel/syscall_runner.rs): Chooses a system call
  by its ID and defers actual syscall execution to code in `src/lib/`.
- [lib/](/src/lib/lib.rs): Library that all Gila's binary programs will be
built against. Some of this code is shared with the kernel.
  - [arch/](/src/lib/arch/mod.rs): Architecture specific functionality like
  system call register storing/loading.
  - [syscall.rs](/src/lib/syscall.rs): System call types common to apps and
  the kernel.

## Building and running

Building a bootable kernel is easy. All you need to do is run
`cargo build --bin kernel`, and a valid, bootable Limine executable will be
generated. However, it cannot be booted without installing it in a bootable
Limine filesystem, and it cannot do anything useful without an initramfs
containing system servers, such as the init server and device drivers.

This project uses [cargo-make](https://github.com/sagiegurari/cargo-make) to
handle building ISOs and managing files not associated with Cargo. You need to
install it before you can build an ISO automatically. To do so, you can run
`cargo install cargo-make`. In addition, you will also need:

- `rustup` command installed
- `limine` command installed
- `xorriso` command installed
- `qemu-system-{your target architecture}` command installed (for running)

Then run `cargo make` to invoke the [Makefile.toml](Makefile.toml).

- `cargo make clean_all`: Cleans all built binaries, libraries, initramfs
files, and ISOs.
- `cargo make lib`: Builds `libgila`, the library that the kernel and user
code are linked against.
- `cargo make kernel`: Builds the kernel ELF file.
- `cargo make initramfs`: Build the init archive.
- `cargo make iso`: Builds the bootable ISO with Limine installed.
- `cargo make run`: Builds the ISO and boots it in QEMU.
- `cargo make debug`: Launch the kernel in QEMU with debugging enabled, and
start and connect GDB.

You do not need to clean any files after making changes. The `lib`, `kernel`,
and `iso` tasks will automatically be rerun if their input files change.

### Configuration

- Variable `LIMINEDIR`: Location of binary files for limine. Default is
`/usr/share/limine`.
- Variable `TARGET`: rustc target triple to compile for. Default is
`x86_64-unknown-none`. Options are listed [in the targets section](#targets).
- Argument `-p`: Rust build profile to use. Default is `dev`. Options are
`dev` and `release`.

> [!NOTE]
> The `-p {profile}` argument must go between `cargo make` and the task
argument.

### Features

Gila has four optional features, which I made optional in anticipation of a
potential future port to older systems which might not support modern
standards like UEFI or ACPI. They are all enabled by default. Disabling them
reduces kernel size and forces the kernel to fall back to other
implementations for some functionality like device discovery.

- `acpi`: Advanced Configuration and Power Interface, for device discovery &
power management
- `dtb`: Device Tree Blob, for device discovery in embedded systems
- `compression`: Compressed initramfs archive
- `uefi`: Universal Extensible Firmware Interface specific bootloader features

### Targets

Gila ~~currently supports~~ aims to support four different CPU architectures:

- `x86_64`
- `aarch64`
- `riscv64`
- `loongarch64`

It currently only builds on `x86_64`, and will continue to only support it
until I can implement support for important features in other architectures.

All these architectures are supported by Limine, and the appropriate backends
are present in the Makefile to compile and build bootable images for each.
While Limine (the bootloader) and rustc also support `IA32` (also referred to
as `i686`), the Limine boot protocol cannot work on a 32-bit architecture, as
it relies heavily on 64-bit pointers. Compilation will fail if a build for an
unsupported target is attempted.

EFI boot is presently supported, at least on `x86_64`. No features depend on
EFI, and as such, the `uefi` feature can be safely disabled when booting
through BIOS.

## Kernel Parameters

Kernel parameters are passed as part of the `cmdline` through
[limine.conf](configs/limine.conf). The parameters are passed as a
space-delimited list of keys and values. Keys begin with a hyphen (`-`), and
keys are separated from their values with equals signs (`=`). Keys can have a
set of multiple values, separated by a comma (`,`). Gila does not currently
support parameter values with spaces. That would require an *actual* parser.

List of current extant kernel parameters:

- `-loglevel`: Can be a number or string corresponding to a log level. Only
one value supported. Current options are `Disabled`, `Trace`, `Info`,
`Warning`, `Error`, and `Critical`. This parameter is case insensitive.
- `-logdev`: A sequence of one or more values representing devices to log to.
Current options are `display` and `serial`. This parameter is case
insensitive.
- `-initramfs`: A valid path to a module to serve as the initramfs (containing
the init binary). Only one value supported. This parameter is case sensitive.

The default behavior for each parameter, when not supplied, is:

`-loglevel=Info -initramfs=/boot/initramfs.tar.lzma`

The `.lzma` extension is removed from the default initramfs name when
compression is disabled. It must also be changed in
[limine.conf](configs/limine.conf) or else Limine will not load it.

## Writing Programs for Gila

Gila's system calls will soon be fully defined in `libgila`. The library is
developed in tandem with, and is used within, the kernel. As Gila does not
currently support any kind of Rust or C standard library, you must compile
your programs for the same architecture as Gila itself, bare metal. Userspace
programs must not use any privileged instructions that would cause an
exception if running in Ring 3 or any other least-privileged mode of a
processor.
