# `kmi`

`kmi` is a ***K***ernel with ***MI***grating threads I've been writing as a hobby project.
To see it in action, have a look at [kmi-example](https://github.com/Kimplul/kmi-example).

Currently the kernel only works in `qemu` with RISC-V 64 bit. There is some code for
RISC-V 32 bit support, but at the moment I'm focusing on 64 bit and 32 is less tested.

# Building

`riscv64-unknown-elf-` is the default toolchain looked for by the build system, but
`riscv64-linux-gnu-` will also work. Either should be available from your package manager.

Simple build with default configuration:
+ `make`

This results in `kmi.bin`, which is the bootloader and the payload kernel.
Currently the kernel expects to be loaded into a specific address, allowing arbitrary boot
locations is on my TODO list. The load address depends on the configured RAM base address,
see `arch/riscv64/conf/kmi.its` and `PM_KERN_BASE`. Dynamically detecting RAM base is also TODO.

Useful `make` flags and targets:

+ `docs`: Generate HTML documentation with doxygen. Currently mostly documents API stuff,
but my intention is to include higher level documentation as well.
Open `docs/output/html/index.html` in your favorite web browser.

+ `RELEASE=<0/1>`: Enable optimizations when set to `1`. Default is `0`.
Note that with `RELEASE=1` the built in serial driver is disabled, as eventually
I'd like to provide it as a separate userspace driver. At the moment only NS16550A and
compatible serial devices are supported.

+ `LLVM=<0/1>`: Use LLVM toolchain when set to `1`. Default is `0`.
Note that due to some bugs and missing features in LLVM RISC-V support, some GNU
binutils tools are still needed, mainly `objcopy`. Also, when combined with `RELEASE`,
disables LTO as `lld` seemed to have issues with some relaxed instructions with LTO enabled.

+ `UBSAN=<0/1>`: Enable undefined behavior sanitizer, outputs a number of warnings at runtime when
undefined behavior is detected. Only available with `RELEASE=0`.

+ `run`: Load a test program into `qemu` and run it. Requires some outside support at the moment,
please see [kmi-example](https://github.com/Kimplul/kmi-example). This command might eventually be moved
out of this repo.

+ `clean/clean_docs/clean_run/clean_all`: Clean compile artefacts, clean documentation artefacts,
clean run artefacts and clean all artefacts, respectively. Note that `clean_run` and `clean_all` may
need root privileges.

# Top-level view

This should eventually be moved into documentation with more details, but in short, `kmi` is a hybrid
kernel with thread migration as the main method of inter-process communication. The kernel itself
handles interrupts, memory management and thread migration, but everything else is inteded to be
up to the operating system built on top of the kernel.

I chose thread migration as the main IPC method because it seemed interesting, somewhat out of the
ordinary and has some parallel features that I think might come in handy as computers slowly but surely
become more parallel.

I chose to implement memory management in the kernel mainly for ease of development. While I am
generally in favor of microkernels and separating responsibilities, I find that memory is central
enough to computers that trying to write a kernel without memory management would be too difficult
for me.

# Future plans

Interrupt and exception handling is still largely TODO.

To keep things interesting I'm working on a heavily simplified Unix-like operating system,
as of yet unreleased, that runs on the `kmi` kernel, with my main goals to provide a shell environment
and disk access. I'm hoping that having a somewhat usable 'full' system will help me run into bugs more
effectively and keep me more motivated to work on the project.