kalorona-kernel

A toy kernel on RISC-V platform.

Features

Implemented

Kernel Features

Module	Description
Kernel Loader - boot
Setting up MMU	Making temporary page-tables for kernel address space, and jumping to kernel at virtual address space.
Console - console	Printing info to UART device, currently through legacy SBI extension.
Device - device
Loading Flatten Device Tree(FDT)	Using libfdt to read info from the FDT given by bootloader.
Detecting System Memory	FDT conatins the info of physical memory space(reservation, availablity).
Hardware Abstraction Layer(HAL)	Console, filesystem etc are able to call hardware through HAL, directing hardware requests to drivers.
Drivers	Supporting RISC-V PLIC and ns16550a Compatible UART device.
Loading Drivers While Walking on FDT	Device discovery, driver loading and device initialization.
Memory Management - memory
Memory Management for Boot Time	Implemented bootmem for early stage memory allocation.
Continuous Page Allocation	Page allocation akin to Buddy.
Object Allocation	Introducing lightweight object allocator.
User Space - proc
Trap/Interrupt Handling	Handling trap/interrupt in kernel/user space.
Timer Interrupt	Supported by SBI firmware, enabling time-span scheduling.
Process Management	Create, kill and reap process control block (PCB).
Context Switcher & Scheduler	Switching between RISC-V supervisor & user mode.
System Call	Supporting kill, exit, fork and other system calls.
System Functionalities
Spinlock
Utilities for Kernel Code - utilities/	Including operations such as bit operations, linked list, sorting algorithms and string/memory related functions.

User SDK

Basic system calls and string utility functions.

Ongoing

• User Space
• File System
• Virtio Driver for I/O
• Device Discovery

Planned

Nothing for now.

Build

Prerequisite

To build, you need the RISC-V GNU Toolchain. Then, set the environment variable CROSS_COMPILE to the prefix of your toolchain name. For example, if the RISC-V GCC is named riscv64-unknown-linux-gnu-gcc, then set CROSS_COMPILE to riscv64-unknown-linux-gnu-.

Besides, you need these packages:

sudo apt install u-boot-tools build-essential gdb-multiarch qemu-system-misc device-tree-compiler

In order to run properly, you also need OpenSBI environment and U-Boot bootloader.

OpenSBI

Please refer to Section "Execution on QEMU RISC-V 64-bit - No Payload Case" of this article to build the OpenSBI. Then, set environment variable SBI_BUILD to the OpenSBI binary fw_jump.elf. Typically, it is located at ${Your OpenSBI root folder}/build/platform/generic/firmware/fw_jump.elf.

U-Boot

Please refer to Section "Running U-Boot SPL" of this article to build U-Boot. You may make a link of the previous OpenSBI build to the root folder of U-Boot. Follow the instructions for 64-bit RISC-V. Then, set environment variable UBOOT_DIR to the directory of the U-Boot repo.

Build Kernel

If you want to build the ELF file only, build the target build/kernel.elf:

cd kernel
make build/kernel.elf

To build a bootable disk image, build the target kernel. This will ask for root privilege to attach the disk image to a loopback device and mount the loopback device to a local folder.

cd kernel
make kernel

Run & Debugging

It is recommended to use a bootloader(e.g U-Boot) to load the kernel ELF file. To run the kernel automatically:

make qemu-uboot

To debug:

make qemu-debug-uboot

A TCP port will be exposed for GDB debugging. It is typically 25501. The .gdbinit file at the root of the repo may be helpful for GDB configuration.

References

• xv6, a simple Unix-like teaching operating system, MIT PDOS, https://github.com/mit-pdos/xv6-riscv
• M. Gorman, Understanding the linux virtual memory manager. Upper Saddle River: Prentice Hall PTR, 2004.