F9 Microkernel

This is F9, an experimental microkernel used to construct flexible embedded systems inspired by famous L4 microkernel. The motivation of F9 microkernel is to deploy modern kernel techniques to support running real-time and time-sharing applications (for example, wireless communications) for ARM Cortex-M series microprocessors with efficiency (performance + power consumption) and security (memory protection + isolated execution) in mind.

Characteristics of F9 Microkernel

• F9 follows the fundamental principles of microkernels in that it implements address spaces, thread management, and IPC only in the privileged kernel.

• Designed and customized for ARM Cortex-M, supporting NVIC (Nested Vectored Interrupt Controller), Bit Banding, MPU (Memory Protection Unit).

• Energy efficient scheduling and tickless timer which allow the ARM Cortex-M to wake up only when needed, either at a scheduled time or on an interrupt event. Therefore, it results in better current consumption than the common approach using the system timer, SysTick, which requires a constantly running and high frequency clock.

• KProbes, dynamic instrumentation system inspired by Linux Kernel, allowing developers to gather additional information about kernel operation without recompiling or rebooting the kernel. It enables locations in the kernel to be instrumented with code, and the instrumentation code runs when the ARM core encounters that probe point. Once the instrumentation code completes execution, the kernel continues normal execution.

• Each thread has its own TCB (Thread Control Block) and addressed by its global id. Also dispatcher is responsible for switching contexts. Threads with the same priority are executed in a round-robin fashion.

• Memory management is split into three concepts:

  • Memory pool, which represent area of physical address space with specific attributes.
  • Flexible page, which describes an always size aligned region of an address space. Unlike other L4 implementations, flexible pages in F9 represent MPU region instead.
  • Address space, which is made up of these flexible pages.

• System calls are provided to manage address spaces:

  • Grant: The memory page is granted to a new user and cannot be used anymore by its former user.
  • Map: This implements shared memory – the memory page is passed to another task but can be used by both tasks.
  • Flush: The memory page that has been mapped to other users will be flushed out of their address space.

• Regarding the interaction between a user thread and the microkernel, the concept of UTCB (user-level thread-control blocks) is being taken on. A UTCB is a small thread-specific region in the thread's virtual address space, which is always mapped. Therefore, the access to the UTCB can never raise a page fault, which makes it perfect for the kernel to access system-call arguments, in particular IPC payload copied from/to user threads.

• The kernel provides synchronous IPC (inter-process communication), for which short IPC carries payload in CPU registers only and full IPC copies message payload via the UTCBs of the communicating parties.

• Debugging and profiling mechanisms:

  • configurable debug console
  • memory dump
  • thread profiling: name, uptime, stack allocated/current/used
  • memory profiling: kernel table, pool free/allocated size, fragmentation

Licensing

F9 Microkernel is freely redistributable under the two-clause BSD License. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.

Quick Start

The current hardware board F9 Microkernel supports is STM32F4DISCOVERY based on ARM Cortex-M4F core, but F9 should work well on any STM32F40x microcontroller.

Building F9 Microkernel requires an arm-none-eabi- toolchain with Cortex-M4F support. The known working toolchains are as following

• Sourcery CodeBench
  • ARM Processors: EABI Release
  • Verified versions: arm-2012.03, arm-2013.05, arm-2013.11
• GNU Tools for ARM Embedded Processors
  • Verified versions: 4.8-2013-q4-major

Other build dependency includes: (for Debian/Ubuntu)

• libncurses5-dev

Configuration is the initial step in the build of F9 Microkernel for your target, and you can use make config to specify the options from which to choose. Regardless of the configuration method you use or the actual configuration options you choose, the build system will generate a .config file at the end of the configuration and will generate a configuration header file, include/autoconf.h for C programs.

Then, just execute make to get the generated files in directory build.

For flashing and debugging on the STM32F40x, stlink is required. With stlink in your path, command "make flash" will flash your STM32F4DISCOVERY board with built F9 binary image.

When developing on top of F9 Microkernel, you do not have the luxury of using a source level debugger such as gdb. There are still a number of techniques at your disposal to assist debugging, however. KDB (in-kernel debugger) is built and run at boot by default, and here are the supported commands:

• a: dump address spaces
• m: dump memory pools
• t: dump threads
• s: show softirqs
• n: show timer (now)
• e: dump ktimer events
• K: print kernel tables

Through USART, KDB can be operated interactively on USART4 (default), USART2, or USART1 of STM32F4DISCOVERY depending on the selected option when you execute make config:

• USART4: PA0 (TX), PA1 (RX)
• USART2: PA2 (TX), PA3 (RX)
• USART1: PA9 (TX), PX10 (RX)

You can established serial connection with the board using a serial to USB converter:

• USB2TTL RX ---> PA0 / PA2 / PA9
• USB2TTL TX ---> PA1 / PA3 / PA10

Select the appropriate terminal emulator and configure it for 115200 baud, 8 data bits, no parity, one stop bit. For GNU/Linux, program screen can be used for such purpose. Installation on Ubuntu / Debian based systems:

sudo apt-get install screen

Then, attach the device file where a serial to USB converter is attached:

screen /dev/ttyUSB0 115200 8n1

Once you want to quit screen, press: Ctrl-a k

Build Configurations

F9 Microkernel deploys Linux Kernel style build system, and the corresponding files are described as following:

• toolchain.mk:
  • toolchain-specific configurations; common cflags and ldflags
• platform/build.mk:
  • platform-specific configurations; cflags/ldflags for CPU and FPU
• board/<BOARD_NAME>/build.mk:
  • board-specific configurations; CHIP model, peripherals
• rules.mk: the magic of build system

You can modify source file board/<BOARD_NAME>/board.[ch] to specify the preferable resource assignment. To get acquainted with the configuration of F9 Microkernel internals, file include/autoconf.h is the entry point:

• CONFIG_DEBUG
  • Enable serial input/out for debugging purpose. An additional service for serial I/O character operations will be included.
• CONFIG_KDB
  • Enable in-kernel debugger.
• CONFIG_KPROBES
  • Enable kernel probes, the lightweight dynamic instrumentation system.
• CONFIG_BITMAP_BITBAND
  • Generate bitmap address in bit-band region
  • Bit-banding maps a complete word of memory onto a single bit in the bit-band region. For example, writing to one of the alias words will set or clear the corresponding bit in the bitband region.
  • When writing to the alias regions bit 0 of the 32 bit word is used to set the value at the bit-banding region. Reading from the alias address will return the value from the bit-band region in bit 0 and the other bits will be cleared.
• CONFIG_MAX_xxx series
  • limits of threads, ktimer events, async events, address spaces, flexible pages.
• CONFIG_PANIC_DUMP_STACK
  • Dump kernel stack while panic.