Kernel Modules and Device Drivers

In Linux

LDD3: Chapters 1 & 2

Questions to answer

• What are devices and drivers?
• What are Linux kernel modules?
• What are key differences between kernelspace and userspace?

What is a device?

• A physical component that is attached to the cpu
• The “organs” of the computer
• A replaceable component
  • Mostly

What is a device driver?

• Bridge OS/Hardware gap
• Allow users to talk to device
• OS <-> driver <-> CPU <--> devices

Overview

http://haifux.org/lectures/86-sil/kernel-modules-drivers/kernel-modules-drivers.html

What is a Kernel Module

• Snippet of code that can be added to a running kernel

Why Kernel Modules for Device Drivers?

• Keep kernel text/data minimal
• Change text/data without reboot
• More customization
• Less error surface
• Easier to develop
• Can add hardware at runtime
  • (if you know its address)

We are not in Kansas anymore

• Goodbye main
• Goodbye C standard library
• Goodbye IEEE754
• Hello ring 0 (x86_64)/EL1 (aarch64)
• Hello asynchronous execution
• Hello kernel API

Kernelspace vs Userspace

• Small, fixed-size stack
• Each mode can have it’s own address space
• Kernel space handles access to hardware
• Execution is transferred to kernel space after a system call

Reentrant code

• Potential confusion: What is called when?
• Kernel code must be able to run in more than one context
• Mutexes mandatory
  
  https://www.cs.mcgill.ca/~cs573/fall2002/notes/lec273/lecture15/re-ent2.GIF