qemu-edu-driver

This is a Linux kernel module written for the QEMU edu device. I found it useful to write a driver for this device while reading Linux Device Drivers, Third Edition, because it allowed me to put the concepts into practice.

Building the module

On Debian-based systems, install the linux-headers-amd64 package, then run:

make -C /lib/modules/$(uname -r)/build M=$(pwd) modules

Alternatively, if you cloned the kernel repository and already have a custom .config, then you can just point the make command to that directory instead:

make -C /path/to/your/linux/tree M=$(pwd) modules

Make sure that the kernel version for which you are building the module is exactly the same as the kernel version running in the VM. If you already built a custom kernel, you can pass it to QEMU using the -kernel option.

Loading the module

Copy the edu.ko file into the VM, then run

insmod edu.ko debug=1 msi=1

The following parameters are supported:

• debug (bool): if true, verbose log messages will be enabled
• msi (bool): if true, MSI will be used instead of the legacy pin-based interrupt

In the dmesg output, you should see a message which looks like this:

edu:edu_init: device number is 250:0

Alternatively, you can grep for "edu" in /proc/devices. Once you have the major/minor device numbers, you can create a device node:

mknod /dev/edu c 250 0

The device node must exist to use edu-cli (see below).

edu-cli

This repository has a CLI program called edu-cli which can be used to interact with the driver via an IOCTL-based interface. To build it, run

make edu-cli

It supports the following commands:

• edu-cli ident: Show the major/minor version of the edu device
• edu-cli liveness <number>: Perform a "liveness check". This just writes a number to the device, and the device computes the bitwise NOT of the value.
• edu-cli factorial <number>: Compute a factorial. This uses interrupts to wait for the calculation to finish.
• edu-cli wait: Wait for any interrupt to arrive, and print the value in the interrupt status register.
• edu-cli raise <number>: Raise an interrupt. The number will appear in the interrupt status register.
• edu-cli dma-write: Write a string into EDU device buffer via DMA. The string is accepted over stdin and can be at most 4096 bytes. Example: echo hello | ./edu-cli dma-write
• edu-cli dma-read <size>: Read size bytes from the EDU device buffer via DMA. The bytes are printed to stdout.