This repository contains an out-of-tree Linux kernel driver for the debugcon (Debug console) device, which is present in the VMMs QEMU and [Cloud Hypervisor)(https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/devices/src/debug_console.rs) (as of >= v38.0). The driver only works on x86, as it needs I/O ports to access the device.
The build process of the driver follows the recommended guidelines.
The driver will provide a character-device file in /dev/debugcon. One process
at a time can open the device file and write bytes to it. Depending on the
configuration of QEMU, the output will land in the configured location.
The driver is packaged in Nix, which allows an easy build setup. Although the driver can be built the regular way, it is recommended to use Nix.
By using the $ nix-build -A runQemuDemoBin command, you get a shell script in
./result/bin/run_qemu_demo that starts a QEMU VM with a minimal Linux kernel
and initrd, that automatically loads the out-of-tree debugcon kernel module at
runtime. Once you are in the VM shell, can write to the device like this:
echo "foo" > /dev/debugcon. By default, the VMM will redirect all output to
./debugcon.txt.
$ nix-build -A runQemuDemo && ./result- (once the prompt of the VM appears)
# echo "foo" > /dev/debugcon - (on the host)
$ cat debugcon.txtwill show what was written to the device
To build the kernel module for your system (without Nix), you can enter the
src directory and type $ make. Ensure that you have installed all relevant
packages for your system that are needed to build a Linux kernel (module).
This resource might be
helpful for you.
If you managed that, you should be able to type $ insmod debugcon.ko. However,
adding this to your main system doesn't make much sense, except you are inside a
QEMU VM.