The objective of this tutorial is to run a Hello World program on QEMU. The prerequisite is to have a cross-compiler for OpenRISC 1000 (or1k). And don’t forget to add it to the PATH .

Intro

QEMU is a generic emulator that supports various target architectures. It can be used to emulate a 32-bit OpenRISC CPU.
QEMU has two different modes of emulation: user mode and system emulation mode. The user mode only allows you to run programs compiled for the target architecture, while the system mode emulates the complete hardware.

User Mode

Install QEMU

Install the pre-built package from your distribution’s package manager. On Ubuntu, run
sudo apt install qemu-user-static
for running statically linked binaries OR
sudo apt install qemu-user
for running dynamically linked binaries. Personally, I am using the static version for this tutorial.

Cross-compile the Program

hello.c is included in this directory. Compile it using the cross-compiler you have. For me, the command is
or1k-none-linux-musl-gcc hello.c -static -o hello.

And check if the output file type looks correct using file command.

file hello
hello: ELF 32-bit MSB executable, OpenRISC, version 1 (SYSV), statically linked, with debug_info, not stripped

Note that -static flag was used when compiling. Without this, the output will be a dynamically linked ELF, which will give an error, qemu-or1k-static: Could not open '/lib/ld-musl-or1k.so.1': No such file or directory, if we try to run it using qemu-or1k-static.

Run the program

qemu-or1k-static hello

If the output is Hello World!, then everything is working correctly.

System Emulation

To run this mode, you need a rootfs and build the Linux kernel with it. See Linux Openrisc for the instructions. Once the kernel is built, you will see vmlinux file in the root directory. Running file vmlinux should give you the following output:

vmlinux: ELF 32-bit MSB executable, OpenRISC, version 1 (SYSV), statically linked, BuildID[sha1]=..., with debug_info, not stripped

Make sure it is an ELF file for OpenRISC, not your host system. If that happened, it means the kernel was built using a wrong toolchain (host system’s toolchain).

Install QEMU

Install the pre-built package from your distribution’s package manager. On Ubuntu, run
sudo apt install qemu-system

Running QEMU

Now that we have all the components we need, we can run QEMU like so:

qemu-system-or1k -cpu or1200 -M or1k-sim -kernel <path-to-your-vmlinux> -serial stdio -nographic -monitor none

Read the output generated and see if everything looks alright, but don’t be alarmed if you see Initial ramdisk not found somewhere in there. It is because we did not explicitly pass one to QEMU using -initrd flag. This is okay, because we have it baked in inside the kernel and kernel uses that when it boots up. As long as the last line says Please press Enter to activate this console and you can see the basic file system inside the QEMU instance, it is all good.

If you want to learn more about QEMU invocation options, please visit this page. OpenRISC-specific QEMU information can be found in this Wiki page.

One thing you will notice once you get to this stage is that you cannot write a C program and compile it inside QEMU, because there is no compiler.

How to Run a Program

For this tutorial, we will use the simplest method of embedding our hello binary in the initramfs, so that when QEMU boots, the binary is already there to run. I put it in <path-to-initramfs>/usr/bin and re-compiled the kernel with it.

Start a QEMU instance with the new kernel and run the hello binary.

Please note that this method is not scalable/sustainable and using SSH, although it is more complicated to set up with QEMU, is recommended.