System emulation using qemu: Raspberry PI 3

Photo by Robert Katzki on Unsplash

Welcome to a short series about using qemu. I wanted to write about this for a while now but just didn’t get the opportunity so I said I will just spend a few weeks getting this sorted. You see, having virtual machines is easy these days: you can use Gnome Boxes, VirtualBox and many others. Some are open source, others free and of course many others paid. The main issue here is not all these virtual machines are created equal.

Gnome Boxes for example is great: just install and run. The catch? It cannot emulate a different system than the one you are currently running. Gnome Boxes simply runs a different Linux distribution on top of yours. There are talks about giving it the ability to at least emulate aarch64 but that’s all they are: talks. Same goes for containers like toolbox: they just allow you to better compartmentalize your work or experiment with technologies that would not be recommended for the live system.

On the other hand qemu, VirtualBox and other stronger emulators allow you to create any kind of system and use hardware acceleration where possible. There is no flatpak for VirtualBox yet, so you need to resort to other tricks to run it in Fedora Silverblue, but qemu runs just fine in a toolbox container. It’s also good to know that qemu is the backbone of many such emulators including VirtualBox and KVM, so learning a bit about how it works never hurts.

Prerequisites for Raspberry PI 3

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Let’s see what we need to run Raspberry PI 3 on our Fedora Silverblue PC. We need qemu because the Raspberry PI is an ARM architecture and we are using an x64 PC so there is a total mismatch between the systems. To emulate the ARM machine we need three things: an ARM kernel, a hardware description file and a prepared disk image to boot (a Linux distribution like Raspbian will to). We will use all these three to configure qemu and have our own emulated Raspberry PI 3.

But first, why do we need a kernel? Doesn’t the prepared Raspbian image have a kernel anyway? Well, we are creating a machine from scratch here which means we have to babysit the whole booting process. To boot, the machine needs to initialize its hardware (hence the hardware description file) and the boot loader needs a kernel to pass control to, hence the kernel image. The kernel then initializes and mounts our Raspbian image. Without a kernel, there is no mount system so our Raspbian image is useless.

Getting the kernel and the hardware description file

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You can grab the kernel and the hardware description file from my Github, but it’s more fun to get them yourself from the official Raspberry PI image. Just head to the web page and download the last image available. I got the 2022–04–04-raspios-bullseye-armhf.img.xz file which I unpacked using xz:

#xz --decompress 2022–04–04-raspios-bullseye-armhf.img.xz

The result will be 2022–04–04-raspios-bullseye-armhf.img, a bootable image of Raspberry PI containing two partitions:

Listing the image partitions using fdisk

The kernel and the hardware description file are in the first partition, starting at position 8192. Let’s mount it so we can extract the needed files:

#sudo mount -o loop,offset=4194304 ./2022-04-04-raspios-bullseye-armhf.img /mnt/test

Note the 4194304: since the partition starts at sector 8192 and the sector size is 512 bytes, the final offset is 512 * 8192, which is 4194304. Now in /mnt/test we have the boot partition of the Raspberry PI image:

All the required files are here

Running qemu for the Raspberry PI 3 hardware

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We can now start qemu with the required files. I am using Fedora Silverblue so I will enter a toolbox and install qemu in it using sudo dnf install qemu. Next we can run:

#qemu-system-aarch64 \
   -machine raspi3b \
   -cpu cortex-a72 \
   -dtb /mnt/test/bcm2710-rpi-3-b-plus.dtb \
   -m 1G -smp 4 -serial stdio \
   -kernel /mnt/test/kernel8.img \
   -append "rw earlyprintk loglevel=8 console=ttyAMA0,115200 dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 rootdelay=1"

The above command will start an aarch64 ARM machine, configured with the Raspberry PI 3B specifications, with a Cortex A72 CPU, using the Raspberry PI 3 hardware description provided by the bcm2710-rpi-3-b-plus.dtb file, with 1 GB of RAM, 4 cores, connected to the standard Linux IO. It will boot using the Raspberry PI 3 kernel provided by the kernel8.img file and will have a few boot options that configure the console and the root drive which for a Raspberry PI is the eMMC device located at /dev/mmcblk0p2.

The command is a mouthful but allows precise configuration of all the aspects of our virtual Raspberry PI 3 machine. One thing is missing though. If we run it, it will break here:

No root block device found

The kernel is panicking because it cannot find any bootable devices. Basically we have the kernel and everything, but we need a disk image from which to boot our system. We configured the root drive to be /dev/mmcblk0p2 but there is nothing there. Our Raspberry PI acts as if there were no SD card to boot from. But we already downloaded the Raspbian OS image, so let’s use it:

#qemu-system-aarch64 \
   -machine raspi3b \
   -cpu cortex-a72 \
   -dtb /mnt/test/bcm2710-rpi-3-b-plus.dtb \
   -m 1G -smp 4 -serial stdio \
   -kernel /mnt/test/kernel8.img \
   -sd ./2022-04-04-raspios-bullseye-armhf.img
   -append "rw earlyprintk loglevel=8 console=ttyAMA0,115200 dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2 rootdelay=1"

Running the above will exit with another error:

qemu-system-aarch64: Invalid SD card size: 4.03 GiB
SD card size has to be a power of 2, e.g. 8 GiB.

So let’s resize the image:

#cp ./2022-04-04-raspios-bullseye-armhf.img ./2022-04-04-raspios-bullseye-armhf-resized.img
#qemu-img resize ./2022-04-04-raspios-bullseye-armhf-resized.img 8G

It will complain that we let it auto detect the image format but in the end it does the job. Our new resized image has 8 GB. Now if we try to run qemu, we will get the expected result:

Raspberry PI 3 emulation started

If the boot sequence runs without errors, we should arrive at the login prompt:

Raspbian waiting for login

Meanwhile, the virtual machine display will show the Raspbian 11 interface:

The Raspbian 11 user interface

We are done for now. We successfully created a virtual Raspberry PI 3 virtual machine on our x86 system using qemu. Next time we will try to add some virtual devices to our setup like a network card and we will try to upgrade our machine to a brand new Raspberry PI 4. See you then!