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Previous Part 4. Devices, VirtIO and more

BIOS & UEFI

Basic Input/Output System (BIOS) and Unifed Extensiable Firmware Interface (UEFI) are essential components that provide the necessary initialization process for machines.12 They are basically the 'first' program that loads when a machine powers up.

These 'programs' are generally stored on a machine's Read-only Memory (ROM). Their main function is to initialize and configure hardware components, perform a Power-On Self-Test (POST), and provide the necessary interfaces for booting the operating system.3. They, in turn, give controls to other programs (a.k.a bootloaders) like GRUB2, SysLinux, MS-DOS.

BIOS and UEFI are two different firmware that tries to solve the same problems.

QEMU, by default, uses a BIOS called SeaBios. It is a pretty good option and most can be used with most bootloaders.4 And naturally, every guest machine is loaded with the SeaBios and you don't have to do anything. However, you might want, or need, to use UEFI instead.

UEFI is the modern replacement of BIOS. And naturally, we want to use UEFI whenever possible;)

OVMF

Open Virtual Machine Firmware, OVMF, is an EDK II based project to enable UEFI support for Virtual Machines. OVMF contains sample UEFI firmware for QEMU and KVM. 5. It is an amazing project with amazing people behind it! I won't talk about it here for obvious reasons. If you want to learn more visit their GitHub repository OVMF - FAQ

Starting from QEMU version 1.6, which was release on December 3 2013, the OVMF project comes pre-installed with many QEMU installations (e.g. Windows). However, package managers like pacman does NOT include it! So, you have to manually install OVMF firmwares.

By time time I'm writing this, your package manager might be updated to also include OVMF packages.

To install the OVMF UEFI firmwares (if you don't have it already):

GNU/Linux

# For 'apt' based distros (e.g. Debian, Ubuntu)
$ apt install qemu-efi-aarch64

# For 'pacman' or 'yay' based distros (e.g. ArchLinux)
$ pacman -S edk2-ovmf

Windows 10/11

Comes pre-installed with the MSI Installer.

macOS 13.0 (Ventura)

Comes pre-installed with Homebrew.

Each CPU architecture (e.g. aarch64, x86_64) has different OVMF firmware. You can see which ones are installed on your system by checking their file locations. Their locations can be found under this repositorie's /Firmwares folder. Access it here Firmwares

UEFI on QEMU

To enable UEFI on a QEMU machine we have to provide two things: UEFI firmware and UEFI variables. In this section I will explain how to do that, but you should know that most of the stuff here is taken from Joonas's article on UEFI, PC boot process and UEFI with QEMU and Arch Linux Wiki.

I highly encourage you to read Joonas's writing. It is totally amazing and will give you a much better context on what we are doing here.

Step 1 - Providing the UEFI firmware itself

This is as easy as adding a -drive to your guest machine. The important thing here is the -drive is of type pflash and is read-only.

$ qemu-system-aarch64 ... -drive if=pflash,format=raw,readonly=on,file=${EFI_FLASH_PATH} ...

# The EFI_FLAS_PATH can be obtained from the previous section

Step 2 - Providing the UEFI variables

The UEFI firmware itself is readonly. We can do some configurations on it like the Boot Order, Secure Boot and Hardware Configurations. UEFI needs to save them somewhere so that they can be accessed after a power reset. This somewhere is basically a -drive called UEFI variables.

The UEFI variables is nothing special. It is a basic image with a very-specific size of 64MiB. So, we need to create a raw image!

Create a raw image with size 64MiB (Windows users can skip this):

$ dd if=/dev/zero of=efi-vars.raw bs=1M count=64

Windows users can just retrieve it from the QEMU installation directory:

$ ls 'C:\Progam Files\qemu\share\edk2-arm-vars.fd'

# Replace 'arm' with 'i386' if your guest machine is x86_64

Now, provide the UEFI variables image:

$ qemu-system-aarch64 ... -drive if=pflash,format=raw,file=${EFI_VARS_PATH} ...

# The EFI_VARS_PATH is the path of the image we've just crated

Step 3 - Enjoy the UEFI (Optional) 💚

Your guest machine should now boot with the UEFI firmware. To test it, just launch your machine and check if TianaCore appears on the boot screen.

Next Part 6. Networking

Footnotes

  1. https://en.wikipedia.org/wiki/BIOS ↩

  2. https://en.wikipedia.org/wiki/UEFI ↩

  3. https://en.wikipedia.org/wiki/Power-on_self-test ↩

  4. https://en.wikipedia.org/wiki/SeaBIOS#Development ↩

  5. https://github.com/tianocore/tianocore.github.io/wiki/OVMF ↩