QEMU x86

Build instructions for bare mode

To build u-boot.rom for QEMU x86 targets, just simply run:

$ make qemu-x86_defconfig (for 32-bit)
$ make qemu-x86_64_defconfig (for 64-bit)
$ make all

Note this default configuration will build a U-Boot for the QEMU x86 i440FX board. To build a U-Boot against QEMU x86 Q35 board, you can change the build configuration during the ‘make menuconfig’ process like below:

Device Tree Control  --->
    ...
    (qemu-x86_q35) Default Device Tree for DT control

Test with QEMU for bare mode

QEMU is a fancy emulator that can enable us to test U-Boot without access to a real x86 board. Please make sure your QEMU version is 2.3.0 or above test U-Boot. To launch QEMU with u-boot.rom, call QEMU as follows:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom

This will instantiate an emulated x86 board with i440FX and PIIX chipset. QEMU also supports emulating an x86 board with Q35 and ICH9 based chipset, which is also supported by U-Boot. To instantiate such a machine, call QEMU with:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom -M q35

Note by default QEMU instantiated boards only have 128 MiB system memory. But it is enough to have U-Boot boot and function correctly. You can increase the system memory by pass ‘-m’ parameter to QEMU if you want more memory:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom -m 1024

This creates a board with 1 GiB system memory. Currently U-Boot for QEMU only supports 3 GiB maximum system memory and reserves the last 1 GiB address space for PCI device memory-mapped I/O and other stuff, so the maximum value of ‘-m’ would be 3072.

QEMU emulates a graphic card which U-Boot supports. Removing ‘-nographic’ will show QEMU’s VGA console window. Note this will disable QEMU’s serial output. If you want to check both consoles, use ‘-serial stdio’.

Multicore is also supported by QEMU via ‘-smp n’ where n is the number of cores to instantiate. Note, the maximum supported CPU number in QEMU is 255.

U-Boot uses ‘distro_bootcmd’ by default when booting on x86 QEMU. This tries to load a boot script, kernel, and ramdisk from several different interfaces. For the default boot order, see ‘qemu-x86.h’. For more information, see ‘doc/develop/distro.rst’. Most Linux distros can be booted by writing a uboot script. For example, Debian (stretch) can be booted by creating a script file named ‘boot.txt’ with the contents:

setenv bootargs root=/dev/sda1 ro
load ${devtype} ${devnum}:${distro_bootpart} ${kernel_addr_r} /vmlinuz
load ${devtype} ${devnum}:${distro_bootpart} ${ramdisk_addr_r} /initrd.img
zboot ${kernel_addr_r} - ${ramdisk_addr_r} ${filesize}

Then compile and install it with:

$ apt install u-boot-tools && \
  mkimage -T script -C none -n "Boot script" -d boot.txt /boot/boot.scr

The fw_cfg interface in QEMU also provides information about kernel data, initrd, command-line arguments and more. U-Boot supports directly accessing these informtion from fw_cfg interface, which saves the time of loading them from hard disk or network again, through emulated devices. To use it , simply providing them in QEMU command line:

$ qemu-system-i386 -nographic -bios path/to/u-boot.rom -m 1024 \
  -kernel /path/to/bzImage -append 'root=/dev/ram console=ttyS0' \
  -initrd /path/to/initrd -smp 8

Note: -initrd and -smp are both optional

Then start QEMU, in U-Boot command line use the following U-Boot command to setup kernel:

=> qfw
qfw - QEMU firmware interface

Usage:
qfw <command>
    - list                             : print firmware(s) currently loaded
    - cpus                             : print online cpu number
    - load <kernel addr> <initrd addr> : load kernel and initrd (if any) and setup for zboot

=> qfw load
loading kernel to address 01000000 size 5d9d30 initrd 04000000 size 1b1ab50

Here the kernel (bzImage) is loaded to 01000000 and initrd is to 04000000. Then, ‘zboot’ can be used to boot the kernel:

=> zboot 01000000 - 04000000 1b1ab50

To run 64-bit U-Boot, qemu-system-x86_64 should be used instead, e.g.:

$ qemu-system-x86_64 -nographic -bios path/to/u-boot.rom

A specific CPU can be specified via the ‘-cpu’ parameter but please make sure the specified CPU supports 64-bit like ‘-cpu core2duo’. Conversely ‘-cpu pentium’ won’t work for obvious reasons that the processor only supports 32-bit.

Booting distros

It is possible to install and boot a standard Linux distribution using qemu-x86_64 by setting up a root disk:

qemu-img create root.img 10G

then using the installer to install. For example, with Ubuntu 2023.04:

qemu-system-x86_64 -m 8G -smp 4 -bios /tmp/b/qemu-x86_64/u-boot.rom \
  -drive file=root.img,if=virtio,driver=raw \
  -drive file=ubuntu-23.04-desktop-amd64.iso,if=virtio,driver=raw

You can also add -serial mon:stdio if you want the serial console to show as well as the video.

The output will be something like this:

U-Boot SPL 2023.07 (Jul 23 2023 - 08:00:12 -0600)
Trying to boot from SPI
Jumping to 64-bit U-Boot


U-Boot 2023.07 (Jul 23 2023 - 08:00:12 -0600)

CPU:   QEMU Virtual CPU version 2.5+
DRAM:  8 GiB
Core:  20 devices, 13 uclasses, devicetree: separate
Loading Environment from nowhere... OK
Model: QEMU x86 (I440FX)
Net:   e1000: 52:54:00:12:34:56
       eth0: e1000#0
Hit any key to stop autoboot:  0
Scanning for bootflows in all bootdevs
Seq  Method       State   Uclass    Part  Name                      Filename
---  -----------  ------  --------  ----  ------------------------  ----------------
Scanning global bootmeth 'efi_mgr':
Hunting with: nvme
Hunting with: qfw
Hunting with: scsi
scanning bus for devices...
Hunting with: virtio
Scanning bootdev 'qfw_pio.bootdev':
fatal: no kernel available
Scanning bootdev 'virtio-blk#0.bootdev':
Scanning bootdev 'virtio-blk#1.bootdev':
  0  efi          ready   virtio       2  virtio-blk#1.bootdev.part efi/boot/bootx64.efi
** Booting bootflow 'virtio-blk#1.bootdev.part_2' with efi
EFI using ACPI tables at f0060
     efi_install_fdt() WARNING: Can't have ACPI table and device tree - ignoring DT.
       efi_run_image() Booting /efi\boot\bootx64.efi
error: file `/boot/' not found.

Standard boot looks through various available devices and finds the virtio disks, then boots from the first one. After a second or so the grub menu appears and you can work through the installer flow normally.

Note that standard boot will not find 32-bit distros, since it looks for a different filename.

Current limitations

Only qemu-x86-64 can be used for booting distros, since qemu-x86 (the 32-bit version of U-Boot) seems to have an EFI bug leading to the boot handing after Linux is selected from grub, e.g. with debian-12.1.0-i386-netinst.iso:

** Booting bootflow 'virtio-blk#1.bootdev.part_2' with efi
EFI using ACPI tables at f0180
     efi_install_fdt() WARNING: Can't have ACPI table and device tree - ignoring DT.
       efi_run_image() Booting /efi\boot\bootia32.efi
Failed to open efi\boot\root=/dev/sdb3 - Not Found
Failed to load image 큀緃: Not Found
start_image() returned Not Found, falling back to default loader
Welcome to GRUB!

The bochs video driver also seems to cause problems before the OS is able to show a display.

The QEMU -cdrom option is intended to work with the original ISO-format images, not the recently invented ISOHybrid image.

Finally, the use of -M accel=kvm is intended to use the native CPU’s virtual-machine features to accelerate operation, but this causes U-Boot to hang when jumping 64-bit mode, at least on AMD machines. This may be a bug in U-Boot or something else.