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Revision as of 14:58, 29 November 2020 by Tperrot (talk | contribs) (Add a guide to populate a minimal sysroot and correct the guide to construct a rootfs image.)
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QEMU is a generic and open source machine emulator and virtualizer, originally developed by Fabrice Bellard.

When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performance.

When used as a virtualizer, QEMU achieves near native performances by executing the guest code directly on the host CPU. Host KVM support is utilized in this case. The virtualizer mode requires that both the host and guest machine use the same instruction set.

QEMU project web site

Use in embedded projects

QEMU is increasingly used to provide an emulator for embedded processors, for testing embedded Linux without the need for actual hardware.

The Embedded Linux From Scratch presentation by Michael Opdenacker has great information about setting up QEMU with embedded Linux.

Also, Aboriginal Linux uses QEMU as part of a "native" build environment to eliminate cross-compilation problems.

Supported architectures

The following architectures are supported as target architectures for system emulation:

Support for new boards or new peripherals can added relatively easily in QEMU, the APIs being quite simple to understand and use.


Some quick useful tips

How to populate a minimal sysroot

Here are the steps to populate a minimal sysroot, assuming you Busybox built on your host, for the target architecture:

cd busybox
mkdir _install/proc _install/sys _install/dev _install/etc _install/etc/init.d
cat > _install/etc/init.d/rcS << EOF
mount -t proc none /proc
mount -t sysfs none /sys
/sbin/mdev -s
[ ! -h /etc/mtab ]  && ln -s /proc/mounts /etc/mtab
[ ! -f /etc/resolv.conf ] && cat /proc/net/pnp > /etc/resolv.conf
chmod +x _install/etc/init.d/rcS

How to build a Ext3 rootfs

Here are the steps for building a Ext3 rootfs image, assuming you Busybox built on your host, for the target architecture:

dd if=/dev/zero of=rootfs.img bs=1M count=10
mke2fs -j rootfs.img
mkdir /mnt/rootfs
mount -o loop rootfs.img /mnt/rootfs
rsync -a _install/ /mnt/rootfs
chown -R root:root /mnt/rootfs/
umount /mnt/rootfs/

Some sample command lines

I got these from Rob Landley at OLS 2008:

qemu -kernel linux-2.6.26/arch/i386/boot/bzImage -hda rootfs.img -append "console=ttyS0 root=/dev/hda" -nographic
qemu -kernel linux-2.6.26/arch/i386/boot/bzImage -hda rootfs.img -append "console=ttyS0 root=/dev/hda init=/bin/ash" -nographic
qemu -kernel linux-2.6.26/arch/i386/boot/bzImage -hda rootfs.img -append "console=ttyS0 root=/dev/hda panic=1" -nographic -no-reboot
killall qemu

Interesting options

  • -kernel <file> = specify the kernel image to use for booting
  • -hda <file> = specify
  • -nographic = don't use graphics, and redirect serial I/O to console
  • -no-reboot = exit instead of rebooting

Resize filesystem image

Not enough space to install anything after you're up and running? Here's how to resize the image.

Host:  qemu-img resize rootfs_debian6_rpi.ext4 +1G
QEMU:  sudo resize2fs /dev/sda  // Be careful not to run this on your host machine

The online resize2fs may corrupt the filesystem so here's an alternative. resize using loopback device