Sunday, 24 January 2010

Debian on an emulated ARM machine


QEMU is a generic and open source processor emulator which can emulate i386, x86_64, ARM, MIPS, PowerPC and SPARC systems. In case of ARM, it can emulate an Integrator or a Versatile platform. The Versatile one is the most interesting as it includes a hard disk SCSI controller, an Ethernet card and a graphical display.
Using a kernel compiled with the right options, it is possible to install a Debian distribution on such an emulated platform. That makes a cheap development platform. The emulated system running on an Athlon 64 X2 3800+ is around 20% faster than the popular NSLU2 and possibly with much more RAM (my emulated system has 256MiB of RAM).
This howto has been written for a Debian host system, but could be easily adapted for other distributions.
Alternatively prebuilt images are also available.

Installing QEMU

QEMU is currently available as a package in the Debian distribution, you will need at least version 0.9.1. If you want to compile it yourself, here is the procedure:
To build QEMU a few packages like SDL needs to be installed on your system. gcc version 3.4 is also needed, as some parts of QEMU do not build with newer gcc versions. As QEMU is present in the archive, just run:
$ su -c "apt-get build-dep qemu"
Then run the configure script:
$ cd qemu-0.9.1
$ ./configure
Then compile it:
$ make
And install it on your system:
$ su -c "make install"

Preparing the installation

First you need to create an image of the hard disk. In my case I have chosen to emulate a 10GB hard-disk, but this size could be changed to correspond to your needs. Note that the file is created in qcow format, so that only the non-empty sectors will be written in the file.
A small tip: create a directory to hold all the files related to the emulated ARM machine.
$ qemu-img create -f qcow hda.img 10G
Debian currently does not support the Versatile platform natively, that means there is no kernel available for this platform. However there is full support for this platform in the upstream kernel. You can either compile your own kernel (using a cross-compiler or an other ARM machine), or use the kernel I have built:
$ wget
$ wget
You also need to get the initrd of the Etch Debian-Installer:
$ wget

Installing Debian Etch

To start the installation process, use the following line:
$ qemu-system-arm -M versatilepb -kernel vmlinuz-2.6.18-6-versatile -initrd initrd.gz -hda hda.img -append "root=/dev/ram"
After a few seconds you should see the kernel booting:
Debian-Installer booting
And then the first screen of the Debian-Installer:
First screen of Debian-Installer
Proceed as a normal installation, until you get to the following screen. If you need some documentation, please refer to the Debian installation guide.
Debian-Installer complains about missing kernel modules
Debian-Installer complains that it can't find kernel modules. This is normal because the initrd of another platform is used. This is not really a problem as the kernel I provide has been compiled with the network driver, the disk driver and ext3 support built-in. However that means you won't be able to install Debian on an XFS partition. This is a known limitation that will disappear if/when the Versatile kernel is integrated in the official Debian kernel.
So in short answer yes, contrarily to what is suggested.
During the installation, Debian installer will complain that it can not found a suitable kernel for this platform, as shown on the screenshot below. This is due to the fact that Debian currently does not support the ARM Versatile platform; the support will be added post-Etch. An unofficial kernel being provided directly to QEMU, you can safely ignore this message and continue the installation.
Debian-Installer complains about missing bootloader
Near to the end of the installation you will get the following error screen:
Debian-Installer complains about missing bootloader
Again consider this message as harmless. There is no need for a bootloader, as QEMU is able to directly load a kernel and an initrd.
Then you will get to the end of the installation. Congratulations!
End of the installation
When the systems reboot, just exit QEMU. Different parameters have to be used to boot the installed system.

Using the system

First boot

To start the system use the following command:
$ qemu-system-arm -M versatilepb -kernel vmlinuz-2.6.18-6-versatile -initrd initrd.img-2.6.18-6-versatile -hda hda.img -append "root=/dev/sda1"
After a few seconds the system should give you a login prompt:
Console login
The first thing to do is to install the kernel image corresponding to the running kernel. This will install all the modules that you may need.
$ apt-get install initramfs-tools
$ wget
$ su -c "dpkg -i linux-image-2.6.18-6-versatile_2.6.18.dfsg.1-18etch1+versatile_arm.deb"

Using Xorg

You now have a full Debian arm system that you can use for development or whatever. You can even run Xorg using the fb device. Note that you have to select a 256-color mode, with a resolution up to 1024x768.
Running Xorg
You can even chat on IRC :)
Running Xchat

QEMU without X

If you don't want to start QEMU in graphic mode, you can use the -nographic option, and ask the kernel to use ttyAMA0 as the console. The command to start the emulated machine then become:
$ qemu-system-arm -M versatilepb -kernel vmlinuz-2.6.18-6-versatile -initrd initrd.img-2.6.18-R-versatile -hda hda.img -append "root=/dev/sda1 console=ttyAMA0" -nographic
To set up a getty on the serial port, and be able to login, you must edit /etc/inittab and add the following line:
T0:23:respawn:/sbin/getty -L ttyAMA0 9600 vt100
All users except root should be able to login on this console. To alswo allow root login you must add the following line in /etc/securetty:

More RAM

By default QEMU emulate a machine with 128MiB of RAM. You can use the -m option to increase or decrease the size of the RAM. It is however limited to 256MiB, greater sizes will crash the kernel.

Connect your emulated machine to a real network

When no option is specified QEMU uses a non priviledged user mode network stack that gives the emulated machine access to the world. But you probably want to make your emulated machine accessible from the outside. It is possible by using the tap mode and bridging the tap interface with the network interface of the host machine.
The first thing to do is to active a bridge on your host machine. For that you have to modify the /etc/network/interfaces file as follow:
auto eth0
iface eth0 inet dhcp

auto br0
iface br0 inet dhcp
  bridge_ports eth0
  bridge_maxwait 0
Then you need to install the bridge-utils package and restart your network interface:
# apt-get install bridge-utils
# ifdown eth0
# ifup br0
Create a script call /etc/qemu-ifup that will be executed upon the start of QEMU:
echo "Executing /etc/qemu-ifup"
echo "Bringing up $1 for bridged mode..."
sudo /sbin/ifconfig $1 promisc up
echo "Adding $1 to br0..."
sudo /usr/sbin/brctl addif br0 $1
sleep 2
As you probably don't want to execute QEMU as root, you need to create a qemu user group and authorize the brctl and ifconfig commands for users of the qemu via sudo. You need to add the following lines to /etc/sudoers (edit the file using visudo):
Cmnd_Alias QEMU = /usr/sbin/brctl, /sbin/ifconfig
Finally you can start your emulated machine using the following command
$ qemu-system-arm -M versatilepb -kernel vmlinuz-2.6.18-6-versatile -initrd initrd.img-2.6.18-6-versatile -hda hda.img -append "root=/dev/sda1" -net nic,macaddr=00:16:3e:00:00:01 -net tap
You don't need to give a MAC address if you are emulating only one machine, as QEMU will use a default one. However if you have more than one emulated machine (don't forget QEMU can also emulate other architectures than ARM), you will have to specify a unique MAC address for each machine. I advise you to select an address from the range 00:16:3e:xx:xx:xx, which has been assigned to Xen.

Other options

QEMU has a lot of other useful options. For a full list, please refer to the documentation


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