ARMv7 Docker emulator in QEMU

# Principle

---

Using Docker on Raspberry Pi provides many different benefits (most explained [here in French](http://raspbian-france.fr/docker-raspberry-pi/)). However when developing new containers, it may require much more CPU and RAM power to build them or even to compile the binaries in armhf format. So this blog post describes how to create an ARM v7 image based on QEMU emulating an ARM Cortex-15 chip with 2Gb of RAM running Debian 8 Jessie.

**Why Debian 8 Jessie ?**

Simply because the embedded kernel is preconfigured for Docker (see the required modules [there](https://github.com/umiddelb/armhf/wiki/Installing,-running,-using-docker-on-armhf-%28ARMv7%29-devices)). I'll probably use [HypriotOS](http://blog.hypriot.com/) in the future but for now that's good enough and i use the Docker packages directly built by Hypriot anyway.

**Why not emulating the Raspberry Pi ?**

Simply because the performances are really really bad and it is limited to 256Mb of RAM. I tried multiple times but it is really too slow so i preferred to switch to a different emulator but keeping the same CPU architecture.

**Why only a Cortex-A15 ?**

Well, it is still an ARMv7 architecture which is the same as the Raspberry Pi 2 and there is no better option in QEMU AFAIK.

**So what is required ?**

You just need the following :

* An x86 host (i use a laptop with Ubuntu 15.10 but can probably run on a VM in the Cloud)
* QEMU emulator (personally i have version 2.3.0)
* Few files from a debian mirror site (see below)

If you don't want to build your own image, just download that [compressed](https://drive.google.com/file/d/0B7XME2kfla9NeHMyUG1lRloxU0U/view?usp=sharing) file or the [torrent](https://drive.google.com/file/d/0B7XME2kfla9NMEI1NHlJMGtXZnM/view?usp=sharing), unzip it and run './run.sh' from within the directory.
PS: user: `root` or `debian`, password: `passw0rd`.

Version of QEMU which i use : 
```
QEMU emulator version 2.3.0 (Debian 1:2.3+dfsg-5ubuntu9.1), Copyright (c) 2003-2008 Fabrice Bellard
```

# Step 1: Set the scene

---

- Collect the kernel and boot files (vmlinuz & initrd) needed to launch a network install
```shell
wget http://ftp.debian.org/debian/dists/jessie/main/installer-armhf/current/images/netboot/initrd.gz
wget http://ftp.debian.org/debian/dists/jessie/main/installer-armhf/current/images/netboot/vmlinuz
wget http://ftp.nl.debian.org/debian/dists/jessie/main/installer-armhf/current/images/device-tree/vexpress-v2p-ca15-tc1.dtb
```

- Create a disk which will host your filesystem where there will be 3 partitions : /boot, / and swap

```shell
qemu-img create -f qcow2 debian-jessie-armhf.img 32G
```

# Step 2: Install the operating system

---

- Start the QEMU emulator of the Cortex-A15 chip

```shell
qemu-system-arm -m 2048M -M vexpress-a15 -dtb vexpress-v2p-ca15-tc1.dtb -cpu cortex-a15 \
        -kernel vmlinuz -initrd initrd.gz \
        -sd debian-jessie-armhf.img \
        -append "root=/dev/ram console=ttyAMA0 earlycon" -no-reboot -nographic
```

- Proceed to the Debian install - please note this takes few hours to complete (i think about 3); here are my basic settings
> 1. Use the whole disk with default install settings
> 2. Create a debian user
> 3. Just install SSH and the minimal system tools

- Now the operating system has been installed it is now time to finalize the boot part by updating the vmlinuz and initrd files

```shell
sudo modprobe nbd max_part=16
sudo qemu-nbd -c /dev/nbd0 debian-jessie-armhf.img 
sudo mkdir /mnt/nbd0p1
sudo mount /dev/nbd0p1 /mnt/nbd0p1
cp /mnt/nbd0p1/initrd.img-3.16.0-4-armmp-lpae .
cp /mnt/nbd0p2/vmlinuz-3.16.0-4-armmp-lpae .
sudo umount /mnt/nbd0p1
```

If you OS didn't have `ndb` module, you can use `libguestfs-tools`:
```shell
sudo yum install libguestfs-tools
LIBGUESTFS_BACKEND=direct guestfish --rw -a debian-jessie-armhf.img
><fs> run
><fs> list-filesystems
/dev/sda1: ext2
/dev/sda2: ext4
/dev/sda3: unknown
/dev/sda5: swap
><fs> mount /dev/sda1 /
><fs> download /initrd.img-3.16.0-4-armmp-lpae ./initrd.img-3.16.0-4-armmp-lpae
><fs> download /vmlinuz-3.16.0-4-armmp-lpae ./vmlinuz-3.16.0-4-armmp-lpae
```

- Just restart the emulator with the new boot files

```shell
qemu-system-arm -m 1024M -M vexpress-a15 -dtb vexpress-v2p-ca15-tc1.dtb -cpu cortex-a15 \
        -kernel vmlinuz-3.16.0-4-armmp-lpae -initrd initrd.img-3.16.0-4-armmp-lpae \
        -sd debian-jessie-armhf.img \
        -append "root=/dev/mmcblk0p2 console=ttyAMA0" -no-reboot -nographic
```

# Step 3: Proceed to Docker set-up

---

Please note that all the actions below are performed as root.

- First update your Debian operating system

```shell
apt-get update && apt-get upgrade
```

- Then install the Docker 1.9.1 package from Hypriot (it will return errors but don't worry at this stage)

```shell
wget http://downloads.hypriot.com/docker-hypriot_1.9.1-1_armhf.deb
dpkg -i docker-hypriot_1.9.1-1_armhf.deb
```

- The vanilla Jessie kernel does not support the Overlay filesystem, so just change the following line in /etc/default/docker :

> From :    DOCKER_OPTS="--storage-driver=overlay -D"
To   :    DOCKER_OPTS="--dns 8.8.8.8 --dns 8.8.4.4"

- You also need to change the Docker service file in /lib/systemd/system/docker.service :
> From :    ExecStart=/usr/bin/docker daemon -H fd:// \$DOCKER_OPTS
To :        ExecStart=/usr/bin/docker daemon -H unix:///var/run/docker.sock \$DOCKER_OPTS

- Add your debian user to the docker group (i assume you know how to do it :) )

- Restart Docker after reloading and enabling

```shell
systemctl daemon-reload
systemctl enable docker.service
systemctl start docker.service
```

Now you should be right there:
![img](https://ooo.0o0.ooo/2017/05/31/592e1cfbbc304.png)
(http://1.bp.blogspot.com/-AcLggf-EaA0/Vp9TIjZQ2XI/AAAAAAAA4gE/olRk-lsYclA/s400/docker-debian-armv7-cortex_a15-qemu.png)

# Few references

ARM Cortex A15: [http://bit.ly/1JUIAik](https://en.wikipedia.org/wiki/ARM_Cortex-A15)
Emulation armhf set-up with QEMU : [http://bit.ly/1PlzWp6](https://mashu.github.io/2015/08/12/QEMU-Debian-armhf.html)
Reference site about installing Debian Jessie on ARM : [http://bit.ly/1S093w8](http://ftp.debian.org/debian/dists/jessie/main/installer-armhf/current/images/)

Repost fron [https://besn0847.blogspot.com/2016/01/armv7-docker-emulator-in-qemu.html](http://besn0847.blogspot.com/2016/01/armv7-docker-emulator-in-qemu.html)

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