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Emulate Rapberry Pi 2 in QEMU

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In the process of planning system testing for one of my clients I found that someone from Microsoft published patches with BCM2836 support to QEMU mailing list. I thought it is very interesting, because if it is possible to setup emulated Raspberry Pi many use cases can be tested faster and in more automatic way. For example checking how application behave when running on more then one device at once, testing massive deployment process, stress testing and finally speed up debug-fix-test process.

So it looks like making RPi 2 working in emulated environment can add a lot of value to some products. In email Andrew mention github repo, which I would like to try in this post

Get QEMU and compile

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git clone https://github.com/0xabu/qemu.git -b raspi
git submodule update --init dtc
./configure
make -j$(nproc)
sudo make install

Prepare to boot

QEMU requires kernel and device tree file to be given as parameters, because of that we have to extract those pieces from existing Raspbian image.

Get kernel and device tree

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wget http://downloads.raspberrypi.org/raspbian/images/raspbian-2015-11-24/2015-11-21-raspbian-jessie.zip
unzip 2015-11-21-raspbian-jessie.zip
[23:35:23] pietrushnic:rpi2_qemu $ sudo /sbin/fdisk -lu 2015-11-21-raspbian-jessie.img 
Disk 2015-11-21-raspbian-jessie.img: 3.7 GiB, 3934257152 bytes, 7684096 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xea0e7380

Device                          Boot  Start     End Sectors  Size Id Type
2015-11-21-raspbian-jessie.img1        8192  131071  122880   60M  c W95 FAT32 (LBA)
2015-11-21-raspbian-jessie.img2      131072 7684095 7553024  3.6G 83 Linux

Check start of W95 FAT32 (LBA) partition. It is 8192. Sector size is 512. So calculate offset in bytes 8192 * 512 = 4194304.

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mkdir tmp
sudo mount -o loop,offset=4194304 2015-11-21-raspbian-jessie.img tmp
mkdir 2015-11-21-raspbian-boot
cp tmp/kernel7.img 2015-11-21-raspbian-boot
cp tmp/bcm2709-rpi-2-b.dtb 2015-11-21-raspbian-boot

Then if you try to boot 2015-11-21 Rapbian with 0xabu code:

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qemu-system-arm -M raspi2 -kernel 2015-11-21-raspbian-boot/kernel7.img \
-sd 2015-11-21-raspbian-jessie.img \
-append "rw earlyprintk loglevel=8 console=ttyAMA0,115200 dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2" \
-dtb 2015-11-21-raspbian-boot/bcm2709-rpi-2-b.dtb -serial stdio

You will experience kernel crash:

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(...)
[    6.021989] Freeing unused kernel memory: 420K (80779000 - 807e2000)
[    7.366232] random: systemd urandom read with 7 bits of entropy available
[    7.383057] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
[    7.383057] 
[    7.384366] CPU: 0 PID: 1 Comm: systemd Not tainted 4.1.13-v7+ #826
[    7.384874] Hardware name: BCM2709
[    7.386615] [<80018444>] (unwind_backtrace) from [<80013e08>] (show_stack+0x20/0x24)
[    7.387307] [<80013e08>] (show_stack) from [<8055a188>] (dump_stack+0x98/0xe0)
[    7.387851] [<8055a188>] (dump_stack) from [<80556340>] (panic+0xa4/0x204)
[    7.388515] [<80556340>] (panic) from [<800293c8>] (do_exit+0xa0c/0xa64)
[    7.389074] [<800293c8>] (do_exit) from [<800294b8>] (do_group_exit+0x4c/0xcc)
[    7.389729] [<800294b8>] (do_group_exit) from [<80033f1c>] (get_signal+0x2b0/0x6e0)
[    7.390388] [<80033f1c>] (get_signal) from [<80013190>] (do_signal+0x98/0x3ac)
[    7.391021] [<80013190>] (do_signal) from [<8001368c>] (do_work_pending+0xb8/0xc8)
[    7.391665] [<8001368c>] (do_work_pending) from [<8000f9e4>] (work_pending+0xc/0x20)
[    7.393209] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000004
[    7.393209] 

To avoid this crash you have to comment /etc/ld.so.preload.

Changing ld.so.preload

First calculate offset in bytes to Raspbian root filesystem partition. According to fdisk output above partition starts with sector 131072, so offset would be 512*131072=67108864.

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sudo mount -o loop,offset=67108864 2015-11-21-raspbian-jessie.img tmp

Use your favourite editor to change tmp/etc/ld.so.preload. Note that you have to edit as superuser. Content of file should looks like this:

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#/usr/lib/arm-linux-gnueabihf/libarmmem.so

Sync and umount partition:

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sync
sudo umount tmp

Final booting

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qemu-system-arm -M raspi2 -kernel 2015-11-21-raspbian-boot/kernel7.img \
-sd 2015-11-21-raspbian-jessie.img \
-append "rw earlyprintk loglevel=8 console=ttyAMA0,115200 dwc_otg.lpm_enable=0 root=/dev/mmcblk0p2" \
-dtb 2015-11-21-raspbian-boot/bcm2709-rpi-2-b.dtb -serial stdio

Summary

There are many problem with existing setup, but from my experience this is best approach and code that I saw during last years. Also it looks like this code is backed by huge corporation so it looks like they see value in providing this code to wide community. Rapid delivery of those patches probably would not be possible without previous work to which Andrew point in his email. It would be great to see community engagement in this effort.

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