Raspberry Pi3 arm64 with kvm

This can be used as a starting point for developing your own image e.g. for running MirageOS on an Pi3

You will need an arm64 / aarch64 cross compiler. You can use e.g. the cross compiler shipped by Ubuntu or get the toolchain of your choice for your OS.

apt-get install gcc-aarch64-linux-gnu

Now we setup the SD card. We need to create two partitions like shown below

Device                                Boot Start     End Sectors  Size Id Type
2018-03-13-raspbian-stretch-lite.img1       8192   93802   85611 41,8M  c W95 FAT32 (LBA)
2018-03-13-raspbian-stretch-lite.img2      98304 3629055 3530752  1,7G 83 Linux

last sector of second partition can be max size of card

You can use fdisk or any other partition tool you like.
Now we need filesystems. The first one needs to be fat32. The second one can be anything a linux kernel can open. We use ext4 here.

  sudo mkfs.vfat /dev/sdc1
  sudo mkfs.ext4 /dev/sdc2

you can know give the paritions names for your convinince.

  sudo fatlabel /dev/sdc1 boot
  sudo tune2fs -L root /dev/sdc2

This is a good time to get a litte familar with the boot process. I found these to posts helpfull
https://raspberrypi.stackexchange.com/questions/39959/raspbian-boot-process-and-the-partition-table?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa
https://raspberrypi.stackexchange.com/questions/10442/what-is-the-boot-sequence

As explained in the linked artickels we need some blobs as this is a broadcom graphics card used as an CPU....

Check out the firmware files you will need
git checkout --depth=1 https://github.com/raspberrypi/firmware

Copy the content of boot to your first partition. There will be some files you dont need like dtb's for older pis and some overlays but for the sake of easy updates in the future and personal lazyness lets ignore that for now.

  cp -r boot/* /<boot partition mount>/

You will find a config.txt which you can alter if you want. https://elinux.org/RPiconfig
gives an good overview on the options.
 
You can start with a default config.ini 
 
 wget https://github.com/RPi-Distro/pi-gen/raw/master/stage1/00-boot-files/files/config.txt
 
You may want to add
 enable_uart=1
 arm_control=0x200
 kernel=Image
to enable the serial console (This will disable you bluetooth for now.) , set the CPU to 64bit mode and choose the name of your kernel image. 

and 
 gpu_mem=16
to reduce the memory for the GPU to the minimum.

We also need a cmdline.txt to tell the kernel some option. We can get a default by
  wget https://github.com/RPi-Distro/pi-gen/raw/master/stage1/00-boot-files/files/cmdline.txt

here we want to set the rootfs to 
  root=/dev/mmcblk0p2 
and you may also want to get rid of predictable device names by adding 
  net.ifnames=0
 
Now we need a root filesystem. We use qemu-debootrap for this as it will give us very plain debian. For this mount the second partition somewhere.
 
  mount /dev/sdc2 /mnt
	 
and run the qemu debootstrap wrapper
	 
You may want to read the deboostrap manpage at this point
	 
  sudo qemu-debootstrap --arch arm64 stretch /mnt
	 
As the kernel that we got from the firmware repo is an rusty old 4.9 with 32bit and no Virtualization we need to build our own. 
	 
	 git clone --depth=1 https://github.com/raspberrypi/linux.git -b rpi-4.16.y
	 
Note that we check out the branch 4.16 which my be outdate at the time you read this. So you may want to use a newer one. 

We use 
  CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 make bcmrpi3_defconfig
to start with an kernel condfig fitting to the raspi. 

Now we need to enable Virtualization. 

  CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 make menuconfig
  -> Virtualization -> 
				-> Kernel-based Virtual Machine (KVM) support *
				-> Host kernel accelerator for virtio net  M

and we are good to go to build our kernel. You may want adjust the -j4 to the number of CPU cores you want to use for this. 

  CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 make -j4 Image dtbs modules
	
We now need to copy the kernel image and the dtbs file to the SD card. Note that we here copy the dtb for an raspberrypi 3 +, so if you use a different pi you may want to copy a different dtb file.

  cp arch/arm64/boot/Image /<boot partition mount>/
  cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b-plus.dtb /<boot partition mount>/

Now we need to copy the modules to the root filesystem
	 sudo CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm64 INSTALL_MOD_PATH=/mnt make modules_install
	 
As debootstrap gives us an unconfigured debian you may want to edit 

/etc/network/interfaces
auto eth0
iface eth0 inet dhcp

/etc/fstab

UUID=31c566e0-0f1d-475d-9908-4740c8ca3653 / ext4    errors=remount-ro 0       1
you can get the uuid by running 
  blkid 
	 
/etc/hostname
 
 finally you want to set a root password
   sudo chroot /mnt
	 passwd 
	 exit