Microdroid

Microdroid is a (very) lightweight version of Android that is intended to run on on-device virtual machines. It is built from the same source code as the regular Android, but it is much smaller; no system server, no HALs, no GUI, etc. It is intended to host headless & native workloads only.

Building

You need a VIM3L board. Instructions for building Android for the target, and flashing the image can be found here.

Then you install com.android.virt APEX. All files needed to run microdroid are included in the APEX, which is already in the yukawa (VIM3L) target. You can of course build and install the APEX manually.

$ source build/envsetup.sh
$ choosecombo 1 aosp_arm64 userdebug // actually, any arm64-based target is ok
$ m com.android.virt
$ adb install $ANDROID_PRODUCT_OUT/system/apex/com.android.virt.apex
$ adb reboot

Running

Create a config file, microdroid.json:

{
  "bootloader": "/data/local/tmp/microdroid/bootloader",
  "disks": [
    {
      "image": "/data/local/tmp/microdroid/os_composite.img",
      "writable": false
    },
    {
      "image": "/data/local/tmp/microdroid/env_composite.img",
      "writable": false
    },
    {
      "image": "/data/local/tmp/microdroid/payload.img",
      "writable": false
    },
    {
      "image": "/data/local/tmp/microdroid/userdata_composite.qcow2",
      "writable": true
    }
  ]
}

Copy the artifacts to the temp directory, create the composite image using mk_cdisk and copy the VM config file. For now, some other files have to be manually created. In the future, you won't need these, and this shall be done via virtmanager.

$ adb root
$ adb shell 'mkdir /data/local/tmp/microdroid'
$ adb shell 'cp /apex/com.android.virt/etc/microdroid_bootloader /data/local/tmp/microdroid/bootloader'
$ adb shell 'cp /apex/com.android.virt/etc/fs/*.img /data/local/tmp/microdroid'
$ adb shell 'cp /apex/com.android.virt/etc/uboot_env.img /data/local/tmp/microdroid'
$ adb shell 'dd if=/dev/zero of=/data/local/tmp/microdroid/misc.img bs=4k count=256'
$ adb shell 'dd if=/dev/zero of=/data/local/tmp/microdroid/userdata.img bs=1 count=0 seek=4G'
$ adb shell 'mkfs.ext4 /data/local/tmp/microdroid/userdata.img'
$ adb shell 'cd /data/local/tmp/microdroid; /apex/com.android.virt/bin/mk_cdisk /apex/com.android.virt/etc/microdroid_cdisk.json os_composite.img'
$ adb shell 'cd /data/local/tmp/microdroid; /apex/com.android.virt/bin/mk_cdisk /apex/com.android.virt/etc/microdroid_cdisk_env.json env_composite.img'
$ adb shell 'cd /data/local/tmp/microdroid; /apex/com.android.virt/bin/mk_cdisk /apex/com.android.virt/etc/microdroid_cdisk_userdata.json userdata_composite.img'
$ adb shell '/apex/com.android.virt/bin/crosvm create_qcow2 --backing_file=/data/local/tmp/microdroid/userdata_composite.img /data/local/tmp/microdroid/userdata_composite.qcow2'
$ adb shell 'cd /data/local/tmp/microdroid; /apex/com.android.virt/bin/mk_payload /apex/com.android.virt/etc/microdroid_payload.json payload.img'
$ adb shell 'chmod go+r /data/local/tmp/microdroid/*-header.img /data/local/tmp/microdroid/*-footer.img'
$ adb push microdroid.json /data/local/tmp/microdroid/microdroid.json

Ensure SELinux is in permissive mode to allow virtmanager and crosvm to open files from /data/local/tmp. Opening files from this directory is neverallow-ed and file descriptors should be passed instead but, before that is supported, adb shell setenforce 0 will put the device in permissive mode.

Now, run the VM and look for adbd starting in the logs.

$ adb shell "start virtmanager"
$ adb shell "RUST_BACKTRACE=1 RUST_LOG=trace /apex/com.android.virt/bin/vm run /data/local/tmp/microdroid/microdroid.json"

ADB

$ CID=10
$ adb forward tcp:8000 vsock:$CID:5555
$ adb connect localhost:8000

CID should be the CID that vm reported was assigned to the VM. You can also check it with adb shell "/apex/com.android.virt/bin/vm list". 5555 must be the value. 8000 however can be any port in the development machine.

Done. Now you can log into microdroid. Have fun!

$ adb -s localhost:8000 shell