May 13, 2019

2331 words 11 mins read



run regular Docker images in KVM/Qemu

repo name gotoz/runq
repo link
language Go
size (curr.) 5573 kB
stars (curr.) 532
created 2018-02-27
license Apache License 2.0

Build Status


runq is a hypervisor-based Docker runtime based on runc to run regular Docker images in a lightweight KVM/Qemu virtual machine. The focus is on solving real problems, not on number of features.

Key differences to other hypervisor-based runtimes:

  • minimalistic design, small code base
  • no modification to existing Docker tools (dockerd, containerd, runc…)
  • coexistence of runq containers and regular runc containers
  • no extra state outside of Docker (no libvirt, no changes to /var/run/…)
  • small init program, no systemd
  • no custom guest kernel or custom qemu needed
  • runs on x86_64 and s390x

runc vs. runq

       runc container                   runq container
       +-------------------------+      +-------------------------+
       |                         |      |                     VM  |
       |                         |      | +---------------------+ |
       |                         |      | |                     | |
       |                         |      | |                     | |
       |                         |      | |                     | |
       |       application       |      | |     application     | |
       |                         |      | |                     | |
       |                         |      | |                     | |
       |                         |      | +---------------------+ |
       |                         |      | |     guest kernel    | |
       |                         |      | +---------------------+ |
       |                         |      |           qemu          |
       +-------------------------+      +-------------------------+
                                host kernel


runq requires a host kernel >= 4.8 with KVM and VHOST_VSOCK support enabled. The easiest way to build runq and to put all dependencies together is using Docker. For fast development cycles a regular build environment might be more efficient. For this refer to section Developing runq below.

# get the source
git clone
cd runq

# compile and create a release tar file in Docker container
make release

# install runq to `/var/lib/runq`
make release-install

Register runq as Docker runtime with appropriate defaults. See daemon.json for more options.

  "runtimes": {
    "runq": {
      "path": "/var/lib/runq/runq",
      "runtimeArgs": [
        "--cpu", "1",
        "--mem", "256",
        "--dns", ",",
        "--tmpfs", "/tmp"

reload Docker config

systemctl reload docker.service

TLS certificates

runq-exec creates a secure connection between host and VM guests. Users of runq-exec are authenticated via a client certificate. Access to the client certificate must be limited to Docker users only.

The CA and server certificates must be installed in /var/lib/runq/qemu/certs. Access must be limited to the root user only.

Examples of server and client TLS certificates can be created with the script:


Note: The host must provide sufficient entropy to the VM guests. If there is not enough entropy available booting of guests can fail with a timeout error. The entropy that’s currently available can be checked with:

cat /proc/sys/kernel/random/entropy_avail

The number returned should always be greater than 1000.

Kernel module vhost_vsock

The kernel module vhost_vsock must be loaded on the host. This can be achieved by creating a config file for the systemd-modules-load service: /etc/modules-load.d/vhost-vsock.conf:

# Load vhost_vsock for runq

Usage examples

the simplest example

docker run --runtime runq -ti busybox sh

custom VM with 512MiB memory and 2 CPUs

docker run --runtime runq -e RUNQ_MEM=512 -e RUNQ_CPU=2 -ti busybox sh

allow loading of extra kernel modules by adding the SYS_MODULE capability

docker run --runtime runq --cap-add sys_module -ti busybox sh -c "modprobe brd && lsmod"

full example PostgreSQL with custom storage

dd if=/dev/zero of=data.img bs=1M count=200
mkfs.ext4 -F data.img

docker run \
    --runtime runq \
    --name pgserver \
    -e RUNQ_CPU=2 \
    -e RUNQ_MEM=512 \
    -e POSTGRES_PASSWORD=mysecret \
    -v $PWD/data.img:/dev/runq/0001/none/ext4/var/lib/postgresql \
    -d postgres:alpine

sleep 10

docker run \
    --runtime runq \
    --link pgserver:postgres \
    --rm \
    -e PGPASSWORD=mysecret \
    postgres:alpine psql -h postgres -U postgres -c "select 42 as answer;"

#  answer
# --------
#      42
# (1 row)

Container with Systemd

For containers that use Systemd as the Docker entry-point the container exit code must be treated differently to ensure that poweroff and reboot executed inside the container work as expected.

with --restart on-failure:1'
poweroff, halt -> SIGINT(2) -> want container restart       -> exit code 0 (forced)
reboot         -> SIGHUP(1) -> don't want container restart -> exit code 1

-e RUNQ_SYSTEMD=1 also prevents runq from mounting cgroups.

See test/examples/Dockerfile.systemd and test/examples/ for an example.


Runq can write the container environment variables in a file named /.runqenv placed in the root directory of the container. This might be useful for containers running Systemd as entry point. This feature can be enabled globally by configuring --runqenv in /etc/docker/daemon.json or for a single container via the environment variable RUNQ_RUNQENV.

runq Components

   docker cli
      dockerd engine
               runq                                           container
              |                                                        |
  docker0     |                                                  VM    |
    `veth <------> veth                 +--------------------------+   |
              |        `<--- macvtap ---|-> eth0                   |   |
              |  proxy  <-----------------> init                   |   |
 runq-exec <-----------tls----------------> `vsockd                |   |
              |                         |+-------------namespace--+|   |
 overlayfs <-----9pfs-------------------||-> /                    ||   |
              |                         ||                        ||   |
 block dev <-----virtio-blk-------------||-> /dev/vdx             ||   |
              |                         ||                        ||   |
              |                         ||                        ||   |
              |                         ||                        ||   |
              |                         ||       application      ||   |
              |                         ||                        ||   |
              |                         |+------------------------+|   |
              |                         |       guest kernel       |   |
              |                         +--------------------------+   |
              |                                     qemu               |

                                host kernel
  • cmd/runq

    • new docker runtime
  • cmd/proxy

    • new Docker entry point
    • first process in container (PID 1)
    • configures and starts Qemu (network, disks, …)
    • forwards signals to VM init
    • receives application exit code
  • cmd/init

    • first process in VM (PID 1)
    • initializes the VM guest (network, disks, …)
    • starts entry-point in PID and Mount namespace
    • sends signals to target application
    • forwards application exit code back to proxy
  • cmd/runq-exec

    • command line utility similar to docker exec
  • cmd/nsenter

    • enters the namespaces of entry-point for runq-exec
  • qemu

    • creates /var/lib/runq/qemu
    • read-only volume attached to every container
    • contains qemu rootfs (proxy, qemu, kernel and initrd)
  • initrd

    • prepares the initrd to boot the VM
  • pkg

    • helper packages


runq-exec (/var/lib/runq/runq-exec) is a command line utility similar to docker exec. It allows running additional commands in existing runq containers executed from the host. It uses VirtioVsock for the communication between host and VMs. TLS is used for encryption and client authorization. Support for runq-exec can be disabled by setting the container environment variable RUNQ_NOEXEC or by --noexec in /etc/docker/daemon.json.

  runq-exec [options] <container> command args

Run a command in a running runq container

  -c, --tlscert string    TLS certificate file (default "/var/lib/runq/cert.pem")
  -k, --tlskey string     TLS private key file (default "/var/lib/runq/key.pem")
  -e, --env stringArray   Set environment variables for command
  -h, --help              Print this help
  -i, --interactive       Keep STDIN open even if not attached
  -t, --tty               Allocate a pseudo-TTY
  -v, --version           Print version

Environment Variable:
  DOCKER_HOST    specifies the Docker daemon socket.

  runq-exec -ti a6c3b7c bash

Qemu and guest Kernel

runq runs Qemu and Linux Kernel from the /var/lib/runq/qemu directory on the host. This directory is populated by make -C qemu. For simplicity Qemu and the Linux kernel are taken from the Ubuntu 18.04 LTS Docker base image. See qemu/x86_64/Dockerfile for details. This makes runq independent of the Linux distribution on the host. Qemu does not need to be installed on the host.

The kernel modules directory (/var/lib/runq/qemu/lib/modules) is bind-mounted into every container to /lib/modules. This allows the loading of extra kernel modules in any container if needed. For this SYS_MODULES capability is required (--cap-add sys_modules).


runq uses Macvtap devices to connect Qemu VirtIO interfaces to Docker bridges. By default a single Ethernet interface is created. Multiple networks can be used by connecting a container to the networks before start. See test/integration/ as an example.

runq container can also be connected to one or more Docker networks of type Macvlan. This allows a direct connection between the VM and the physical host network without bridge and without NAT. See for details.

For custom networks the docker daemon implements an embedded DNS server which provides built-in service discovery for any container created with a valid container name. This Docker DNS server (listen address is reachable only by runc containers and not by runq containers. A work-around is to run one or more DNS proxy container in the custom network with runc and use the proxy IP address for DNS of runq containers. See test/examples/ for details on how to setup a DNS proxy.

DNS configuration without proxy can be done globally via runtime options specified in ‘/etc/docker/daemon.json’ (see example above) or via environment variables for each container at container start. The environment variables are RUNQ_DNS, RUNQ_DNS_OPT and RUNQ_DNS_SEARCH. Environment variables have priority over global options.


Extra storage can be added in the form of Qcow2 images, raw file images or regular block devices. Storage devices will be mounted automatically if a filesystem and a mount point has been specified. Supported filesystems are ext2, ext3, ext4, xfs and btrfs. Cache type must be writeback, writethrough, none or unsafe. Cache type “none” is recommended for filesystems that support O_DIRECT. See man qemu(1) for details about different cache types.


--volume <image  name>:/dev/runq/<id>/<cache type>[/<filesystem type><mount point>]
--device <device name>:/dev/runq/<id>/<cache type>[/<filesystem type><mount point>]

<id> is used to create symbolic links inside the VM guest that point to the Qemu Virtio device files. The id can be any character string that matches the regex pattern "^[a-zA-Z0-9-_]{1,36}$" but it must be unique within a container.

/dev/disk/by-runq-id/0001 -> ../../vda

Storage examples

Mount the existing Qcow image /data.qcow2 with xfs filesystem to /mnt/data:

docker run -v /data.qcow2:/dev/runq/0001/none/xfs/mnt/data ...

Attach the host device /dev/sdb1 formatted with ext4 to /mnt/data2:

docker run --device /dev/sdb1:/dev/runq/0002/writethrough/ext4/mnt/data2 ...

Attach the host device /dev/sdb2 without mounting:

docker run --device /dev/sdb2:/dev/runq/0003/writethrough ...


A block device or a raw file with an EXT2 or EXT4 filesystem can be used as rootdisk of the VM. On first boot of the container the content of the Docker image is copied into the rootdisk. The block device or raw file will then be used as root filesystem via virtio-blk instead of 9pfs. But be aware that changes to the root filesystem will not be reflected in the source docker container filesystem. (docker cp will no longer work as expected)

# existing block device with empty ext4 filesystem
docker run --runtime runq --device /dev/sdb1:/dev/runq/0001/none/ext4 -e RUNQ_ROOTDISK=0001 -ti alpine sh

# new raw file
fallocate -l 1G disk.raw
mkfs.ext4 disk.raw
docker run --runtime runq --volume $PWD/disk.raw:/dev/runq/0001/none/ext4 -e RUNQ_ROOTDISK=0001 -ti alpine sh

Directories can be excluded from being copied with the RUNQ_ROOTDISK_EXCLUDE environment variable. E.g. -e RUNQ_ROOTDISK_EXCLUDE="/foo,/bar"

See Dockerfile.rootdisk and as a further example.


By default runq drops all capabilities except those needed (same as regular Docker does). The white list of the remaining capabilities is provided by the Docker engine.


See man capabilities for a list of all available capabilities. Additional Capabilities can be added to the white list at container start:

docker run --cap-add SYS_TIME --cap-add SYS_MODULE ...`


runq supports the default Docker seccomp profile as well as custom profiles.

docker run --security-opt seccomp=<profile-file> ...

The default profile is defined by the Docker daemon and gets applied automatically. Note: Only the runq init binary is statically linked against libseccomp. Therefore libseccomp is needed only at compile time.

If the host operating system where runq is being built does not provide static libseccomp libraries one can also simply build and install libseccomp from the sources.

Seccomp can be disabled at container start:

docker run --security-opt seccomp=unconfined ...

Note: Some Docker daemon don’t support custom Seccomp profiles. Run docker info to verify that Seccomp is supported by your daemon. If it is supported the output of docker info looks like this:

Security Options:
  Profile: default

AP adapter passthrough (s390x only)

AP devices provide cryptographic functions to all CPUs assigned to a Linux system running in an IBM Z system LPAR. AP devices can be made available to a runq container by passing a VFIO mediated device from the host through Qemu into the runq VM guest. VFIO mediated devices are enabled by the vfio_ap kernel module and allow for partitioning of AP devices and domains. The environment variable RUNQ_APUUID specifies the VFIO mediated device UUID. runq automatically loads the required zcrypt kernel modules inside the VM. E.g.:

docker run --runtime runq -e RUNQ_APUUID=b34543ee-496b-4769-8312-83707033e1de ...

For details on how to setup mediated devices on the host see


Most docker commands and options work as expected. However, due to the fact that the target application runs inside a Qemu VM which itself runs inside a Docker container and because of the minimalistic design principle of runq some docker commands and options don’t work. E.g:

  • adding / removing networks and storage dynamically
  • docker exec (see runq-exec)
  • docker swarm
  • privileged mode
  • apparmor, selinux, ambient
  • docker HEALTHCHECK

The following common options of docker run are supported:

--attach                    --name
--cap-add                   --network
--cap-drop                  --publish
--cpus                      --restart
--cpuset-cpus               --rm
--detach                    --runtime
--entrypoint                --sysctl
--env                       --security-opt seccomp=unconfined
--env-file                  --security-opt no-new-privileges
--expose                    --security-opt seccomp=<filter-file>
--group-add                 --tmpfs
--help                      --tty
--hostname                  --ulimit
--init                      --user
--interactive               --volume
--ip                        --volumes-from
--link                      --workdir

Nested VM

A nested VM is a virtual machine that runs inside of a virtual machine. In plain KVM this feature is considered working but not meant for production use. Running KVM guests inside guests of other hypervisors such as VMware might not work as expected or might not work at all. However to try out runq in a VM guest the (experimental) runq runtime configuration parameter --nestedvm can be used. It modifies the parameters of the Qemu process.

Developing runq

For fast development cycles runq can be build on the host as follows:

  1. Prerequisites:
  • Docker >= 17.09.x-ce
  • Go >= 1.11
  • /var/lib/runq must be writable by the current user
  • Libseccomp static library. E.g. libseccomp-dev for Ubuntu or libseccomp-static for Fedora
  1. Download runc and runq source code
    go get -d -u
    go get -d -u
  2. Install Qemu and guest kernel to /var/lib/runq/qemu All files are taken from the Ubuntu 18.04 LTS Docker base image.
    cd $GOPATH/src/
    make -C qemu all install
  3. Compile and install runq components to /var/lib/runq
    make install
  4. Create TLS certificates
  5. Adjust file and directory permissions
    sudo chown -R root:root /var/lib/runq
  6. Register runq as Docker runtime with appropriate defaults as shown in section Installation above.


See CONTRIBUTING for details.


The code is licensed under the Apache License 2.0. See LICENSE for further details.

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