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Direct Kernel Boot

Virtink uses Cloud Hypervisor as the underlying hypervisor for VMs. Cloud Hypervisor supports direct kernel boot into a vmlinux ELF kernel. It should work with a rootfs from most distributions and does not require an EFI system partition. To use the direct kernel boot feature in Virtink, both a kernel image and a rootfs volume are required.

Kernel Images

In order to support virtio-watchdog, Cloud Hypervisor has its own kernel development branch. Currently, we provide a pre-built v5.15.12 kernel image in the Docker Hub. In the future, more versions of pre-built kernels may be added.

Kernel is configured in spec.instance.kernel. You should specify both the image name of the kernel, and the Linux cmdline to start the kernel. Below is an example to use the pre-built kernel image for direct kernel booting:

apiVersion: virt.virtink.smartx.com/v1alpha1
kind: VirtualMachine
spec:
  instance:
    kernel:
      image: smartxworks/virtink-kernel-5.15.12
      cmdline: "console=ttyS0 root=/dev/vda rw"
    disks:
      - name: ubuntu
  volumes:
    - name: ubuntu
      containerRootfs:
        image: smartxworks/virtink-container-rootfs-ubuntu
        size: 4Gi

Building and Using Your Own Kernel

It's possible to build and use your own kernel for direct kernel booting. To build a kernel that can work properly with Cloud Hypervisor, refer to the Cloud Hypervisor documentation.

After successfully built your kernel, you should have a vmlinux file ready to be injected into a container image. The vmlinux file must be placed at exactly the /vmlinux path and the image must be based on smartxworks/virtink-kernel-base.

Below is an example of injecting a local kernel into a container image:

FROM smartxworks/virtink-kernel-base
COPY vmlinux /vmlinux

Rootfs Volumes

The rootfs defines the root filesystem of the VM. The root parition from most distributions should work for direct kernel booting. However, Virtink does provide a more effortless way to build and use a rootfs using Docker with the containerRootfs volume feature.

containerRootfs Volume

The containerRootfs feature provides the ability to store and distribute VM rootfs in the container image registry. Everything you need to build a containerRootfs image is the Docker toolchain. No raw or QCOW2 images are involved. For building and using a containerRootfs image with direct kernel booting, refer to the containerRootfs volume documentation.

Other Types of Volumes

Other types of volumes can also be a valid rootfs source for direct kernel booting, as long as the containing disk image has a root partition and you specify it correctly in the spec.instance.kernel.cmdline. Below is an example of direct kernel booting a Ubuntu VM using the Ubuntu cloud image inside a containerDisk volume:

apiVersion: virt.virtink.smartx.com/v1alpha1
kind: VirtualMachine
spec:
  instance:
    kernel:
      image: smartxworks/virtink-kernel-5.15.12
      cmdline: "console=ttyS0 root=/dev/vda1 rw"
    disks:
      - name: ubuntu
  volumes:
    - name: ubuntu
      containerDisk:
        image: smartxworks/virtink-container-disk-ubuntu