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platform-qemu.md

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LinuxKit with qemu/kvm

The qemu backend is the most versatile run backend for linuxkit. It can boot both x86_64 and arm64 images, runs on macOS and Linux (and possibly Windows), and can boot most types of output formats. On Linux, kvm acceleration is enabled by default if available. On macOS, hvf acceleration (using the Hypervisor framework) is used if your qemu version supports it (versions released after Jan/Feb 2018 should support it). s390x is currently only supported in kvm mode as the emulated s390x architecture (aka tcg mode) does not seem to support several required platform features. Further, on s390x platforms you need to set vm.allocate_pgste=1 via sysctl (or use echo 1 > /proc/sys/vm/allocate_pgste).

Boot

By default linuxkit run qemu will boot with the host architecture (e.g., aarch64 on arm64 systems). The architecture can be specified with -arch and currently accepts x86_64, aarch64, and s390x as arguments.

linuxkit run qemu can boot in different types of images:

  • kernel+initrd: This is the default mode of linuxkit run qemu [x86_64, arm64, s390x]
  • kernel+squashfs: linuxkit run qemu -squashfs <path to directory>. This expects a kernel and a squashfs image. [x86_64, arm64, s390x]
  • iso-bios: linuxkit run qemu -iso <path to iso> [x86_64]
  • iso-efi: linuxkit run qemu -iso -uefi <path to iso>. This looks in /usr/share/ovmf/bios.bin for the EFI firmware by default. Can be overwritten with -fw. [x86_64, arm64]
  • qcow-bios: linuxkit run qemu disk.qcow2 [x86_64]
  • raw-bios: linuxkit run qemu disk.img [x86_64]
  • aws: linuxkit run qemu disk.img boots a raw AWS disk image. [x86_64]

The formats qcow-efi and raw-efi may also work, but are currently not tested.

The default kernel+initrd boot uses a RAM disk for the root filesystem. If you have RAM constraints or large images we recommend using one of the other methods, such as kernel+squashfs or booting via a ISO image.

Console

With linuxkit run qemu the serial console is redirected to stdio, providing interactive access to the VM. You can specify -gui to get a console window.

Disks

The qemu backend supports multiple disks to be attached to the VM using the standard linuxkit -disk syntax. The qemu backend supports a number of different disk formats.

Networking

The qemu backend supports a number of networking options, depending on the platform you are running. The default is the userspace networking which provides the VM with a internal DHCP server and network connectivity, but does not provide access to the VMs network from the outside.

With user mode networking you can publish selected VM ports on the host, using the -publish option. It uses the same syntax as the qemu binary. For example linuxkit run qemu -publish 8080:80 linuxkit exposes port 80 from the VM as port 8080 on the host.

On Linux, you can attach the VM either to an existing bridge or tap interface. These require root privileges and you may want to use the qemu-bridge-helper. To attach to an existing bridge br0 (e.g., one created with virt-manager) you can use linuxkit run qemu -networking bridge,br0 linuxkit.

Integration services and Metadata

The qemu backend also allows passing custom userdata into the metadata package using either the -data or -data-file command-line option. This attaches a CD device with the data on.

If the linuxkit/qemu-ga package is added to the YAML the Qemu Guest Agent will be enabled. This provides better integration with libvirt.