‘microvm’ virtual platform (microvm)

microvm is a machine type inspired by Firecracker and constructed after its machine model.

It’s a minimalist machine type without PCI nor ACPI support, designed for short-lived guests. microvm also establishes a baseline for benchmarking and optimizing both QEMU and guest operating systems, since it is optimized for both boot time and footprint.

Supported devices

The microvm machine type supports the following devices:

  • ISA bus
  • i8259 PIC (optional)
  • i8254 PIT (optional)
  • MC146818 RTC (optional)
  • One ISA serial port (optional)
  • LAPIC
  • IOAPIC (with kernel-irqchip=split by default)
  • kvmclock (if using KVM)
  • fw_cfg
  • Up to eight virtio-mmio devices (configured by the user)

Limitations

Currently, microvm does not support the following features:

  • PCI-only devices.
  • Hotplug of any kind.
  • Live migration across QEMU versions.

Using the microvm machine type

Machine-specific options

It supports the following machine-specific options:

  • microvm.x-option-roms=bool (Set off to disable loading option ROMs)
  • microvm.pit=OnOffAuto (Enable i8254 PIT)
  • microvm.isa-serial=bool (Set off to disable the instantiation an ISA serial port)
  • microvm.pic=OnOffAuto (Enable i8259 PIC)
  • microvm.rtc=OnOffAuto (Enable MC146818 RTC)
  • microvm.auto-kernel-cmdline=bool (Set off to disable adding virtio-mmio devices to the kernel cmdline)

Boot options

By default, microvm uses qboot as its BIOS, to obtain better boot times, but it’s also compatible with SeaBIOS.

As no current FW is able to boot from a block device using virtio-mmio as its transport, a microvm-based VM needs to be run using a host-side kernel and, optionally, an initrd image.

Running a microvm-based VM

By default, microvm aims for maximum compatibility, enabling both legacy and non-legacy devices. In this example, a VM is created without passing any additional machine-specific option, using the legacy ISA serial device as console:

$ qemu-system-x86_64 -M microvm \
   -enable-kvm -cpu host -m 512m -smp 2 \
   -kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda" \
   -nodefaults -no-user-config -nographic \
   -serial stdio \
   -drive id=test,file=test.img,format=raw,if=none \
   -device virtio-blk-device,drive=test \
   -netdev tap,id=tap0,script=no,downscript=no \
   -device virtio-net-device,netdev=tap0

While the example above works, you might be interested in reducing the footprint further by disabling some legacy devices. If you’re using KVM, you can disable the RTC, making the Guest rely on kvmclock exclusively. Additionally, if your host’s CPUs have the TSC_DEADLINE feature, you can also disable both the i8259 PIC and the i8254 PIT (make sure you’re also emulating a CPU with such feature in the guest).

This is an example of a VM with all optional legacy features disabled:

$ qemu-system-x86_64 \
   -M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off \
   -enable-kvm -cpu host -m 512m -smp 2 \
   -kernel vmlinux -append "console=hvc0 root=/dev/vda" \
   -nodefaults -no-user-config -nographic \
   -chardev stdio,id=virtiocon0 \
   -device virtio-serial-device \
   -device virtconsole,chardev=virtiocon0 \
   -drive id=test,file=test.img,format=raw,if=none \
   -device virtio-blk-device,drive=test \
   -netdev tap,id=tap0,script=no,downscript=no \
   -device virtio-net-device,netdev=tap0

Triggering a guest-initiated shut down

As the microvm machine type includes just a small set of system devices, some x86 mechanisms for rebooting or shutting down the system, like sending a key sequence to the keyboard or writing to an ACPI register, doesn’t have any effect in the VM.

The recommended way to trigger a guest-initiated shut down is by generating a triple-fault, which will cause the VM to initiate a reboot. Additionally, if the -no-reboot argument is present in the command line, QEMU will detect this event and terminate its own execution gracefully.

Linux does support this mechanism, but by default will only be used after other options have been tried and failed, causing the reboot to be delayed by a small number of seconds. It’s possible to instruct it to try the triple-fault mechanism first, by adding reboot=t to the kernel’s command line.