QEMU microvm machine type

A QEMU machine type (-M microvm) explicitly modelled on Firecracker. It strips PCI, ACPI, and most legacy buses out of the i386 platform and replaces device discovery with a fixed virtio-mmio bus, so QEMU itself can serve as a microVM substrate alongside the Rust-rewritten alternatives in rust-vmm. Positioned in the upstream docs as "a baseline for benchmarking and optimizing both QEMU and guest operating systems" — useful as the C-vintage reference point against which Firecracker and Cloud Hypervisor are measured.

What's in, what's out

The machine includes ISA bus, LAPIC, IOAPIC (kernel-irqchip=split by default), kvmclock under KVM, fw_cfg, and up to eight virtio-mmio slots. Optional legacy devices: i8259 PIC, i8254 PIT, MC146818 RTC, and a single ISA serial port — each toggleable via machine properties.

Explicitly unsupported:

PCI-only devices (anything that needs PCI discovery)
Hotplug
Live migration across QEMU versions (snapshot/migrate is intra-version only)

The "no PCI, no ACPI" choice is the one that matters. PCI gets you device enumeration and hotplug at the cost of a fairly large emulation surface; ACPI gets you tables, power management, and the standard shutdown path. Dropping both forces virtio-mmio for I/O and triple-fault for shutdown, which is exactly the trade microvm platforms make.

Boot flow

QEMU microvm uses qboot (a SeaBIOS-compatible minimal firmware) by default, but the firmware cannot boot from virtio-mmio block devices. So the host supplies the kernel directly via -kernel vmlinux (and optionally -initrd), the same direct-kernel-boot path Firecracker uses. There is no BIOS-level disk boot; the rootfs comes in over virtio-mmio after the kernel is already running.

Machine properties

Configurable via -M microvm,<prop>=<value>:

x-option-roms=bool — disable option ROM loading
pit=OnOffAuto — i8254 PIT
pic=OnOffAuto — i8259 PIC
rtc=OnOffAuto — MC146818 RTC
isa-serial=bool — ISA serial port
auto-kernel-cmdline=bool — auto-inject virtio-mmio device descriptors into the kernel cmdline

The OnOffAuto properties default to auto, which keeps legacy devices on for compatibility. Turning them all off gets the minimal footprint variant, which depends on the host CPU having TSC_DEADLINE for the LAPIC timer to replace the PIT.

Usage

Basic invocation with legacy devices enabled:

qemu-system-x86_64 -M microvm \
  -enable-kvm -cpu host -m 512m -smp 2 \
  -kernel vmlinux -append "earlyprintk=ttyS0 console=ttyS0 root=/dev/vda"

Minimal-footprint variant — assumes KVM with TSC_DEADLINE, disables every optional legacy device:

qemu-system-x86_64 \
  -M microvm,x-option-roms=off,pit=off,pic=off,isa-serial=off,rtc=off

Shutdown

Without ACPI and without an emulated keyboard there is no soft-off path. The convention is to trigger a triple fault to reboot. Add reboot=t to the kernel cmdline so Linux prefers the triple-fault path; pair with -no-reboot on the QEMU side so QEMU terminates instead of restarting the guest. This is the same pattern Firecracker uses, modulo the host-side wiring.

Where this sits next to Firecracker

QEMU microvm and Firecracker overlap on the architectural choices — virtio-mmio, direct kernel boot, no PCI/ACPI — but differ on the surrounding code. QEMU is the C-vintage VMM (~1.7M LOC) with microvm carved out of it; rust-vmm is the modern Rust ecosystem rewritten from scratch around the same architectural decisions. Firecracker is ~83K LOC of Rust, Cloud Hypervisor ~106K — both an order of magnitude smaller than QEMU's whole codebase, even though the microvm machine type only exercises a slice of it.

The practical implication for sandboxing-ai-agents use cases: QEMU microvm is useful when you want minimal-microVM semantics on top of an existing QEMU deployment, when matryoshka-isolation needs a Kata-Containers backend, or when you need a feature Firecracker has refused to add. For green-field agent sandboxes the Rust VMMs are the live choices — see microvm-2026 for the platform survey.