# QEMU microvm Machine Type Documentation

# QEMU microvm Machine Type Documentation

## Overview

The microvm is a minimalist x86 machine type inspired by Firecracker, designed as "a baseline for benchmarking and optimizing both QEMU and guest operating systems" with emphasis on boot time and footprint reduction.

## Design Rationale

Microvm deliberately omits PCI and ACPI support to achieve a stripped-down architecture suitable for short-lived guests. It establishes performance benchmarking baselines while maintaining reasonable compatibility through optional legacy device support.

## Supported Devices

The machine includes these components:

- ISA bus
- i8259 PIC (optional)
- i8254 PIT (optional)
- MC146818 RTC (optional)
- Single ISA serial port (optional)
- LAPIC and IOAPIC (latter using kernel-irqchip=split by default)
- kvmclock (KVM only)
- fw_cfg
- Up to eight virtio-mmio devices

## Unsupported Features

Currently excluded:

- PCI-only devices
- Hotplug functionality
- Live migration across QEMU versions

## Boot Flow

Microvm uses `qboot` BIOS by default (compatible with SeaBIOS), but requires host-side kernel and optional initrd. Firmware cannot boot virtio-mmio block devices, necessitating direct kernel boot methodology.

## Machine-Specific Options

Configuration properties include:

- `microvm.x-option-roms=bool` (disable option ROM loading)
- `microvm.pit=OnOffAuto` (i8254 PIT control)
- `microvm.isa-serial=bool` (ISA serial port toggle)
- `microvm.pic=OnOffAuto` (i8259 PIC control)
- `microvm.rtc=OnOffAuto` (RTC control)
- `microvm.auto-kernel-cmdline=bool` (automatic virtio-mmio kernel cmdline insertion)

## Usage Examples

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 disabling RTC, PIC, PIT, ISA serial, and option ROMs when using KVM with TSC_DEADLINE support:

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

## Guest Shutdown

Without ACPI or keyboard mechanisms, microvm relies on triple-fault triggering system reboot. Adding `reboot=t` to kernel cmdline prioritizes this mechanism. The `-no-reboot` flag allows QEMU to detect and gracefully terminate on shutdown events.