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Device model

ignition wires every device through one uniform path. A DeviceManager owns the set of devices, and each device implements the MmioDevice trait. That single abstraction handles MMIO-window and SPI allocation, bus dispatch, FDT node emission, and snapshot enumeration, so adding a device does not mean touching the boot path, the FDT generator, and the snapshot writer separately.

DeviceManager and the MmioDevice trait

DeviceManager centralizes what would otherwise be scattered per-device plumbing:

  • MMIO / SPI allocation. The manager hands each device a slice of the MMIO address window and an SPI line, so device placement is decided in one place instead of being hard-coded per device.
  • Bus dispatch. A guest MMIO access (decoded from a Data Abort in the run loop) is routed to the device whose window contains the faulting address.
  • FDT node emission. Each device describes its own FDT node (reg, interrupt, compatible string). The FDT generator walks the manager rather than hard-listing devices, so the device tree the guest sees always matches the devices that are actually wired.
  • Snapshot hooks. Each device emits a DeviceRecord at snapshot time and is reconstructed from one at restore. The snapshot format is a self-describing list of these records rather than a hand-maintained struct of device fields.

Because the same DeviceManager describes devices for both a fresh boot and a restore, there is a single device-wiring site. Boot and restore drive the same code to allocate windows, register on the bus, and produce or consume device records, which keeps the two paths from drifting apart.

The shipped device set

ignition implements the full Firecracker aarch64 device set:

  • virtio-blk for the root filesystem.
  • virtio-net over a vmnet NAT backend.
  • virtio-rng backed by host entropy.
  • virtio-balloon for on-demand memory reclaim.
  • virtio-vsock for guest-to-host streams.
  • PL031 RTC for wall-clock time.
  • boot-timer, a magic-MMIO probe that reports guest boot time (and that the fuzzer reuses as a control-plane doorbell).

For the per-device behavior, the networking model, and the SMP wiring, see Devices, SMP & networking.