weft-sim

Board simulation. The real Linux drivers run in a small VM and talk to simulated chips from weft-parts, bound through the same device tree overlays as on hardware.

Note

Prototype. One chip (ADS1115) on one simulated I2C bus, x86-64 guest. Like all of weft, it is built heavily with AI; see How weft is made.

How it works

┌──────────────── guest Linux (QEMU, KVM / HVF / WHPX) ──────────┐
│ real drivers: ti-ads1015, ...      bound by the board overlay  │
│        │ I2C transfers                                         │
│ weft_sim.ko: I2C adapters that forward each transfer           │
│        │ /dev/weft-sim                                         │
│ weft-sim serve: weft-parts chip models                         │
└────────────────────────────────────────────────────────────────┘

1. weft_sim.ko (C, about 400 lines) creates /dev/weft-sim. It holds no chip logic.
2. weft-sim serve (Rust, static binary) attaches the chip models and starts answering transfers.
3. The runner applies the base overlay (guest/base.dts), which creates a simulated bus node published as &i2c1, registers I2C adapter 1 for it, then applies the board overlay, exactly as written for a Raspberry Pi (tests/boards/ads1115.dts).
4. The upstream driver probes, and the test reads the ADC through sysfs.

The models run inside the guest, so bus traffic never leaves the VM and the host needs only a stock QEMU. The guest's CPU follows the host's, so the hypervisor always accelerates it. Why this design and not vhost-user: weft-design docs/04-decisions.md.

Run

nix develop      # Linux: toolchain, QEMU, guest kernel, busybox
just sim         # build module, runner and initramfs; boot; expect "SIM PASS"
just sim weft.trace    # also log every I2C transfer

On a laptop with KVM, a run takes under a second, boot included.

Layout

Path	What
module/	Kernel module and the interface header shared with the runner
src/	The runner: wire format, ioctls, command line
guest/	Base overlay, init script, initramfs builder
tests/boards/	Board overlays under test

Next

• Our own minimal guest kernel for x86-64 and arm64, published as a ready-made image.
• Host harness (weft-sim run) for Linux, macOS and Windows: picks KVM, HVF or WHPX and steers tests over a virtio-serial control channel.
• More chips: MCP4728, 24C32, SPI (AD5592R), GPIO and interrupts through gpio-sim.

Licence

MIT, like all weft repositories. The kernel module is MIT OR GPL-2.0-only, so it can also be built into Linux. See LICENSE and LICENSES/.