[TASK] Phase 4.3.2: virtio-net Driver (QEMU NIC)

[iamvirul]

Overview

Implement a user-space virtio-net driver for QEMU's paravirtual NIC. virtio-net is the standard NIC in QEMU/KVM environments and is the correct first target — it is simpler than real PCIe NICs and avoids PCI enumeration complexity.

Background

virtio-net uses shared memory virtqueues (TX and RX rings in DMA buffers) for zero-copy packet transfer. The driver places packet buffers in the RX virtqueue; QEMU fills them with incoming frames and fires an interrupt. For TX, the driver places frames in the TX virtqueue; QEMU drains them.

Design

Virtqueue

pub struct Virtqueue {
    descriptors: &'static mut [VirtqDesc],   // DMA buffer
    available:   &'static mut VirtqAvail,
    used:        &'static mut VirtqUsed,
    queue_size:  u16,
    last_used:   u16,
}

Driver Lifecycle

1. Enumerate PCI bus for virtio-net device (vendor=0x1AF4, device=0x1000)
2. Read virtio device config (MAC address, features)
3. Negotiate features (VIRTIO_NET_F_MAC minimum)
4. Allocate TX/RX virtqueues via sys_dma_alloc
5. Register IrqCapability for virtio interrupt
6. Populate RX queue with empty buffers; notify device
7. On IRQ: drain used RX descriptors, pass frames to sys_nic_send (to kernel RX ring)
8. On TX: get frame from kernel TX ring via sys_nic_recv, place in TX virtqueue, kick device

Acceptance Criteria

• PCI device enumeration finds virtio-net device in QEMU
• Virtqueues allocated via sys_dma_alloc (physically contiguous)
• MAC address read from virtio config space
• RX: incoming frames received and forwarded to kernel net stack
• TX: outgoing frames from kernel net stack transmitted to QEMU
• Driver sends ReadySignal after init
• QEMU -netdev user mode: ping to host gateway succeeds (verified via ARP/ICMP in Phase 4.3.3)
• Serial log: "virtio-net: found PCI device at bus 0 slot N", "virtio-net: MAC=52:54:00:xx:xx:xx, MTU=1500", "virtio-net: ready"

Files

• userspace/drivers/virtio-net/src/main.rs
• userspace/drivers/virtio-net/src/pci.rs — PCI enumeration
• userspace/drivers/virtio-net/src/virtqueue.rs
• userspace/drivers/virtio-net/src/net.rs — TX/RX loops
• userspace/drivers/virtio-net/Cargo.toml

Dependencies

• Phase 4.3.1 (NIC abstraction): sys_nic_send / sys_nic_recv
• Phase 3.1.4: sys_dma_alloc for virtqueue memory
• Phase 3.1.3: sys_irq_register for virtio IRQ