lkl: add libslirp network backend for user-mode networking

[xdqi]

Summary

Add a libslirp-based virtio-net backend to LKL, enabling user-mode TCP/IP networking without root privileges or pre-configured TAP interfaces.

Motivation

The existing LKL network backends (TAP, macvtap, DPDK, VDE, wintap) all require either root privileges, pre-configured host interfaces, or specific hardware. libslirp provides user-mode NAT networking — the same stack used by QEMU's -netdev user — allowing LKL guests to access the network without any host configuration.

Changes

File	Description
tools/lkl/lib/virtio_net_slirp.c	New slirp backend (648 lines), POSIX + MinGW portable
tools/lkl/include/lkl.h	Slirp API: lkl_netdev_slirp_create, add_hostfwd, remove_hostfwd
tools/lkl/Makefile.autoconf	Auto-detection: header check (POSIX) or pkg-config (NT64)
tools/lkl/lib/Build	Build integration for virtio_net_slirp.o
tools/lkl/include/mingw32/sys/uio.h	Win32 stub for struct iovec

Architecture

• Guest network: 10.0.2.0/24, gateway 10.0.2.2, DNS 10.0.2.3
• Packet ring buffer (256 entries) between slirp's send_packet callback and LKL RX path
• Dedicated poll thread drives slirp's internal socket I/O
• Platform abstraction layer supports both POSIX (pthread/poll/pipe) and Win32 (Winsock/WSA/CreateThread)

API

struct lkl_netdev *lkl_netdev_slirp_create(void);
int lkl_netdev_slirp_add_hostfwd(struct lkl_netdev *nd, int is_udp,
    const char *host_addr, int host_port,
    const char *guest_addr, int guest_port);
int lkl_netdev_slirp_remove_hostfwd(struct lkl_netdev *nd, int is_udp,
    const char *host_addr, int host_port);

Build

Requires libslirp-dev (pkg-config: slirp). The backend is auto-detected at build time — no extra flags needed. If slirp is not installed, the backend is silently disabled.

Example usage

// Create slirp netdev — no root required
struct lkl_netdev *nd = lkl_netdev_slirp_create();

// Forward host port 12345 to guest 10.0.2.15:12345
lkl_netdev_slirp_add_hostfwd(nd, 0, "0.0.0.0", 12345, "10.0.2.15", 12345);

// Start kernel, add netdev, configure interface (same as other backends)
lkl_init(&lkl_host_ops);
lkl_start_kernel("mem=32M");
int nd_id = lkl_netdev_add(nd, NULL);
lkl_if_up(lkl_netdev_get_ifindex(nd_id));
lkl_if_set_ipv4(lkl_netdev_get_ifindex(nd_id),
    inet_addr("10.0.2.15"), 24);
lkl_set_ipv4_gateway(inet_addr("10.0.2.2"));

Disclaimer

🤖 Generated with Claude Code but I've audited the code.

[tavip]

This is awesome, thanks @xdqi ! Overall looks good, I'll take a closer look a bit later.

Please check the windows test failure and the checkpatch errors.

Also, please add tests similar with what we have for the other backends.