Add Linux KVM slirp networking

[misaelzapata]

Summary

• Add Linux/KVM libslirp user-mode networking for --net slirp / network: "slirp", including TCP host forwarding.
• Preserve Linux TAP/NAT --net auto by advertising the KVM virtio-net device and letting the guest init configure eth0.
• Update native build detection, CI/prebuild dependencies, docs, capabilities, and e2e/unit coverage.

Validation

• npm run test:unit
• NODE_VMM_KERNEL=/home/misael/.cache/node-vmm/kernels/gocracker-guest-standard-vmlinux npm run test:e2e
• npm run test:coverage
• node --test dist/test/native.test.js
• npm run test:consumers
• npm run test:js-apps
• sudo -n env PATH="$PATH" NODE_VMM_KERNEL=/home/misael/.cache/node-vmm/kernels/gocracker-guest-standard-vmlinux npm run test:real-apps
• npm run pack:check
• Manual CLI smoke: --net slirp -p <host>:3000 returned manual-linux-slirp-ok
• Manual CLI smoke: --net auto -p <host>:3000 returned manual-linux-tap-ok

[Copilot]

Pull request overview

Adds Linux/KVM libslirp user-mode networking support to complement existing TAP/NAT (--net auto) behavior, aligning KVM networking features more closely with WHP/HVF while extending port publishing coverage and documentation.

Changes:

• Extend host capabilities / CLI / docs to advertise and describe Linux --net slirp (with TCP host forwarding).
• Update native build + GYP wiring to optionally link libslirp on Linux (pkg-config-driven) and update CI deps accordingly.
• Add/adjust unit + e2e coverage for slirp networking and interactive console behavior.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
test/unit.test.ts	Expands kernel candidate/capabilities assertions and adds a probeHvf assertion for unsupported hosts.
test/e2e.test.ts	Adds HTTP port-publish e2e for KVM slirp and adjusts interactive run args / PTY input line endings.
src/rootfs.ts	Reorders interactive console selection blocks (PTY vs getty ordering).
src/net.ts	Adjusts Linux TAP/NAT network config (removes kernel ip= arg for Linux tap path).
src/kvm.ts	Adds coverage ignores around console notification control logic.
src/index.ts	Advertises slirp as a network mode and ensures virtio-net kernel args are added for non-none networking (non-HVF).
src/cli.ts	Updates help text to reflect WHP/HVF auto→slirp behavior.
scripts/build-native.mjs	Adds Linux pkg-config detection and env wiring for libslirp flags.
native/kvm/backend.cc	Implements KVM virtio-net slirp integration + host forwarding + new N-API config parsing.
docs/testing.md	Documents rootless considerations and adds Linux slirp port-publish smoke example.
docs/sdk.md	Documents net: "slirp" as a rootless-friendly option when available.
docs/features.md	Updates feature matrix and networking descriptions for Linux slirp support.
binding.gyp	Adds Linux conditional compile/link configuration for libslirp.
README.md	Updates Linux deps and networking notes to include slirp.
.github/workflows/release-prebuilds.yml	Adds libglib2.0-dev dependency for Linux prebuild job.
.github/workflows/ci.yml	Adds libslirp-dev + libglib2.0-dev to Linux CI jobs.

Comments suppressed due to low confidence (1)

native/kvm/backend.cc:2203

• tap_readable() returns true unconditionally when slirp is enabled. The watchdog thread checks net->tap_readable() every 20ms and calls interrupt_all() when it’s true, so this will force continuous interrupts even when there is no network traffic, leading to unnecessary KVM_RUN exits and high host CPU usage. Make the slirp path only report readable when there is pending slirp I/O (e.g., queued RX frames, or poll slirp fds for readiness) instead of always true.

	  bool tap_readable() const {
    if (slirp_enabled()) {
      return true;
    }
	    struct pollfd pfd {};
	    pfd.fd = tap_.get();
	    pfd.events = POLLIN;
	    int rc = poll(&pfd, 1, 0);
	    return rc > 0 && (pfd.revents & POLLIN);

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