Migrate virtio-net from semu

[Charlie-Tsai1123]

Summary

This PR migrates virtio-net support from semu and follows the existing virtio-blk and virtio-rng integration in rv32emu.

The implementation adds a TAP-backed virtio-net device model for system emulation mode, including MMIO register handling, queue setup, feature negotiation, RX/TX virtqueue handling, virtio-net header processing, interrupt delivery through the PLIC, dynamic DTB node creation, and a runtime option for enabling the device.

Implementation notes

• Add a TAP backend helper for host network I/O.
• Add a virtio-net device model with RX and TX virtqueue handling.
• Handle the virtio-net header on both guest TX and RX paths.
• Route virtio-net MMIO accesses through the system MMIO path.
• Update virtio-net interrupts through the PLIC.
• Dynamically create a virtio,mmio DTB node for virtio-net.
• Ensure virtio-net can coexist with existing virtio-blk and virtio-rng devices without reusing MMIO bases or IRQs.
• Add a runtime option for enabling virtio-net with a TAP backend.
• Note that opening a TAP device usually requires elevated privileges, so rv32emu should be executed with sudo or equivalent permissions when using the TAP backend.

Test

Build:

make system_defconfig
make ENABLE_SYSTEM=1

sudo build/rv32emu \
  -k build/linux-image/Image \
  -i build/linux-image/rootfs.cpio \
  -x vblk:build/disks/disk.img \
  -x vrng \
  -x vnet:tap

After run rv32emu with -x vnet:tap rv32emu would build TAP, so host linux doesn't need to build TAP again.
Host TAP setup:

ip link | grep tap
sudo ip addr add 192.168.100.1/24 dev tap0
sudo ip link set  tap0 up
sudo tcpdump -i tap0 -n -vv

Guest device verification:

readlink /sys/bus/virtio/devices/virtio0/driver
ip link set eth0 up
ip addr add 192.168.100.2/24 dev eth0
ip addr show eth0
ping -c 3 192.168.100.1

Expected Result:

# readlink /sys/bus/virtio/devices/virtio0/driver
../../../../../bus/virtio/drivers/virtio_net
# ip link set eth0 up
# ip addr add 192.168.100.2/24 dev eth0
# ip addr show eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
    link/ether aa:e0:56:5b:58:bc brd ff:ff:ff:ff:ff:ff
    inet 192.168.100.2/24 scope global eth0
       valid_lft forever preferred_lft forever
# ping -c 3 192.168.100.1
PING 192.168.100.1 (192.168.100.1): 56 data bytes
64 bytes from 192.168.100.1: seq=0 ttl=64 time=1.200 ms

64 bytes from 192.168.100.1: seq=1 ttl=64 time=1001.085 ms
64 bytes from 192.168.100.1: seq=2 ttl=64 time=1.334 ms

--- 192.168.100.1 ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 1.200/334.539/1001.085 ms
# 

---

Summary by cubic

Adds a TAP-backed virtio-net device for system emulation. It’s wired through VirtIO-MMIO and the PLIC, auto-added to the DTB, and enabled via -x vnet:tap for guest networking.

• New Features

  • Virtio-net: feature negotiation, RX/TX queues, virtio-net header (TX skip, RX prepend), QueueNotify/used ring, and reset.
  • Linux TAP backend with non-blocking I/O; interface auto-created (tapN); polled in the main loop; MMIO and PLIC interrupt wiring.
  • DTB node for virtio,mmio with unique MMIO base/IRQ; coexists with virtio-blk/virtio-rng.

• Migration

  • Run with -x vnet:tap; Linux hosts only. Use elevated privileges to create TAP. Bring the host TAP up and assign an IP if needed.

^{Written for commit 56a5c0b. Summary will update on new commits.}

[jserv]

Benchmarks

Details

Benchmark suite	Current: 56a5c0b	Previous: b775142	Ratio
Dhrystone	1576.667 DMIPS	1585.333 DMIPS	1.01
CoreMark	1115.023 iterations/sec	1120.589 iterations/sec	1.00

This comment was automatically generated by workflow using github-action-benchmark.

[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}

[shengwen-tw]

Hi @Charlie-Tsai1123,

I’m not the original author of the virtio-net device. Please check the commit history via git log --follow -- virtio-net.c; I believe the original author is @jserv.

[Charlie-Tsai1123]

Thanks for the clarification, and sorry for the incorrect attribution.
I checked the semu history with git log --follow -- virtio-net.c. The original virtio-net implementation appears to have been introduced by jserv. I will update the commit message accordingly.