[WIP] Support virtio-console

[shengwen-tw]

Overview

This adds a virtio-console device as a secondary console.

The existing 8250 UART remains the primary console and early boot path. The virtio-console device exposes /dev/hvc0 inside the guest and can be connected from a host PTY endpoint.

This is still a draft while I evaluate the best integration model of virtio-consolefor semu.

Test procedures

1. Build the Linux image with virtio-console enabled:

scripts/build-image.sh --all --clean-build

2. Launch semu:

cd semu/
make check

3. Connect to the virtio-console from another host terminal:

screen /tmp/semu-hvc0

4. Confirm inside the guest:

cat /proc/consoles
ls -l /dev/hvc0

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Summary by cubic

Add virtio-console as a secondary console that exposes /dev/hvc0 and connects to a host PTY. The 8250 UART remains the primary console; new CLI and build options let you choose which consoles to enable.

• New Features
  • MMIO virtio-console device with RX/TX queues and PLIC IRQ 10; integrated into the emulator loop and interrupt path.
  • New CLI flag --virtio-console-path to create a symlink to the host PTY (used by make check as /tmp/semu-hvc0).
  • Build option SEMU_CONSOLE={virtio|uart8250|both} to select consoles; default is both.
  • Device tree adds virtio@5200000; Linux enables CONFIG_VIRTIO_CONSOLE=y.
  • Buildroot post-build script adds a getty on /dev/hvc0; README updated with usage and examples.

^{Written for commit 1ab73d0. Summary will update on new commits.}