docs: add SSH access guide for guest networking

Add section 4.5 to the cosim usage guide (both en/zh) documenting
how to configure guest networking and SSH into the guest via the
port forwarding enabled in PR #6.

Steps: identify NIC name with `ip a`, configure netplan DHCP,
then `ssh -p 2222 gem5@localhost` from the host.

Signed-off-by: Chao Liu <[email protected]>

Author: zevorn

docs/en/cosim-usage-guide.md

@@ -271,6 +271,50 @@ screen -r qemu-cosim
 # Detach from screen: Ctrl-A D
 ```
 
+#### 4.5 SSH Access to Guest
+
+The `cosim_launch.sh` script enables user networking and SSH port forwarding by default (`-netdev user,id=net0,hostfwd=tcp::2222-:22` + `virtio-net-pci`). To use SSH access to the guest, configure networking inside the guest first.
+
+**1. Identify the network interface name:**
+
+```bash
+ip a
+```
+
+Look for the virtio NIC interface (e.g., `enp0s2`). The exact name may vary depending on the PCI topology.
+
+**2. Configure netplan:**
+
+Edit `/etc/netplan/50-cloud-init.yaml`:
+
+```yaml
+network:
+  version: 2
+  ethernets:
+    enp0s2:
+      dhcp4: true
+```
+
+> **Note:** Replace `enp0s2` with the actual interface name from the `ip a` output.
+
+**3. Apply the configuration:**
+
+```bash
+netplan apply
+```
+
+**4. SSH from the host:**
+
+Open another terminal on the host and connect:
+
+```bash
+ssh -p 2222 gem5@localhost
+```
+
+Default password: `12345`.
+
+> **Tip:** SSH access is much more convenient than the QEMU serial console for interactive use, file transfers (`scp -P 2222`), and running multiple sessions.
+
 ---
 
 ## Step 5: Load the GPU Driver

docs/zh/cosim-usage-guide.md

@@ -271,6 +271,50 @@ screen -r qemu-cosim
 # 退出 screen: Ctrl-A D（分离）
 ```
 
+#### 4.5 SSH 访问 Guest
+
+`cosim_launch.sh` 脚本默认启用了用户态网络和 SSH 端口转发（`-netdev user,id=net0,hostfwd=tcp::2222-:22` + `virtio-net-pci`）。要通过 SSH 访问 Guest，需要先在 Guest 内配置网络。
+
+**1. 查看网卡名称：**
+
+```bash
+ip a
+```
+
+找到 virtio 网卡接口（如 `enp0s2`），具体名称取决于 PCI 拓扑，可能不同。
+
+**2. 配置 netplan：**
+
+编辑 `/etc/netplan/50-cloud-init.yaml`：
+
+```yaml
+network:
+  version: 2
+  ethernets:
+    enp0s2:
+      dhcp4: true
+```
+
+> **注意：** 将 `enp0s2` 替换为 `ip a` 输出中的实际接口名称。
+
+**3. 应用配置：**
+
+```bash
+netplan apply
+```
+
+**4. 从宿主机 SSH 登录：**
+
+在宿主机上打开另一个终端，执行：
+
+```bash
+ssh -p 2222 gem5@localhost
+```
+
+默认密码：`12345`。
+
+> **提示：** 相比 QEMU 串口控制台，SSH 访问在交互操作、文件传输（`scp -P 2222`）以及多会话场景下更加方便。
+
 ---
 
 ## 第五步：加载 GPU 驱动