dev-docs: document method for obtaining serial console on CoCo AKS

dev-docs/aks/serial-console.md

@@ -0,0 +1,41 @@
+# Obtain a serial console inside the podvm
+
+Get a shell on the AKS node. If in doubt, use [nsenter-node.sh](https://github.com/alexei-led/nsenter/blob/master/nsenter-node.sh).
+Now run the following commands to use a debug [igvm](https://docs.google.com/presentation/d/1uWeyqtYV53Vtxd3ayYWWTLbxasYNr35a/) and enable debugging for the kata runtime.
+
+```sh
+sed -i -e 's#^igvm = "\(.*\)"#igvm = "/opt/confidential-containers/share/kata-containers/kata-containers-igvm-debug.img"#g' /opt/confidential-containers/share/defaults/kata-containers/configuration-clh-snp.toml
+sed -i -e 's/^#enable_debug = true/enable_debug = true/g' /opt/confidential-containers/share/defaults/kata-containers/configuration-clh-snp.toml
+systemctl restart containerd
+```
+
+Now you need to reconnect to the host. Use the following commands to print the sandbox ids of kata VMs.
+Please note that only the pause container of every pod has the `clh.sock`.  Other containers are part of the same VM.
+
+```shell-session
+$ ctr --namespace k8s.io container ls "runtime.name==io.containerd.kata-cc.v2"
+$ sandbox_id=ENTER_SANDBOX_ID_HERE
+```
+
+And attach to the serial console using socat. You need to type `CONNECT 1026<ENTER>` to get a shell.
+
+```shell-session
+$ cd /var/run/vc/vm/${sandbox_id}/ && socat stdin unix-connect:clh.sock
+CONNECT 1026
+```
+
+Alternatively, you can use `kata-runtime exec`, which gives you proper tty behavior including tab completion.
+First ensure the kata-monitor is running, then connect.
+
+```
+$ kata-monitor &
+$ kata-runtime exec ${sandbox_id}
+```
+
+If you are done, use the following commands to go back to a release igvm.
+
+```sh
+sed -i -e 's#^igvm = "\(.*\)"#igvm = "/opt/confidential-containers/share/kata-containers/kata-containers-igvm.img"#g' /opt/confidential-containers/share/defaults/kata-containers/configuration-clh-snp.toml
+sed -i -e 's/^enable_debug = true/#enable_debug = true/g' /opt/confidential-containers/share/defaults/kata-containers/configuration-clh-snp.toml
+systemctl restart containerd
+```