docs: Add document for memory hotplug on arm64

After some enablement work, memory hotplug can be used on arm64.
Here we offer a document to instruct user to enable it.

Fixes: #1854
Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

docs/how-to/README.md

@@ -37,3 +37,4 @@
 - [How to use Kata Containers with `virtio-mem`](how-to-use-virtio-mem-with-kata.md)
 - [How to set sandbox Kata Containers configurations with pod annotations](how-to-set-sandbox-config-kata.md)
 - [How to monitor Kata Containers in K8s](how-to-set-prometheus-in-k8s.md)
+- [How to use hotplug memory on arm64 in Kata Containers](how-to-hotplug-memory-arm64.md)

docs/how-to/how-to-hotplug-memory-arm64.md

@@ -0,0 +1,32 @@
+# How to use memory hotplug feature in Kata Containers on arm64
+
+- [Introduction](#introduction)
+- [Install UEFI ROM](#install-uefi-rom)
+- [Run for test](#run-for-test)
+
+## Introduction
+
+Memory hotplug is a key feature for containers to allocate memory dynamically in deployment.
+As Kata Container bases on VM, this feature needs support both from VMM and guest kernel. Luckily, it has been fully supported for the current default version of QEMU and guest kernel used by Kata on arm64. For other VMMs, e.g, Cloud Hypervisor, the enablement work is on the road. Apart from VMM and guest kernel, memory hotplug also depends on ACPI which depends on firmware either. On x86, you can boot a VM using QEMU with ACPI enabled directly, because it boots up with firmware implicitly. For arm64, however, you need specify firmware explicitly. That is to say, if you are ready to run a normal Kata Container on arm64, what you need extra to do is to install the UEFI ROM before use the memory hotplug feature.
+
+## Install UEFI ROM
+
+We have offered a helper script for you to install the UEFI ROM. If you have installed Kata normally on your host, you just need to run the script as fellows:
+
+```bash
+$ pushd $GOPATH/src/github.com/kata-containers/tests
+$ sudo .ci/aarch64/install_rom_aarch64.sh
+$ popd
+```
+
+## Run for test
+
+Let's test if the memory hotplug is ready for Kata after install the UEFI ROM. Make sure containerd is ready to run Kata before test.
+
+```bash
+$ sudo ctr image pull docker.io/library/ubuntu:latest
+$ sudo ctr run --runtime io.containerd.run.kata.v2 -t --rm docker.io/library/ubuntu:latest hello sh -c "free -h"
+$ sudo ctr run --runtime io.containerd.run.kata.v2 -t --memory-limit 536870912 --rm docker.io/library/ubuntu:latest hello sh -c "free -h"
+```
+
+Compare the results between the two tests. If the latter is 0.5G larger than the former, you have done what you want, and congratulation!