docs: Add a guide for testing on AArch64

The support of AArch64 is in very early stage. The steps in building and
runing on X86 and AArch64 can not align well yet. Adding AArch64 content
to README.md would produce much divergence.
Adding a guide in docs/ folder could be a better way to start now.

Signed-off-by: Michael Zhao <michael.zhao@arm.com>

docs/arm64.md

@@ -0,0 +1,64 @@
+# How to build and run Cloud-hypervisor on Arm64
+
+Cloud-hypervisor is partially enabled on AArch64 architecture.
+Although all features are not ready yet, you can begin to test Cloud-hypervisor on a Arm64 host by following this guide.
+
+## Prerequisites
+
+On Arm64 machines, Cloud-hypervisor depends on an external library `libfdt-dev` for generating Flatted Device Tree (FDT).
+
+The long-term plan is to replace `libfdt-dev` with some pure-Rust component to get rid of such dependency.
+
+```bash
+sudo apt-get update
+sudo apt-get install libfdt-dev
+```
+
+## Build
+
+Before building, a hack trick need to be performed to get rid of some build error in vmm component. See [this](https://github.com/cloud-hypervisor/kvm-bindings/pull/1) for more info about this temporary workaround.
+
+```bash
+sed -i 's/"with-serde",\ //g' vmm/Cargo.toml
+```
+
+The support of AArch64 is in very early stage, only Virtio devices with MMIO tranport is available.
+
+```bash
+cargo build --no-default-features --features "mmio"
+```
+
+## Image
+
+Download kernel binary and rootfs image from AWS.
+
+```bash
+wget https://s3.amazonaws.com/spec.ccfc.min/img/aarch64/ubuntu_with_ssh/fsfiles/xenial.rootfs.ext4 -O rootfs.img
+wget https://s3.amazonaws.com/spec.ccfc.min/img/aarch64/ubuntu_with_ssh/kernel/vmlinux.bin -O kernel.bin
+```
+
+## Containerized build
+
+If you want to build and test Cloud Hypervisor without having to install all the required dependencies, you can also turn to the development script: dev_cli.sh.
+
+To build the development container:
+
+```bash
+./scripts/dev_cli.sh build-container
+```
+
+To build Cloud-hypervisor in the container:
+
+```bash
+./scripts/dev_cli.sh build
+```
+
+## Run
+
+Assuming you have built Cloud-hypervisor with the development container, a VM can be started with command:
+
+```bash
+sudo target/debug/cloud-hypervisor --kernel kernel.bin --disk path=rootfs.ext4 --cmdline "keep_bootcon console=hvc0 reboot=k panic=1 pci=off root=/dev/vda rw" --cpus boot=4 --memory size=512M --seccomp false --serial file=serial.log --log-file log.log -vvv
+```
+
+If the build was done out of the container, replace the binary path with `build/cargo_target/aarch64-unknown-linux-gnu/debug/cloud-hypervisor`.

scripts/dev_cli.sh

@@ -208,6 +208,11 @@ cmd_build() {
         rustflags="-C link-arg=-lgcc"
     fi
 
+    # A workaround on Arm64 to avoid build errors in kvm-bindings
+    if [ $(uname -m) = "aarch64" ]; then
+        sed -i 's/"with-serde",\ //g' "$CLH_ROOT_DIR"/vmm/Cargo.toml
+    fi
+
     $DOCKER_RUNTIME run \
 	   --user "$(id -u):$(id -g)" \
 	   --workdir "$CTR_CLH_ROOT_DIR" \