docs: Add guide for direct boot VM on riscv64

Add `riscv.md` to guide developers/users on preparing guest kernel,
image, and direct boot VM on riscv64 platforms; document support status
and known limitations.

Co-authored-by: Bo Chen <bchen@crusoe.ai>
Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

docs/riscv.md

@@ -0,0 +1,88 @@
+# How to build and test Cloud Hypervisor on riscv64
+
+This document introduces how to build and test Cloud Hypervisor on `riscv64`.
+All instructions here are tested with Ubuntu 24.04.2 as the host OS.
+
+## Hardware requirements
+
+- riscv64 servers (recommended) or development boards equipped with the AIA
+(Advance Interrupt Architecture) interrupt controller.
+
+## Getting started
+
+We create a folder to build and run Cloud Hypervisor at `$HOME/cloud-hypervisor`
+
+```console
+export CLOUDH=$HOME/cloud-hypervisor
+mkdir $CLOUDH
+```
+
+## Prerequisites
+
+You need to install some prerequisite packages to build and test Cloud Hypervisor.
+
+### Tools
+
+```console
+# Install rust tool chain
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+# Install the tools used for building guest kernel, EDK2 and converting guest disk
+sudo apt-get update
+sudo apt-get install git build-essential m4 bison flex uuid-dev qemu-utils
+```
+
+### Building Cloud Hypervisor
+
+```console
+pushd $CLOUDH
+git clone https://github.com/cloud-hypervisor/cloud-hypervisor.git
+cd cloud-hypervisor
+cargo build
+popd
+```
+
+### Disk image
+
+Download the Ubuntu cloud image and convert the image type.
+
+```console
+pushd $CLOUDH
+wget https://cloud-images.ubuntu.com/jammy/current/jammy-server-cloudimg-riscv64.img
+qemu-img convert -p -f qcow2 -O raw jammy-server-cloudimg-riscv64.img jammy-server-cloudimg-riscv64.raw
+popd
+```
+
+## Direct-kernel booting
+
+### Building kernel
+
+```console
+pushd $CLOUDH
+git clone --depth 1 "https://github.com/cloud-hypervisor/linux.git" -b ch-6.12.8
+cd linux
+make ch_defconfig
+make -j `nproc`
+popd
+```
+
+### Booting the guest VM
+
+```console
+pushd $CLOUDH
+sudo $CLOUDH/cloud-hypervisor/target/debug/cloud-hypervisor \
+           --kernel $CLOUDH/linux/arch/riscv64/boot/Image \
+           --disk path=jammy-server-cloudimg-riscv64.raw \
+           --cmdline "console=hvc0 root=/dev/vda rw" \
+           --cpus boot=1 \
+           --memory size=1024M \
+           --seccomp false \
+           --log-file boot.log -vv
+popd
+```
+
+## Known limitations
+
+- Direct kernel boot only
+- `64-bit Linux` guest OS only
+- For more details, see
+  [here](https://github.com/cloud-hypervisor/cloud-hypervisor/issues/6978).