# UEFI Boot

Cloud Hypervisor supports UEFI boot through the utilization of the EDK II based UEFI firmware. 

## Building UEFI Firmware

To avoid any unnecessary issues, it is recommended to use Ubuntu 18.04 and its default toolset. Any other compatible Linux distribution is otherwise suitable, however it is suggested to use a temporary Docker container with Ubuntu 18.04 for a quick build on an existing Linux machine.

The commands below will compile an OVMF firmware suitable for Cloud Hypervisor.

```shell
sudo apt-get update
sudo apt-get install uuid-dev nasm iasl build-essential python3-distutils git

git clone https://github.com/cloud-hypervisor/edk2 -b ch
cd edk2
. edksetup.sh
git submodule update --init

echo "ACTIVE_PLATFORM=OvmfPkg/OvmfPkgX64.dsc" >> Conf/target.txt
echo "TARGET_ARCH=X64" >> Conf/target.txt
echo "TOOL_CHAIN_TAG=GCC5" >> Conf/target.txt

make -C ./BaseTools
build
```

After the successful build, the resulting firmware binaries are available under `Build/OvmfX64/DEBUG_GCC5/FV` underneath the edk2 checkout.

## Using OVMF Binaries

Any UEFI capable image can be booted using the Cloud Hypervisor specific firmware. Windows guests under Cloud Hypervisor only support UEFI boot, therefore OVMF is mandatory there.

To make Cloud Hypervisor use UEFI boot, pass the `OVMF.fd` file path as an argument to the `--kernel` option. The firmware file will be opened in read only mode.

The same firmware can be used with Cloud Hypervisor or with QEMU. This is particularly useful if using QEMU for the preparation phase.

## Building UEFI Firmware with Compatibility Support Module (CSM)

CSM is a module that allows to boot legacy operating systems using the OVMF firmware. OVMF can embed a CSM build of SeaBIOS. To build the SeaBIOS with CSM support, add `CONFIG_CSM=y` to `.config` before the build. The outcome `out/Csm16.bin` is to be moved into `OvmfPkg/Csm/Csm16/Csm16.bin` before OVMF is built. Then, the OVMF build will have to be passed the `-D CSM_ENABLE` option in order to generate a legacy aware UEFI firmware. At the current stage, all the necessary patches are included in the Cloud Hypervisor specific [SeaBIOS branch](https://github.com/cloud-hypervisor/seabios/tree/ch). Taking into account the previous instructions, the modified command sequence to compile an OVMF binary with CSM support is the following one:

```shell
sudo apt-get update
sudo apt-get install uuid-dev nasm iasl build-essential python3-distutils git libncurses-dev

git clone https://github.com/cloud-hypervisor/seabios -b ch
cd seabios
make menuconfig
# Enable `CONFIG_CSM` and `CONFIG_QEMU_HARDWARE`
# Please make sure `python` is in your path
make CONFIG_CSM=y CONFIG_QEMU_HARDWARE=y
cd ..

git clone https://github.com/cloud-hypervisor/edk2 -b ch
cd edk2
. edksetup.sh
git submodule update --init --recursive
cp ../seabios/out/Csm16.bin OvmfPkg/Csm/Csm16/

echo "ACTIVE_PLATFORM=OvmfPkg/OvmfPkgX64.dsc" >> Conf/target.txt
echo "TARGET_ARCH=X64" >> Conf/target.txt
echo "TOOL_CHAIN_TAG=GCC5" >> Conf/target.txt

make -C ./BaseTools
build -DCSM_ENABLE

```

Please note, that the CSM support has currently only been tested with Linux guests. There are no plans to provide legacy support for other OSes (e.g. Windows).

# Links

- [OVMF wiki](https://github.com/tianocore/tianocore.github.io/wiki/OVMF) 
- [Cloud Hypervisor specific tree](https://github.com/cloud-hypervisor/edk2/tree/ch)
- [Redhat OVMF Status Report](https://access.redhat.com/sites/default/files/attachments/ovmf-whtepaper-031815.pdf)
- [SeaBIOS Build Overview](https://www.seabios.org/Build_overview#Build_as_a_UEFI_Compatibility_Support_Module_.28CSM.29)