Building a custom OpenHCL Linux Kernel

Note

This step is NOT required in order to build OpenHCL!

Unless you have a specific reason, it is strongly recommended to stick to the pre-built Kernel image which is automatically downloaded as part of the OpenHCL build process.

Cloning the kernel repository

If you need to rebuild the kernel, the sources are available in the OHCL-Linux-Kernel repo :

Unless you need the entire repo history, cloning just one branch and depth = 1, saves significant time and disk space:

git clone https://github.com/microsoft/OHCL-Linux-Kernel.git -b product/hcl-main/6.6 --depth=1

Cloning under Windows is likely to fail due to some files using names that Windows reserves, e.g. aux.c, and NTFS being non case-sensitive by default as there are few files in the Linux kernel repo whose names differ in their case only. To clone successfully under Windows, need a fix in ntdll (merged in Ni?), and a case-sensitive NTFS partition. Best to start with the default WSL2 if there is no existing working setup.

Building the kernel locally

The following instructions for building the kernel locally target the Ubuntu distributions. In order to build on rpm-based systems the only changes are likely to be the way the package manager is invoked and installing the kernel-devel package instead of what is needed for Ubuntu.

The paths below are relative to the cloned kernel repository root.

Do once for every machine that hasn't run this step successfully:

./Microsoft/install-deps.sh

If you plan to use your custom kernel to work with confidential VMs, you need to enable a few more options in the kernel. Do once after cloning the kernel repository or every time you remove local changes from your kernel configuration file:

./Microsoft/merge-cvm-config.sh

Every time the kernel needs to be rebuilt:

./Microsoft/build-hcl-kernel.sh

The output directory is ./out, it contains the kernel binary, the kernel modules, and the debug symbols, and cargo xflowey build-igvm can be pointed to it to use as a source of the kernel binary and the modules.

In the case you are iterating on a change, install ccache for decreasing the kernel build time significantly, might be close to an order of magnitude. To use the compiler cache, prepend CC="ccache gcc" to the build command. To see if the cache integrates itself into the toolchain:

host:~$ which gcc

/usr/lib64/ccache/gcc