reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/iommu/linux.git/commit/?id=20858d4ebb423826b7a978d54cde195f51d87e20
        git remote add iommu https://git.kernel.org/pub/scm/linux/kernel/git/iommu/linux.git
        git fetch --no-tags iommu core
        git checkout 20858d4ebb423826b7a978d54cde195f51d87e20
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash