Distribution Kernels: module rebuilds, better ZFS support and UEFI executables

The primary goal of the Distribution Kernel project is provide a seamless kernel upgrade experience to Gentoo users. Initially, this meant configuring, building and installing the kernel during the @world upgrade. However, you had to manually rebuild the installed kernel modules (and @module-rebuild is still broken), and sometimes additionally rebuild the initramfs after doing that.

To address this, we have introduced a new dist-kernel USE flag. This flag is automatically added to all ebuilds installing kernel modules. When it is enabled, the linux-mod eclass adds a dependency on virtual/dist-kernel package. This virtual, in turn, is bound to the newest version of dist-kernel installed. As a result, whenever you upgrade your dist-kernel all the module packages will also be rebuilt via slot rebuilds. The manual @module-rebuild should no longer be necessary!

ZFS users have pointed out that after rebuilding sys-fs/zfs-kmod package, they need to rebuild the initramfs for Dracut to include the new module. We have combined the dist-kernel rebuild feature with pkg_postinst() to rebuild the initramfs whenever zfs-kmod is being rebuilt (and the dist-kernel is used). As a result, ZFS should no longer require any manual attention — as long as rebuilds succeed, the new kernel and initramfs should be capable of running on ZFS root once the @world upgrade finishes.

Finally, we have been asked to provide support for uefi=yes Dracut option. When this option is enabled, Dracut combines the EFI stub, kernel and generated initramfs into a single UEFI executable that can be booted directly. The dist-kernels now detect this scenario, and install the generated executable in place of the kernel, so everything works as expected. Note that due to implementation limitations, we also install an empty initramfs as otherwise kernel-install.d scripts would insist on creating another initramfs. Also note that until Dracut is fixed to use correct EFI stub path, you have to set the path manually in /etc/dracut.conf: