Gentoo Live CD Adventures
Background
To study Intel’s latest Linear Address Masking feature, I have my supervisor bought me a ASUS Laptop with latest Intel Ultra 258V CPU. As my usual development environment is built upon Gentoo, I decided to install Gentoo on it and test the LAM feature.
However, when I booted into Gentoo’s Live CD, I found that the kernel is too old and nothing works. The network card is gone, the kernel is spitting out tons of errors in dmesg.
Gentoo’s Live CD
So, I learnt that Gentoo provides tools to build your own Live CD, named catalyst (not AMD’s driver). This is the guide (https://wiki.gentoo.org/wiki/Catalyst/Custom_Media_Image). The guide is … well … OK.
It just misses the things I need. Long story short, I need solve the following things to get a usable Live CD.
- Linux Firmware is too old, only the latest one includes the iwlwifi-bz-***-92 firmware.
- The kernel is too old, it supports neither the firmware nor the CPU.
- Catalyst provides neither guides on how to tweak with the kernel nor the firmware.
Linux Firmware
The Linux firmware is rather easy to solve. Gentoo has synced with upstream and provides sys-kernel/linux-firmware-20240909-r1.
By catalyst’s guide, one can change its portage behavior by modifying releng’s config. So, I changed package.mask to force using the latest firmware.
<sys-kernel/linux-firmware-20240909-r1
From distkernel to Source Kernel
Let alone the Catalyst’s configuration, we first need to let Catalyst to use the latest kernel package.
This is still done via portage’s configuration. I changed package.mask to force using the latest kernel.
<sys-kernel/gentoo-sources-6.11.0
and allow the latest kernel in package.accept_keywords.
=sys-kernel/gentoo-sources-6.11.0 ~amd64
The hard part — Source Kernel
The kernel used by Catalyst is the gentoo-kernel, which is a pre-built kernel and the latest version is 6.10.12. Sadly, this version
does not support the iwlwifi-bz-***-92 firmware. It only supports iwlwifi-bz-***-90, which does not exist at all…
So, I need to change the Catalyst’s specification to do the following things.
- Add necessary tools for building from the source.
- Change the kernel configuration.
- Change the Catalyst configuration.
Add necessary tools
I added the following packages to /var/tmp/catalyst/installcd-stage1.spec
sys-kernel/genkernel # for building the kernel
dev-util/pahole # for checking the kernel's data structure
sys-fs/squashfs-tools # for mounting the rootfs
Change the kernel configuration
To begin with, the Catalyst does not actually provide a kernel config for sources. So, I just copied the 6.10.12 configuration from distkernel.
You can find it in gentoo-kernel’s ebuild
After I boot into the kernel, the rootfs is not mounted and the Live CD complains not valid rootfs. Basically, the kernel does not recognize the squashfs filesystem, we need to enable it, not as a module, but as a built-in feature.
CONFIG_BLK_DEV_LOOP=y
CONFIG_SQUASHFS=m
Change Catalyst configuration
The original Catalyst configuration contains a part of dracut, which is for generating the initramfs. However, if you’re using gentoo-sources, the genkernel is used to generate the initramfs. So, I removed the dracut part.
subarch: amd64
version_stamp: custom_livecd
target: livecd-stage2
rel_type: 23.0-default
profile: default/linux/amd64/23.0/no-multilib
snapshot_treeish: current.xz
source_subpath: 23.0-default/livecd-stage1-amd64-custom_livecd
portage_confdir: /home/john/documents/releng/releases/portage/isos
livecd/bootargs: dokeymap
livecd/fstype: squashfs
livecd/iso: install-amd64-minimal.iso
livecd/type: gentoo-release-minimal
livecd/volid: Gentoo-amd64
boot/kernel: gentoo
boot/kernel/gentoo/config: /var/tmp/catalyst/kconfig
boot/kernel/gentoo/packages: net-wireless/broadcom-sta
And finally, the Live CD boots up with the network card.