Kernel
Kernel series
Void Linux provides many kernel series in the default repository. These are
named linux<x>.<y>: for example, linux4.19. You can query for all available
kernel series by running:
$ xbps-query --regex -Rs '^linux[0-9.]+-[0-9._]+'
The linux meta package, installed by default, depends on one of the kernel
packages, usually the package containing the latest mainline kernel that works
with all DKMS modules. Newer kernels might be available in the repository, but
are not necessarily considered stable enough to be the default; use these at
your own risk. If you wish to use a more recent kernel and have DKMS modules
that you need to build, install the relevant linux<x>.<y>-headers package,
then use xbps-reconfigure(1) to
reconfigure the linux<x>.<y> package you installed. This will build the DKMS
modules.
Removing old kernels
When updating the kernel, old versions are left behind in case it is necessary
to roll back to an older version. Over time, old kernel versions can accumulate,
consuming disk space and increasing the time taken by DKMS module updates.
Furthermore, if /boot is a separate partition and fills up with old kernels,
updating can fail or result in incomplete initramfs filesystems to be generated
and result in kernel panics if they are being booted. Thus, it may be advisable
to clean old kernels from time to time.
Removing old kernels is done using the
vkpurge(8) utility. vkpurge comes
pre-installed on every Void Linux system. This utility runs the necessary
hooks when removing old kernels. Note that vkpurge does not
remove kernel packages, only particular kernels.
Removing the default kernel series
If you've installed a kernel package for a series other than the default, and
want to remove the default kernel packages, you should install the linux-base
package or mark it as a manual package
in case it is already installed. After this procedure, you can remove the
default kernel packages with
xbps-remove(1). It might be necessary
to add linux and linux-headers to an ignorepkg entry in
xbps.d(5), since base packages can depend
on them.
Switching to another kernel series
If you'd like to use the linux-lts or linux-mainline kernel series instead
of the default linux, first install the desired series metapackage (and the
linux-lts-headers or linux-mainline-headers metapackage if needed). Then you
can add linux and linux-headers to an ignorepkg entry in
xbps.d(5) and uninstall those packages.
Changing the default initramfs generator
By default, Void Linux uses dracut to
prepare initramfs images for installed kernels. Alternatives such as
mkinitcpio are available. Each
initramfs generator registers an XBPS
alternative in the initramfs
group to link its kernel hooks to be used when creating or
removing initramfs images for a given kernel.
To replace dracut with, e.g., mkinitcpio, install the mkinitcpio package;
confirm that mkinitcpio appears in the list of available alternatives by
running
$ xbps-alternatives -l -g initramfs
Issue the command
# xbps-alternatives -s mkinitcpio
to replace the dracut kernel hooks with those provided by mkinitcpio. With subsequent kernel updates (or updates to DKMS packages that trigger initramfs regeneration), mkinitcpio will be used instead of dracut to prepare initramfs images. To force images to regenerate, reconfigure your kernel packages by invoking
# xbps-reconfigure -f linux<x>.<y>
for each linux<x>.<y> package that is currently installed.
cmdline
The kernel, the initial RAM disk (initrd) and some system programs can be configured at boot by kernel command line arguments. The parameters understood by the kernel are explained in the kernel-parameters documentation and by bootparam(7). Parameters understood by dracut can be found in dracut.cmdline(7).
Once the system is booted, the current kernel command line parameters can be
found in the /proc/cmdline file. Some system programs can change their
behavior based on the parameters passed in the command line, which is what
happens when booting a different
runsvdir, for example.
There are different ways of setting these parameters, some of which are explained below.
GRUB
Kernel command line arguments can be added through the GRUB bootloader by
editing /etc/default/grub, changing the GRUB_CMDLINE_LINUX_DEFAULT variable
and then running update-grub.
dracut
Dracut offers a kernel_cmdline configuration
option and --kernel-cmdline
command-line option that will encode
command-line arguments directly in the initramfs image. When dracut is used to
create a UEFI executable, arguments set with these options will be passed to the
kernel. However, when an ordinary initramfs is produced, these options will
not be passed to the kernel at boot. Instead, they will be written to a
configuration file in /etc/cmdline.d within the image. While dracut parses
this configuration to control its own boot-time behavior, the kernel itself will
not be aware of anything set via this mechanism.
After modifying a dracut configuration, regenerate the initramfs to ensure that it includes the changes.
Kernel hardening
Void Linux ships with some kernel security options enabled by default. This was
originally provided by kernel command line arguments slub_debug=P page_poison=1, but since kernel series 5.3, these have been replaced with
init_on_alloc and init_on_free (see this
commit).
Void's kernels come with the init_on_alloc option enabled by default where
available (i.e. linux5.4 and greater). In most cases you should usually not
disable it, as it has a fairly minimal impact on performance (within 1%). The
init_on_free option is more expensive (around 5% on average) and needs to be
enabled manually by passing init_on_free=1 on the kernel command line. If you
need to disable init_on_alloc, you can do that similarly by passing
init_on_alloc=0.
There is a chance that your existing system still has the old options enabled.
They still work in newer kernels, but have a performance impact more in line
with init_on_free=1. On older hardware this can be quite noticeable. If you
are running a kernel series older than 5.4, you can keep them (or add them) for
extra security at the cost of speed; otherwise you should remove them.
Kernel modules
Kernel modules are typically drivers for devices or filesystems.
Loading kernel modules during boot
Normally the kernel automatically loads required modules, but sometimes it may be necessary to explicitly specify modules to be loaded during boot.
To load kernel modules during boot, a .conf file like
/etc/modules-load.d/virtio.conf needs to be created with the contents:
# load virtio-net
virtio-net
Blacklisting kernel modules
Blacklisting kernel modules is a method for preventing modules from being loaded by the kernel. There are two different methods for blacklisting kernel modules, one for modules loaded by the initramfs and one for modules loaded after the initramfs process is done. Modules loaded by the initramfs have to be blacklisted in the initramfs configuration.
To blacklist modules loaded after the initramfs process, create a .conf file,
like /etc/modprobe.d/radeon.conf, with the contents:
blacklist radeon
Blacklisting modules in the initramfs
After making the necessary changes to the configuration files, the initramfs needs to be regenerated for the changes to take effect on the next boot.
dracut
Dracut can be configured to not include kernel modules through a configuration
file. To blacklist modules from being included in a dracut initramfs, create a
.conf file, like /etc/dracut.conf.d/radeon.conf, with the contents:
omit_drivers+=" radeon "
mkinitcpio
To blacklist modules from being included in a mkinitcpio initramfs a .conf
file needs to be created like /etc/modprobe.d/radeon.conf with the contents:
blacklist radeon
Kernel hooks
Void Linux provides directories for kernel hooks in
/etc/kernel.d/{pre-install,post-install,pre-remove,post-remove}.
These hooks are used to update the boot menus for bootloaders like grub,
gummiboot and lilo.
Install hooks
The {pre,post}-install hooks are executed by
xbps-reconfigure(1) when
configuring a Linux kernel, such as building its initramfs. This happens when a
kernel series is installed for the first time or updated, but can also be run
manually:
# xbps-reconfigure --force linux<x>.<y>
If run manually, they serve to apply initramfs configuration changes to the next boot.
Remove hooks
The {pre,post}-remove hooks are executed by
vkpurge(8) when removing old kernels.
Dynamic Kernel Module Support (DKMS)
There are kernel modules that are not part of the Linux source tree that are
built at install time using DKMS and kernel hooks. The
available modules can be listed by searching for dkms in the package
repositories.
DKMS build logs are available in /var/lib/dkms/.