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Add example for use of kernel/initrd/cmdline options (#1620)

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Provide a simple working example of how to set the kernel, initrd,
cmdline options and where to retrieve the required files when working
with disk images.

Fixes: #920
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Darragh Bailey 2022-09-30 17:39:56 +01:00 committed by GitHub
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@ -83,6 +83,58 @@ Vagrant.configure("2") do |config|
end
```
## Using kernel and initrd
It's possible to use a direct kernel boot to modify the kernel boot parameters used to boot the VM. This typically involves either downloading the kernel/initrd directly and placing somewhere locally for use, or making use of a tool such as `virt-copy-out` to extract the relevant files from a disk image file.
Looking at a generic/fedora35 image with the following contents of /boot and /boot/grub2, it should be possible to copy out the kernel, initrd, and the grub.cfg file (provides a starting cmdline).
```shell
BOX_DIR="${VAGRANT_HOME:-~/.vagrant.d}/boxes/generic-VAGRANTSLASH-fedora35/4.1.10/libvirt"
virt-ls -a ${BOX_DIR}/box.img /boot/ /boot/grub2
.vmlinuz-5.18.19-100.fc35.x86_64.hmac
System.map-5.18.19-100.fc35.x86_64
config-5.18.19-100.fc35.x86_64
efi
grub2
initramfs-0-rescue-5cbe0655dcd04b46a88f5a424135fbb8.img
initramfs-5.18.19-100.fc35.x86_64.img
loader
symvers-5.18.19-100.fc35.x86_64.gz
vmlinuz-0-rescue-5cbe0655dcd04b46a88f5a424135fbb8
vmlinuz-5.18.19-100.fc35.x86_64
device.map
fonts
grub.cfg
grubenv
i386-pc
locale
```
Assuming you run something like the following:
```shell
BOX_DIR="${VAGRANT_HOME:-~/.vagrant.d}/boxes/generic-VAGRANTSLASH-fedora35/4.1.10/libvirt"
virt-copy-out -a ${BOX_DIR}/box.img \
/boot/vmlinuz-5.18.19-100.fc35.x86_64 \
/boot/initramfs-5.18.19-100.fc35.x86_64.img \
/boot/grub2/grub.cfg \
.
```
The final Vagrantfile should contain something like the following:
```ruby
Vagrant.configure("2") do |config|
config.vm.box = "generic/fedora35"
config.vm.provider :libvirt do |libvirt|
libvirt.kernel = "#{Dir.pwd}/vmlinuz-5.18.19-100.fc35.x86_64"
libvirt.initrd = "#{Dir.pwd}/initramfs-5.18.19-100.fc35.x86_64.img"
# cmd_line is taken from the grub.cfg to ensure starting from a working value
libvirt.cmd_line = 'root=/dev/mapper/fedora-root ro biosdevname=0 no_timer_check ' +
'resume=/dev/mapper/fedora-swap rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap net.ifnames=0'
end
end
```
## SSH Access To VM
vagrant-libvirt supports vagrant's [standard ssh