Implement a subset of the multiboot specification in order to boot Xen
and a FreeBSD Dom0 from the FreeBSD bootloader. This multiboot
implementation is tailored to boot Xen and FreeBSD Dom0, and it will
most surely fail to boot any other multiboot compilant kernel.
In order to detect and boot the Xen microkernel, two new file formats
are added to the bootloader, multiboot and multiboot_obj. Multiboot
support must be tested before regular ELF support, since Xen is a
multiboot kernel that also uses ELF. After a multiboot kernel is
detected, all the other loaded kernels/modules are parsed by the
multiboot_obj format.
The layout of the loaded objects in memory is the following; first the
Xen kernel is loaded as a 32bit ELF into memory (Xen will switch to
long mode by itself), after that the FreeBSD kernel is loaded as a RAW
file (Xen will parse and load it using it's internal ELF loader), and
finally the metadata and the modules are loaded using the native
FreeBSD way. After everything is loaded we jump into Xen's entry point
using a small trampoline. The order of the multiboot modules passed to
Xen is the following, the first module is the RAW FreeBSD kernel, and
the second module is the metadata and the FreeBSD modules.
Since Xen will relocate the memory position of the second
multiboot module (the one that contains the metadata and native
FreeBSD modules), we need to stash the original modulep address inside
of the metadata itself in order to recalculate its position once
booted. This also means the metadata must come before the loaded
modules, so after loading the FreeBSD kernel a portion of memory is
reserved in order to place the metadata before booting.
The way to load a FreeBSD Dom0 from the bootloader is the following,
first we must make sure nothing else has been loaded, and then we can
load in the following order the Xen microkernel, FreeBSD and whatever
modules are needed in order to boot:
unload
load /boot/xen
load kernel
load zfs
load if_tap
load ...
boot
The arguments that are passed to the Xen kernel can be set from both
the command line at load time:
load /boot/xen dom0_mem=1024M dom0pvh=1
Or by setting the following variable in the /boot/loader.conf file:
xen_cmdline="dom0_mem=1024M dom0_max_vcpus=2 dom0pvh=1 com1=115200,8n1 console=com1"
The only missing piece here is the ability to tell the loader to boot
directly into Xen. I was thinking of adding something like the
following to /boot/loader.conf to make it boot automatically into Xen:
xen_kernel="/boot/xen"
However, I haven't been able to implement it because AFAICT it should
be done in forth, and I'm not even able to figure out what's a
function, a variable or a conditional in that language.
A couple of notes on the coding style:
Several files on sys/boot have a very bad style, mixing hard tabs
with spaces, the newly added multiboot files only use hard tabs
following the FreeBSD kernel coding style.
For the other bits that I've modified, I've tried to do my best to
adapt to the style of the surrounding code.