Might it be possible to get a u-boot-xen-arm64 package built?  While
"PyGRUB" is great for Linux, it isn't so good for booting other OSes.
I'm proposing "xen-domu-bootloader" as a virtual package name for other
DomU bootloaders to use...

Control: tags 979548 moreinfo

Do you mean:

https://gitlab.denx.de/u-boot/u-boot/-/blob/master/doc/board/xen/xenguest_arm64.rst

This doesn't describe how to use it or, importantly, what files we would
need to ship in the package. If you could help clarify that (possibly
provide a patch), and ideally get it clarified in the upstream
documentation, then I would think we would be able to ship such a
package.


live well,
  vagrant

I'm less than 100% sure myself.  :-)

Most likely you simply configure xenguest_arm64_defconfig, build the
configuration and the package would be the copyright plus one output
file.

In order to use this you setup a VM/domain configuration file where the
single output file is specified as the "kernel" parameter.  This would
cause the U-Boot image to be loaded as if it was an OS kernel and be
loaded into the resultant VM and started.  Then in theory U-Boot loads
configuration parameters from VM disk devices as it normally would.

I think 2 or 3 files would be useful to ship in such a package.  First
would be "u-boot.bin" or whatever the output filename is.  Second might
be a README mentioning the 3 values needing to be set in a domu.cfg file.
Third might be a /etc/xen/xlexample.u-boot file.

The 3 values which need to be set in the domain configuration file are:

kernel = "/usr/lib/u-boot/xen/u-boot.bin"
# ramdisk =
# extra =

Mainly the "ramdisk" and "extra" settings should be left unset, while
"kernel" points at the U-Boot image.  A /etc/xen/xlexample.u-boot would
be a copy of Xen's /etc/xen/xlexample.pvlinux with the 3 values set
appropriately.

Then https://wiki.debian.org/Xen should be adjusted to mention U-Boot
being available to boot user domains for Xen.  In fact I'm trying to
find out whether Xen/U-Boot can load OSes besides Linux.

Note, this is presently theory as src:u-boot has a problematic set of
package requirements.

Presently libfdt-dev doesn't allow installation of multiple architecture
versions.  My build VM is setup to target Xen which needs the host
package, while the u-boot build needs the build package.  Grr!

Appears the build issue wasn't libfdt-dev, but instead `dwz` and
`debhelper`.  I suspect libfdt-dev:any or libfdt-dev may now be
sufficient for building (I'm not 100% sure since I have a workaround in
place).

Anyway I now have something which looks like a first pass at having
U-Boot/ARM boot a Xen VM.  Some progress has been made, but it I haven't
confirmed full operation yet.

The build was achieved by copying configs/xenguest_arm64_defconfig over
qemu_arm64_defconfig and then cross-building for arm64.  This suggests
extra steps for "qemu" are also appropriate for "xenguest".

Once complete, the file ./usr/lib/u-boot/qemu_arm64/u-boot.bin was
copied to the host machine.  A configuration file was created for xl,
the value for "kernel" was pointed at the u-boot.bin file, both
"bootloader" and "ramdisk" options were left unset.

Upon attempt to boot this VM (`xl create -c u-boot.cfg`) I ended up at a
prompt "xenguest# ".  The command-line appeared to act how I would expect
U-Boot to act, so I conclude U-Boot had successfully loaded.

The next task is to get the OS I wish to run in the VM loaded by U-Boot.

As of 2020.10+dfsg-2, appears the "xenguest" defconfig disables all
EFI/GPT support.  I must recommend the U-Boot maintainers advise upstream
to set CONFIG_EFI_LOADER=y and CONFIG_CMD_PART=y in the Xen defconfig.

While some smaller VMs may not need EFI support, it appears to be gaining
traction everywhere with ARM64.  I note SuSE uses it as an intermediate
stage between U-Boot and GRUB.  FreeBSD's ARM64 VM images appear to
*assume* EFI is in use.

I haven't gotten U-Boot/Xen to successfully load FreeBSD's bootloader
yet, but progress is being made.