How to boot FreeBSD for arm 32 bit as DomU with u-boot on my ARM Chromebook

Hello.

I'm trying to boot FreeBSD for arm32 bit as DomU on my ARM Chromebook.
Basically there are two ways to accomplish this task :

1) to write a patch that allows the FreeBSD kernel to boot as a zImage
file. This could be accomplished applying this patch to a specific file
that's on the source code of FreeBSD :


https://xenbits.xen.org/gitweb/?p=p...8;hb=0782e25d98cc1391472717035f986c979edef0c9
<https://xenbits.xen.org/gitweb/?p=people/julieng/freebsd.git;a=blob_plain;f=sys/arm/conf/XENVIRT;h=ff6433392e6fc2d5fa7c1ad1f7c40155003af9a8;hb=0782e25d98cc1391472717035f986c979edef0c9>


This patch was written by Julien Grall a lot of time ago and now it does
not work anymore. This is the reason :


It appears FreeBSD-CURRENT removed the last step converting the kernel file
to kernel.bin. The patch can be readily rebased, but without kernel.bin
that doesn't do too much.



So,without a rebase of that patch the first option is not applicable. And
I'm not able to fix it.

2) booting FreeBSD using U-Boot,as explained to me by a xen developer :


I was trying to explain why and how Julien's patch works so that you could
be the one to re-do something similar or fix the patch on the FreeBSD
kernel that you are working with. I am happy to help review and write
patches but I don't work with the FreeBSD kernel so I wouldn't be able to
help you quickly. However, I might have a suggestion. Do you know if
FreeBSD can be booted by U-Boot ? Because U-Boot definitely boots as Xen on
ARM guest firmware/bootloader. You should be able to build U-Boot and use
the U-Boot binary as Xen guest kernel, then U-Boot could load FreeBSD from
disk or network and start it. For instance as domU config file:

kernel="/home/petalinux/u-boot.bin"
disk = [ '/home/petalinux/test.img,raw,xvda' ]

I know it is important to build u-boot with the following config to make it
work on Xen.

CONFIG_CMO_BY_VA_ONLY=y



This option seems more doable to me according to my knowledge. But I need
to understand how to do it.

Well,let's say that on the ARM Chromebook I'm forced to use and install a
customized version of u-boot,created by virtual open systems,because it is
the only one that allows bypassing its bootloader protection. You can find
more information here :

http://www.virtualopensystems.com/en/solutions/guides/kvm-on-chromebook/?vos=tech

This is the relevant section to read :


Bootloader :

If you wish to skip this chapter you can download a pre-compiled binary of
the bootloader:


$ wget
http://www.virtualopensystems.com/downloads/guides/kvm_on_chromebook/nv_u-boot-snow.kpart


To be able to run KVM on ARM platforms, the kernel has to be booted in
hypervisor mode. Because of this relatively recent requirement (due to the
introduction of the virtualization extensions), up until now all booting
methods would boot the kernel in the standard Supervisor mode. For the ARM
Chromebook the default boot procedure doesn't allow us to boot in
hypervisor mode. Although the laptop's boot mechanism is based on the
frequently used u-boot, the binary is located in RO memory. Fortunately, a
chained u-boot mechanism can be used (i.e. starting another u-boot after
the original). We can then enter hypervisor mode from our custom iteration
of u-boot and subsequently load our kernel and userspace.

Checkout the needed u-boot code :


$ git clone git://github.com/virtualopensystems/u-boot.git$ cd u-boot$
./scripts/build.sh


If successful, a message about how to copy the bootloader on the USB flash
disk or SD card will appear. We will use it later when preparing the boot
medium to start our system. If you have followed the Setting up the boot
medium chapter and you have a prepared boot device, then you can update
u-boot by running :


$ sudo dd if=nv_uboot-snow.kpart of=/dev/sdX1



so,the needed u-boot that we must use should be installed on the first
partition of the sd card.

There is another relevant section to read :


Setting up the boot medium

Now it is time to copy all the relevant files that we created in the
previous chapters,and use them to boot Chromebook with a different kernel
and OS. In all these examples the device /dev/sdX is used. Take extra care
to change the examples to the device that you have attached. Insert the
boot medium on your workstation and carefully execute the following step.
First we need to properly format the boot medium.

In the uboot source directory :


$ sudo ./scripts/sdcard.sh /dev/sdX


This will erase all data and create 4 partitions in the medium, along with
copying the u-boot binary to the first partition:


Partition 1 = ChromeOS signed binary (V.O.S chained u-boot)
Partition 2 = not used
Partition 3 = EXT2 partition for u-boot files (uImage and
exynos5250-snow.dtb)
Partition 4 = EXT4 partition for userspace files


With u-boot being copied, next is the kernel image and DTB file. From the
kernel source execute :


$ mkdir ../mnt/
$ sudo mount /dev/sdX3 ../mnt/
$ sudo cp arch/arm/boot/uImage ../mnt/
$ sudo cp arch/arm/boot/dts/exynos5250-snow.dtb ../mnt/
$ sudo umount /dev/sdX3


Finally, we have to copy the Ubuntu userspace filesystem that we created
earlier:


$ sudo mount /dev/sdX4 mnt/$ sudo cp -a ./precise/* mnt/$ sudo umount
/dev/sdX4



Now,my idea is to chainload the already chain loaded u-boot created by
V.O.S to the new u-boot that we need for booting FreeBSD and that can be
installed in the partition n.2,as shown in this scheme,because it is not
used :


Partition 1 = ChromeOS signed binary (V.O.S chained u-boot)
Partition 2 = not used (maybe we can install the u-boot for arm 32
bit,compatible with FreeBSD on this partition)
Partition 3 = EXT2 partition for u-boot files (uImage and
exynos5250-snow.dtb)
Partition 4 = EXT4 partition for userspace files


Take in consideration that default boot string is hardcoded here,in the
snow.h file of the custom u-boot created by VOS :


https://github.com/virtualopensyste...18a39b6c177dff58a/include/configs/snow.h#L101
<https://github.com/virtualopensystems/u-boot/blob/eecfeb578e296ef3b739ac918a39b6c177dff58a/include/configs/snow.h#L101>


and it needs to be recompiled because it should point to the partition
n.2,where I will install the u-boot files as explained here :


https://wiki.freebsd.org/arm/Chromebook


I have some questions to ask before I start working on this.

1) The xen developer said :


You should be able to build U-Boot and use the U-Boot binary as Xen guest
kernel...



where is the u-boot binary,according to this document ?

https://wiki.freebsd.org/arm/Chromebook

I don't see it.


2) where is the source code of the file that I can get here :

http://commondatastorage.googleapis.com/chromeos-localmirror/distfiles/nv_uboot-snow-simplefb.kpart.bz2

I need the source code if I want to recompile u-boot so that it can point
to the partition 4.

Maybe it can be found on this link :

http://linux-exynos.org/dist/chromebook/nv_uboot/

but it can't be opened....


3) in this specific scenario the source code of u-boot should run on arm 32
bit,not on arm 64,because I have the Samsung Chromebook "SNOW" model
XE303C12,that's powered by a Samsung Exynos 5250 (ARMv7 32 bit Cortex A15)
Soc.


4) I'm not sure if I can chainload the customized u-boot created by V.O.S
that should be installed on the first partition with the u-boot tailored
for booting FreeBSD that should be installed on the partition 2....


5) the xen developer said that u-boot should be compiled enabling this
option :


Code:

CONFIG_CMO_BY_VA_ONLY=y



Well,can you provide some good source that can help me to understand how I
can recompile u-boot for FreeBSD ? thanks.

-- 
Mario.