Re: Should we boot the FreeBSD kernel in ELF format or in zImage format ? How?

On Sat, Nov 25, 2023 at 4:41 AM Mario Marietto <marietto2008@gmail.com>
wrote:

> Hello to everyone.
>
> we have just virtualized Debian 12 on our arm (32 bit) Chromebook. As host
> / dom0 we have chosen Devuan 5,and for guest / domU,Debian 12. It works
> great. But our goal is different. We want to virtualize FreeBSD as domU.
> Can we have a working Xen PV network driver for a FreeBSD arm guest ?. I
> found that Julien Grall has ported the Xen drivers to FreeBSD on arm. I
> would like to know if Julien's work was accepted upstream by FreeBSD, in
> which case FreeBSD as a Xen guest on arm should work if we enable the Xen
> PV drivers in the FreeBSD on arm kernel. If Julien's work was not accepted
> upstream by FreeBSD, we will have to find his patches and apply them
> ourselves to the FreeBSD on arm kernel.
>
> We found these slides :
>
>
> https://events.static.linuxfound.org/sites/events/files/slides/Porting%20FreeBSD%20on%20Xen%20on%20ARM%20.pdf
>
> Slide 13 refers to a XENHVM FreeBSD on arm kernel config - that is what we
> want to find.
>
> It looks like when that slide presentation was written, there were some
> limitations on FreeBSD Xen guests. For example, for our debian bookworm
> guest, I am using vcpus = '2' to match the number of real cpus on our
> Chromebook, but slide 13 mentions support for only 1 VCPU with a FreeBSD
> guest, so I will need to change that vcpus = '1' in the FreeBSD guest
> config unless support for 2 or more vcpus was added later, which is
> possible because that slide presentation is 9 years old.
>
> Here is where I would expect to find the XENHVM FreeBSD on arm kernel
> config file:
>
> https://cgit.freebsd.org/src/tree/sys/arm/conf
>
> But it is not there unless I am not understanding something correctly. For
> now, unfortunately conclude that the support for Xen on arm that Julien
> Grall mentioned in that slide presentation 9 years ago was never added to
> the official FreeBSD source code. I am searching the web now to see if the
> patches that Julien Grall wrote are still posted somewhere online. If we
> cannot find them, we can ask here and on the xen-users mailing list. Julien
> regularly reads that list and responds to question about Xen on arm, so I
> think he will tell us how to find the patches if we cannot find them online.
>
> According to this page from the FreeBSD wiki:
>
> https://wiki.freebsd.org/Xen
>
> I think FreeBSD only supports Xen on x86, not arm. So this is going to be
> a bit of a challenge to get a Xen FreeBSD guest on arm working. We know
> Julien Grall has some patches that made it work in the past !
>
> I found a slightly newer slide presentation by Julien here:
>
> https://www.slideshare.net/xen_com_mgr/bsdcan-2015-how-to-port-your-bsd
>
> It is about the same, but it mentions the GENERIC FreeBSD kernel supports
> Xen on arm64, but still says we need the XENHVM FreeBSD config for Xen on
> arm 32 bit, which I haven't found online yet.
>
> Please,take a look at this output of the linux kernel that can boot on
> Xen, and the FreeBSD kernel that cannot :
>
>
> % file zImage-6.1.59-stb-xen-cbe+
> zImage-6.1.59-stb-xen-cbe+: Linux kernel ARM boot executable zImage (little-endian)
>
> % file FREEBSD-XENVIRT
> FREEBSD-XENVIRT: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /red/herring, for FreeBSD 11.0 (1100048), not stripped
>
>
> The FreeBSD kernel that won't boot is in ELF format but the Linux kernel
> that does boot is in zImage format.
>
> I spent time reading the docs on xenbits.xenproject.org, and according to
> those docs Xen on arm only knows how to boot a kernel in the zImage format,
> so the FreeBSD kernel is in a format that modern Xen incorrectly detects as
> an x86 kernel.
>
> I also watched Julien Grall's 30 minute video presentation of his work to
> boot FreeBSD/arm on Xen at FOSDEM 2014 here :
>
> https://archive.fosdem.org/2014/schedule/event/freebsd_xen_arm/
>
> In that video, and in other places, Julien mentions that the boot ABI for
> FreeBSD/arm on Xen was not yet developed and he was getting occasional
> crashes and needed to investigate the problem. He mentioned the zImage ABI
> that Linux uses, but pointed out FreeBSD does not use that format, and back
> then it was an open question which format to use to boot FreeBSD/arm on
> Xen. Unfortunately, nine years later, the only supported format is still
> the zImage format that Linux uses.
>
> It looks like Julien's work back then was using an ELF binary to boot
> FreeBSD/arm on Xen instead of the supported zImage format that Linux uses
> and the modern Xen toolstack exits with an error when trying to boot the
> FreeBSD ELF formatted binary that Julien's patch creates. So the best
> solution would be to try to port the rules to build a FreeBSD kernel in the
> zImage format instead of the ELF format. I have been studying the Makefiles
> in Linux to see how Linux builds the Linux arm kernel in the zImage format,
> but it is not trivial to understand
>

Look at kernel.bin in FreeBSD's kernel. It's enabled -DWITH_KERNEL_BIN. It
should be easy to adapt the target to build that. I've done similar things
with u-boot formats in the past, but that was 4 employers and 20 years ago
now.

This path is not well trod. I do know that arm64 virtualization with bhyve
is hitting the tree. I'm not sure how easy/hard this will be to modernize.
I'm interested to see how your explorations go.

Warner