Re: We are not able to virtualize FreeBSD using xen 4.17 on Arm 32 bit

[Henry Wang <Henry.Wang@xxxxxxx>]

+(xen-devel and Arm maintainers, including Julien)

> On Nov 27, 2023, at 18:03, Mario Marietto <marietto2008@xxxxxxxxx> wrote:
> 
> Hello.
> We have just virtualized Debian 12 on our arm (32 bit) Chromebook model 
> xe303c12 . 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 questions 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. This is the error we get :
> # xl create freebsd.cfg 
> Parsing config from freebsd.cfg xc: error: panic: xg_dom_elfloader.c:63: 
> xc_dom_guest_type: image not capable of booting inside a HV
> M container: Invalid kernel 
> libxl: error: libxl_dom.c:571:libxl__build_dom: xc_dom_parse_image failed 
> libxl: error: libxl_create.c:1640:domcreate_rebuild_done: Domain 4:cannot 
> (re-)build domain: -3 
> libxl: error: libxl_domain.c:1183:libxl__destroy_domid: Domain 4:Non-existant 
> domain 
> libxl: error: libxl_domain.c:1137:domain_destroy_callback: Domain 4:Unable to 
> destroy guest 
> libxl: error: libxl_domain.c:1064:domain_destroy_cb: Domain 4:Destruction of 
> domain failed
> 
> We are spending 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.
> -- 
> Mario.