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List: freebsd-hackers
Subject: Re: Should we boot the FreeBSD kernel in ELF format or in zImage format ? How?
From: Mario Marietto <marietto2008 () gmail ! com>
Date: 2023-11-25 22:20:33
Message-ID: CA+1FSijGW9Mxp6N5H2q+jPJmozEtDmtvqr_LQf1SHvQnO33LaQ () mail ! gmail ! com
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I really don't know where to start. It sounds too technical for me. I don't
even know what to ask to understand where to start. mmm...can someone
provide some documentation ? but not too much dense with technicalities.
Thanks.
On Sat, Nov 25, 2023 at 6:02 PM Warner Losh <imp@bsdimp.com> wrote:
>
>
> 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
>
--
Mario.
[Attachment #3 (text/html)]
<div dir="ltr">I really don't know where to start. It sounds too technical for \
me. I don't even know what to ask to understand where to start. mmm...can someone \
provide some documentation ? but not too much dense with technicalities. Thanks. \
<br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Nov \
25, 2023 at 6:02 PM Warner Losh <<a \
href="mailto:imp@bsdimp.com">imp@bsdimp.com</a>> wrote:<br></div><blockquote \
class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid \
rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div dir="ltr"><br></div><br><div \
class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Nov 25, 2023 at \
4:41 AM Mario Marietto <<a href="mailto:marietto2008@gmail.com" \
target="_blank">marietto2008@gmail.com</a>> wrote:<br></div><blockquote \
class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid \
rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div class="gmail_quote"><div \
dir="ltr">Hello to everyone.<div><div><p>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.</p>
<p>We found these slides :</p>
<p><a href="https://events.static.linuxfound.org/sites/events/files/slides/Porting%20FreeBSD%20on%20Xen%20on%20ARM%20.pdf" \
rel="nofollow noreferrer" \
target="_blank">https://events.static.linuxfound.org/sites/events/files/slides/Porting%20FreeBSD%20on%20Xen%20on%20ARM%20.pdf</a></p>
<p>Slide 13 refers to a XENHVM FreeBSD on arm kernel config - that is what we want \
to find.</p> <p>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.</p>
<p>Here is where I would expect to find the XENHVM FreeBSD on arm kernel config \
file:</p> <p><a href="https://cgit.freebsd.org/src/tree/sys/arm/conf" rel="nofollow \
noreferrer" target="_blank">https://cgit.freebsd.org/src/tree/sys/arm/conf</a></p> \
<p>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.</p>
<p>According to this page from the FreeBSD wiki:</p>
<p><a href="https://wiki.freebsd.org/Xen" rel="nofollow noreferrer" \
target="_blank">https://wiki.freebsd.org/Xen</a></p> <p>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 !</p> <p>I found a slightly newer slide presentation by Julien \
here:</p> <p><a href="https://www.slideshare.net/xen_com_mgr/bsdcan-2015-how-to-port-your-bsd" \
rel="nofollow noreferrer" \
target="_blank">https://www.slideshare.net/xen_com_mgr/bsdcan-2015-how-to-port-your-bsd</a></p>
<p>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.</p>
<p>Please,take a look at this output of the linux kernel that can boot on Xen, and \
the FreeBSD kernel that cannot :</p><p><br></p> <pre><code>% 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 \
</code></pre> <p><br></p><p>The FreeBSD kernel that won't boot is in ELF format \
but the Linux kernel that does boot is in zImage format.</p> <p>I spent time reading \
the docs on <a href="http://xenbits.xenproject.org" \
target="_blank">xenbits.xenproject.org</a>, 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.</p>
<p>I also watched Julien Grall's 30 minute video presentation of his work to boot \
FreeBSD/arm on Xen at FOSDEM 2014 here :</p> <p><a \
href="https://archive.fosdem.org/2014/schedule/event/freebsd_xen_arm/" rel="nofollow \
noreferrer" target="_blank">https://archive.fosdem.org/2014/schedule/event/freebsd_xen_arm/</a></p>
<p>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.</p>
<p>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</p></div></div></div></div></div></blockquote><div><br></div><div>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.<br></div><div><br></div><div>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.<br></div><div><br></div><div>Warner <br></div></div></div> </blockquote></div><br \
clear="all"><br><span class="gmail_signature_prefix">-- </span><br><div dir="ltr" \
class="gmail_signature">Mario.<br></div>
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