Re: Xen DomU Bootloader Experiences

[Elliott Mitchell <ehem+xen@m5p.com>]

On Thu, Nov 13, 2025 at 10:23:20PM +0000, Teddy Astie wrote:
> Le 12/11/2025 à 22:16, Elliott Mitchell a écrit :
> > A few times there have been mentions of a need to choose between boot
> > methods for DomUs.  There is a need to decide on ones to recommend and
> > put effort into supportting.  I may not have tried that many nor done
> > particularly great amounts of experimentation, but I do have some
> > experience with multiple User Domain bootloaders.
> > 
> > PyGRUB
> > Xen's bootloader.  PyGRUB is quite functional within its limits.  In
> > particular it simulates the domain's environment in Domain 0.  This means
> > the security exposure is problematic.  Another big concern is that it
> > only does GRUB v1 syntax.  For a long while Debian had a package for
> > generating those files on a modern system, but that package was dropped.
> > 
> > Yet PyGRUB does avoid needing to use external tools to retrieve the
> > kernel.  If the kernel is updated inside the domain, this does get the
> > new kernel.  Further being architecture-independent this works on x86,
> > ARM*, RISC-V and PowerPC.
> > 
> > As it is the only GRUB-flavor loader available on ARM*, that is the only
> > place where I've used PyGRUB.
> > 
> 
> To be fair, I don't really like PyGRUB, it hasn't really worked well for 
> me (perhaps I'm trying too modern distros ?). It's still what's being 
> used for XAPI, which doesn't make things practical and even sometimes 
> brittle.
> 
> I'm more tending to the other alternatives.
> 
> > 
> > PvGRUB
> > I'm sure nearly everyone knows about PvGRUB.  By being a proper port of
> > GRUB to run directly on Xen, it overcomes PyGRUB's disadvantages.  The
> > one disadvantage is needing to get patches into an external project for
> > changes in Xen.
> > 
> > Two changes to Xen urgently need propogation to PvGRUB.  I'm unsure
> > whether PvGRUB unmaps its mapping of vcpu_info data.  The second is
> 
> I don't think it maps vcpu_info, otherwise, most (all ?) Linux kernels 
> will immediately break.

I thought multiply mapping the vcpu_info was deprecated, but still
allowed on x86.  Only on ARM{,64} was it actually disabled.  As such it
might still have a private mapping, but it is possible it never had one
or was cleaned up at some point.

I keep mentioning this as Tianocore/EDK2 does a private mapping, but
fails to remove it when bootloading finishes.  Since support for multiply
mapped vcpu_info was removed, Tianocore/EDK2 got broken by Xen.  This is
still unfixed.

> > needing to work on ARM*, RISC-V and PowerPC.  The latter is the one and
> > only way in which PvGRUB is inferior to PyGRUB.
> 
> Perhaps it could be a good idea; although I'm more tending to EDK2 
> instead which works well with it and cover more cases (unless EDK2 stops 
> working for some obscure reason). For a more long-term solution, I would 
> prefer something different to GRUB but basing on "Boot Loader 
> Specification", maybe Edera's Sprout [1] or rust-hypervisor-firmware [2] 
> as a PVH firmware.

I concur with Tianocore/EDK2 covering more cases.  I do suspect keeping
it functional may be more difficult so I'm slightly wary of it.  Whereas
PvGRUB is lower overhead and faster for *Linux*.


> > EDK2/Tianocore
> > Quite well-known for being the basis of most x86 firmwares, plus being
> > part of a typical Qemu setup.  Not nearly as well known for being a Xen
> > DomU bootloader.
> > 
> > When it was working you would build their ArmVirtXen.dsc file and get
> > XEN_EFI.fd as output.  You would then use XEN_EFI.fd for the domain's
> > kernel.  If you looked at the console you saw something which looked and
> > acted pretty similar to a UEFI firmware on x86 machines.  This was
> > extremely functional for OSes which didn't particularly like GRUB.
> > Notably I've read of it being able to load a Redmond OS and it was quite
> > functional for booting an ARM64 port of FreeBSD.
> > 
> > Sometime after November 16th, 2022 or commit fff6d81270.  The built
> > images stopped functioning.  This is actually rather concerning since it
> > may also effects firmwares built for x86 HVM domains.  I don't presently
> > know whether there are multiple bugs, or a single one effecting all Xen
> > builds.
> 
> Actually, you can use it for x86 like you do for ARM, with PVH (using 
> the same binary as for HVM). But it broke at some point, and I'm mostly 
> living with a older but working custom built version of it.
> 
> We definitely want to sort-out this, as it is going to be a blocker for 
> providing better foundations for PVH guests on XAPI/XCP-ng.

Well, Tianocore/EDK2's Xen support was broken sometime mid-late 2022.
The only working firmware images I have are 5 years old and fail to load
as PVH.  Being used as DomU kernel may have worked on x86 at some point,
but I've never seen it.

Meanwhile Tianocore/EDK2 ARM was broken on *Xen*'s side.  Usually that
would mean the Xen developers are supposed to get updates into
Tianocore/EDK2.


> > Recommendations:
> > PyGRUB is functional within its limits.  Problems are GRUBv1 syntax and
> > running within Domain 0.  Given this I feel the Xen Project should be
> > heading towards deprecating PyGRUB.  Since PvGRUB works for x86 now, I
> > would default to neither building nor installing PyGRUB on x86.  For
> > other architectures PyGRUB is still useful.
> 
> +1. At least making it deprecated, and at the same time improving the 
> ergonomics of alternatives (i.e PVH+EDK2, legacy grub-pv and/or "PV-GRUB").

Trying to used legacy GRUB syntax is hopeless.  Most distribution scripts
will have been updated to be GRUBv2 syntax-only.  As such PV GRUB is
pretty clearly the way to go.


> > The ability to configure XEN_EFI.fd as a domain kernel is a feature
> > highly worthy of being ported to x86.  For OSes which don't particularly
> > like GRUB, but do have PV drivers this is an ideal boot method.
> > 
> 
> AFAICT it is already there :)

I've never seen it working.  It may have worked at one time, but it has
remained broken for rather a long time.

Perhaps time to add CI on Xen's side?


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