Re: [PATCH v2 20/22] x86/slaunch: support EFI boot

[Sergii Dmytruk <sergii.dmytruk@3mdeb.com>]

On Tue, May 13, 2025 at 09:25:44PM -0400, Demi Marie Obenour wrote:
> On 5/13/25 1:05 PM, Sergii Dmytruk wrote:
> > When running on an EFI-enabled system, Xen needs to have access to Boot
> > Services in order to initialize itself properly and reach a state in
> > which a dom0 kernel can operate without issues.
> >
> > This means that DRTM must be started in the middle of Xen's
> > initialization process.  This effect is achieved via a callback into
> > bootloader (GRUB) which is responsible for initiating DRTM and
> > continuing Xen's initialization process.  The latter is done by
> > branching in Slaunch entry point on a flag to switch back into long mode
> > before calling the same function which Xen would execute as the next
> > step without DRTM.
>
> Depending on the bootloader for this unnecessarily ties DRTM to GRUB.
> Instead, it would be much better for Xen to be able to perform DRTM
> itself, which would allow DRTM to work without GRUB.  Pop! OS already
> uses systemd-boot and the trend seems to be from GRUB to systemd-boot.
> Furthermore, this would allow DRTM with Xen launched directly from
> the UEFI firmware.
> --
> Sincerely,
> Demi Marie Obenour (she/her/hers)

That sentence in the commit message is worth rewording.  GRUB isn't a
requirement, any TrenchBoot-enabled bootloader (or anything that wants
to act as a bootloader) can be used.  systemd-boot could implement
Secure Launch specification [0] and start Xen/Linux/something else via
DRTM.  Usage without a real bootloader could be implemented similarly
via some EFI stub that has binaries embedded into it or that can load
them from a drive.

Mind that at least Intel and AMD DRTM implementations require a DCE [1]
binary that depends on a vendor, firmware version or a CPU generation.
So even embedding all code into every kernel-like software won't produce
self-contained DRTM-capable images.

[0]: https://trenchboot.org/specifications/Secure_Launch/
[1]: https://trenchboot.org/theory/Glossary/#dynamic-configuration-environment-dce

Regards