Xen.efi "must be loaded below 4Gb"

[Andrew Cooper <andrew.cooper3@citrix.com>]

Hello,

Forwarding a bug report from IRC from before Christmas:

---
bit of a random question, but: has anyone had any luck booting efi
builds of Xen? over the last year or so I've tried 4.18 and 4.19, Alpine
and Fedora builds, and on a Dell PowerEdge R430 and an Optiplex 7010
Plus, and in every case received an error that "Xen must be loaded below
4Gb"
---

The Xen.efi path does expect to be loaded below 4G, and does give up
rather than relocating itself.

Right now, I'm aware of at least one blocker to xen.efi being able to
relocate itself, and that is because it populates the MB1 metadata with
physical pointers into the ebmalloc[] region, which is in .bss.  Fallout
related to this was the subject of c/s 0fe607b2a1 "x86/boot: Fix PVH
boot during boot_info transition period" and a protective ASSERT() included.

The ProperFix(tm) is to remove ebmalloc(), and the scratch space in the
trampoline, and instead have a range in initdata to stash the bootloader
metadata, and use virtual pointers rather than physical.  This also
avoids us double/triple handling the bootloader metadata, simplifying
all aspects of the startup logic.

~Andrew