Re: Xen.efi "must be loaded below 4Gb"

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Jan 02, 2025 at 04:26:28PM +0000, Andrew Cooper wrote:
> 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.  

What about not touching anything MB1-related in the EFI boot path? MB1
can't possibly work on EFI, right?

> 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.

This obviously would be better

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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