Re: [Xen-devel] [PATCH v4 4/5] x86: add multiboot2 protocol support for EFI platforms

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Thu, Jan 19, 2017 at 02:09:37AM -0700, Jan Beulich wrote:
> >>> On 18.01.17 at 20:51, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 18/01/17 14:17, Doug Goldstein wrote:
> >> +.Lefi_multiboot2_proto:
> >> +        /* Zero EFI SystemTable and EFI ImageHandle addresses. */
> >> +        xor     %esi,%esi
> >> +        xor     %edi,%edi
> >> +
> >> +        /* Skip Multiboot2 information fixed part. */
> >> +        lea     (MB2_fixed_sizeof+MULTIBOOT2_TAG_ALIGN-1)(%rbx),%ecx
> >> +        and     $~(MULTIBOOT2_TAG_ALIGN-1),%ecx
> >
> > Are we strictly guaranteed to have the entire image, including multiboot
> > tags, loaded below 4GB virtual even in the 64bit case? Even on non-EFI
> > capable grub2's ?
>
> I can't speak for GrUB, but I've recently learned that EFI alone

GRUB is able to enforce 4 GiB restriction. At least right now.
However, I am afraid that sooner or later machines will appear
in the wild which do not have memory below 4 GiB (well, they must
have some below 1 MiB but everything above is not a must as I know)
or everything will be reserved/used by EFI boot/runtime services, etc.
Then GRUB will also fail and we will be forced to extend multiboot2
proto further. However, it looks that current solution should not
change much then.

> doesn't appear to make such guarantees, and also doesn't look
> to have ways to request such address restricted loading of an

Why? Due to lack of memory below 4 GiB, it being reserved for
something, EFI implementation bugs or anything else?

Daniel

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