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Re: [Xen-devel] Request to revert superpage adjustments

On Fri, Mar 11, 2016 at 09:35:19AM -0500, Konrad Rzeszutek Wilk wrote:
> > >>  Nor would
> > >> that deal with avoiding reserved regions not too far above 1Mb,
> > >> since at best the main payload can be relocatable (but certainly
> > >> not the binary seen by the boot loader, as at least multiboot1
> > >> doesn't support anything like that).
> > >
> > > The only reason Xen sits at the 1MB boundary is because of its ELF header.
> > >
> > > A plain binary with a multiboot header has no such restriction, although
> > > we flag an interested to have 4k alignment using the multiboot flags.
> > > There is no technical limitation causing Xen to be linked to run at 1MB;
> > > its just the way its alway been.  There is nothing preventing the entry
> > > point becoming properly relocatable.
> And one can make it be at 2MB - which is what Daniel's patches did.

IIRC, with my patches it could be anything which is multiple of 2 MiB.

> (CC-ing Daniel in case I got it wrong). We also needed GRUB2 patches
> to take into account relocating Xen at a different location.

Konrad is correct.

By the way, I would like to mention that probably next week all required
multiboot2 functionality will be available in upstream git tree and later
in GRUB2 2.02 release. This means that new multiboot2 features are almost
set in stone.

> > Without proper container format, how would the boot loader
> > know where to place the binary, or how to relocate it if it doesn't
> > get placed at its linked address? The only alternatives I see in
> > grub1 are a.out and a kludge of a.out, both of which - at the first
> > glance - also don't appear to do any relocation. And for the Linux
> > variant, as said, it doesn't look like it's compatible with us needing
> > multiple modules.

multiboot and multiboot2 (without my extensions) put image exactly at address
specified in ELF or multiboot2 header. This means that boot loader is not able
to relocate image during load. I do not mention that current version of Xen
(without my EFI/multiboot2 patches) is not able to understand relocation
(if it would be provided/executed by loader) too.


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