Re: [PATCH, RFC] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

[Kirill A. Shutemov]

On Thu, Oct 12, 2017 at 06:07:36PM -0500, Eric W. Biederman wrote:
> "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> writes:
> 
> > On Mon, Oct 09, 2017 at 09:54:53AM -0700, Dave Hansen wrote:
> >> On 10/09/2017 09:09 AM, Kirill A. Shutemov wrote:
> >> > Apart from trampoline itself we also need place to store top level page
> >> > table in lower memory as we don't have a way to load 64-bit value into
> >> > CR3 from 32-bit mode. We only really need 8-bytes there as we only use
> >> > the very first entry of the page table.
> >> 
> >> Oh, and this is why you have to move "lvl5_pgtable" out of the kernel image?
> >
> > Right. I initialize the new location of top level page table directly.
> 
> So just a quick note.  I have a fuzzy memory of people loading their
> kernels above 4G physical because they did not have any memory below
> 4G.
> 
> That might be a very specialized case if my memory is correct because
> cpu startup has to have a trampoline below 1MB.  So I don't know how
> that works.  But I do seem to remember someone mentioning it.
> 
> Is there really no way to switch to 5 level paging other than to drop to
> 32bit mode and disable paging?    The x86 architecture does some very
> bizarre things so I can believe it but that seems like a lot of work to
> get somewhere.

The spec[1] is pretty clear on this, see section 2.2.2:

	The processor allows software to modify CR4.LA57 only outside of
	IA-32e mode. In IA-32e mode, an attempt to modify CR4.LA57 using
	the MOV CR instruction causes a general-protection exception
	(#GP).

[1] https://software.intel.com/sites/default/files/managed/2b/80/5-level_paging_white_paper.pdf

-- 
 Kirill A. Shutemov