Make it possible to restore a hibernation image on x86_64 with the help of a
kernel different from the one in the image.

The idea is to split the core restoration code into two separate parts and to
place each of them in a different page.   The first part belongs to the boot
kernel and is executed as the last step of the image kernel's memory
restoration procedure.   Before being executed, it is relocated to a safe page
that won't be overwritten while copying the image kernel pages.

The final operation performed by it is a jump to the second part of the core
restoration code that belongs to the image kernel and has just been restored.
This code makes the CPU switch to the image kernel's page tables and restores
the state of general purpose registers (including the stack pointer) from
before the hibernation.

The main issue with this idea is that in order to jump to the second part of
the core restoration code the boot kernel needs to know its address.
 However, this address may be passed to it in the image header.   Namely, the
part of the image header previously used for checking if the version of the
image kernel is correct can be replaced with some architecture specific data
that will allow the boot kernel to jump to the right address within the image
kernel.   These data should also be used for checking if the image kernel is
compatible with the boot kernel (as far as the memory restroration procedure
is concerned).  It can be done, for example, with the help of a "magic" value
that has to be equal in both kernels, so that they can be regarded as
compatible.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>