This patch is the meat of the PDA change.  This patch makes several related
changes:

1: Most significantly, %gs is now used in the kernel.  This means that on
   entry, the old value of %gs is saved away, and it is reloaded with
   __KERNEL_PDA.

2: entry.S constructs the stack in the shape of struct pt_regs, and this
   is passed around the kernel so that the process's saved register
   state can be accessed.

   Unfortunately struct pt_regs doesn't currently have space for %gs
   (or %fs). This patch extends pt_regs to add space for gs (no space
   is allocated for %fs, since it won't be used, and it would just
   complicate the code in entry.S to work around the space).

3: Because %gs is now saved on the stack like %ds, %es and the integer
   registers, there are a number of places where it no longer needs to
   be handled specially; namely context switch, and saving/restoring the
   register state in a signal context.

4: And since kernel threads run in kernel space and call normal kernel
   code, they need to be created with their %gs == __KERNEL_PDA.
Signed-off-by: NJeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: NAndi Kleen <ak@suse.de>
Cc: Chuck Ebbert <76306.1226@compuserve.com>
Cc: Zachary Amsden <zach@vmware.com>
Cc: Jan Beulich <jbeulich@novell.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>

This patch removes the default_ldt[] array, as it has been unused since
iBCS stopped being supported.  This means it is now possible to actually
set an empty LDT segment.

In order to deal with this, the set_ldt_desc/load_LDT pair has been
replaced with a single set_ldt() operation which is responsible for both
setting up the LDT descriptor in the GDT, and reloading the LDT register.
If there are no LDT entries, the LDT register is loaded with a NULL
descriptor.
Signed-off-by: NJeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: NAndi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>
Acked-by: NZachary Amsden <zach@vmware.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>

Signed-off-by: NDavid Woodhouse <dwmw2@infradead.org>

Initial git repository build. I'm not bothering with the full history,
even though we have it. We can create a separate "historical" git
archive of that later if we want to, and in the meantime it's about
3.2GB when imported into git - space that would just make the early
git days unnecessarily complicated, when we don't have a lot of good
infrastructure for it.

Let it rip!