x86, realmode: header for trampoline code

Added header for trampoline code that can be used to supply
input data to it. This makes interface between real mode code
and kernel cleaner and simpler. Replaced two confusing pointers
to level4 pgt in trampoline_64.S with a single pointer to the
beginning of the page table.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Link: http://lkml.kernel.org/r/1336501366-28617-21-git-send-email-jarkko.sakkinen@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/realmode/rm/trampoline_common.S

@@ -0,0 +1,23 @@
+	.section ".rodata","a"
+
+	.balign	4
+tr_idt: .fill 1, 6, 0
+
+	.bss
+
+	.balign	4
+GLOBAL(trampoline_status)	.space	4
+
+GLOBAL(trampoline_header)
+#ifdef CONFIG_X86_32
+	tr_start:		.space	4
+	tr_gdt:			.space	6
+#else
+	tr_start:		.space	8
+#endif
+END(trampoline_header)
+
+#ifdef CONFIG_X86_64
+	.balign	PAGE_SIZE
+GLOBAL(trampoline_pgd)		.space	PAGE_SIZE
+#endif