efi/x86: Add true mixed mode entry point into .compat section

Currently, mixed mode is closely tied to the EFI handover protocol
and relies on intimate knowledge of the bootparams structure, setup
header etc, all of which are rather byzantine and entirely specific
to x86.

Even though no other EFI supported architectures are currently known
that could support something like mixed mode, it still makes sense to
abstract a bit from this, and make it part of a generic Linux on EFI
boot protocol.

To that end, add a .compat section to the mixed mode binary, and populate
it with the PE machine type and entry point address, allowing firmware
implementations to match it to their native machine type, and invoke
non-native binaries using a secondary entry point.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/boot/header.S

@@ -106,7 +106,7 @@ coff_header:
 #else
 	.word	0x8664				# x86-64
 #endif
-	.word	3				# nr_sections
+	.word	section_count			# nr_sections
 	.long	0 				# TimeDateStamp
 	.long	0				# PointerToSymbolTable
 	.long	1				# NumberOfSymbols
@@ -230,6 +230,23 @@ section_table:
 	.word	0				# NumberOfLineNumbers
 	.long	0x42100040			# Characteristics (section flags)
 
+#ifdef CONFIG_EFI_MIXED
+	#
+	# The offset & size fields are filled in by build.c.
+	#
+	.asciz	".compat"
+	.long	0
+	.long	0x0
+	.long	0				# Size of initialized data
+						# on disk
+	.long	0x0
+	.long	0				# PointerToRelocations
+	.long	0				# PointerToLineNumbers
+	.word	0				# NumberOfRelocations
+	.word	0				# NumberOfLineNumbers
+	.long	0x42100040			# Characteristics (section flags)
+#endif
+
 	#
 	# The offset & size fields are filled in by build.c.
 	#
@@ -248,6 +265,7 @@ section_table:
 	.word	0				# NumberOfLineNumbers
 	.long	0x60500020			# Characteristics (section flags)
 
+	.set	section_count, (. - section_table) / 40
 #endif /* CONFIG_EFI_STUB */
 
 	# Kernel attributes; used by setup.  This is part 1 of the