x86, espfix: Make espfix64 a Kconfig option, fix UML

Make espfix64 a hidden Kconfig option.  This fixes the x86-64 UML
build which had broken due to the non-existence of init_espfix_bsp()
in UML: since UML uses its own Kconfig, this option does not appear in
the UML build.

This also makes it possible to make support for 16-bit segments a
configuration option, for the people who want to minimize the size of
the kernel.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Richard Weinberger <richard@nod.at>
Link: http://lkml.kernel.org/r/1398816946-3351-1-git-send-email-hpa@linux.intel.com

arch/x86/Kconfig

@@ -914,6 +914,10 @@ config VM86
 	  XFree86 to initialize some video cards via BIOS. Disabling this
 	  option saves about 6k.
 
+config X86_ESPFIX64
+	def_bool y
+	depends on X86_64
+
 config TOSHIBA
 	tristate "Toshiba Laptop support"
 	depends on X86_32

arch/x86/kernel/Makefile

@@ -29,7 +29,7 @@ obj-$(CONFIG_X86_64)	+= sys_x86_64.o x8664_ksyms_64.o
 obj-y			+= syscall_$(BITS).o vsyscall_gtod.o
 obj-$(CONFIG_X86_64)	+= vsyscall_64.o
 obj-$(CONFIG_X86_64)	+= vsyscall_emu_64.o
-obj-$(CONFIG_X86_64)	+= espfix_64.o
+obj-$(CONFIG_X86_ESPFIX64)	+= espfix_64.o
 obj-$(CONFIG_SYSFS)	+= ksysfs.o
 obj-y			+= bootflag.o e820.o
 obj-y			+= pci-dma.o quirks.o topology.o kdebugfs.o

arch/x86/kernel/smpboot.c

@@ -246,7 +246,7 @@ static void notrace start_secondary(void *unused)
 	/*
 	 * Enable the espfix hack for this CPU
 	 */
-#ifdef CONFIG_X86_64
+#ifdef CONFIG_X86_ESPFIX64
 	init_espfix_ap();
 #endif
 

init/main.c

@@ -617,7 +617,7 @@ asmlinkage void __init start_kernel(void)
 	if (efi_enabled(EFI_RUNTIME_SERVICES))
 		efi_enter_virtual_mode();
 #endif
-#ifdef CONFIG_X86_64
+#ifdef CONFIG_X86_ESPFIX64
 	/* Should be run before the first non-init thread is created */
 	init_espfix_bsp();
 #endif