x86_64: display more intuitive error message if kernel is not 2MB aligned

o x86_64 kernel needs to be compiled for 2MB aligned addresses. Currently
  we are using BUILD_BUG_ON() to warn the user if he has not done so. But
  looks like folks are not finding message very intutive and don't open
  the respective c file to find problem source. (Bug 8439)

arch/x86_64/kernel/head64.c: In function 'x86_64_start_kernel':
arch/x86_64/kernel/head64.c:70: error: size of array 'type name' is negative

o Using preprocessor directive #error to print a better message if
  CONFIG_PHYSICAL_START is not aligned to 2MB boundary.
Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
Cc: Andi Kleen <ak@suse.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86_64/kernel/head64.c

@@ -62,13 +62,6 @@ void __init x86_64_start_kernel(char * real_mode_data)
 {
 	int i;
 
-	/*
-	 * Make sure kernel is aligned to 2MB address. Catching it at compile
-	 * time is better. Change your config file and compile the kernel
-	 * for a 2MB aligned address (CONFIG_PHYSICAL_START)
-	 */
-	BUILD_BUG_ON(CONFIG_PHYSICAL_START & (__KERNEL_ALIGN - 1));
-
 	/* clear bss before set_intr_gate with early_idt_handler */
 	clear_bss();
 

include/asm-x86_64/page.h

@@ -80,6 +80,15 @@ extern unsigned long phys_base;
 #define __PHYSICAL_START	CONFIG_PHYSICAL_START
 #define __KERNEL_ALIGN		0x200000
 
+/*
+ * Make sure kernel is aligned to 2MB address. Catching it at compile
+ * time is better. Change your config file and compile the kernel
+ * for a 2MB aligned address (CONFIG_PHYSICAL_START)
+ */
+#if (CONFIG_PHYSICAL_START % __KERNEL_ALIGN) != 0
+#error "CONFIG_PHYSICAL_START must be a multiple of 2MB"
+#endif
+
 #define __START_KERNEL		(__START_KERNEL_map + __PHYSICAL_START)
 #define __START_KERNEL_map	_AC(0xffffffff80000000, UL)
 #define __PAGE_OFFSET           _AC(0xffff810000000000, UL)