x86-64: don't set the early IDT to point directly to 'early_idt_handler'

The code requires the use of the proper per-exception-vector stub
functions (set up as the early_idt_handlers[] array - note the 's') that
make sure to set up the error vector number.  This is true regardless of
whether CONFIG_EARLY_PRINTK is set or not.

Why? The stack offset for the comparison of __KERNEL_CS won't be right
otherwise, nor will the new check (from commit 8170e6be: "x86,
64bit: Use a #PF handler to materialize early mappings on demand") for
the page fault exception vector.
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/include/asm/proto.h

@@ -5,8 +5,6 @@
 
 /* misc architecture specific prototypes */
 
-void early_idt_handler(void);
-
 void system_call(void);
 void syscall_init(void);
 

arch/x86/kernel/head64.c

@@ -159,13 +159,8 @@ void __init x86_64_start_kernel(char * real_mode_data)
 	/* clear bss before set_intr_gate with early_idt_handler */
 	clear_bss();
 
-	for (i = 0; i < NUM_EXCEPTION_VECTORS; i++) {
-#ifdef CONFIG_EARLY_PRINTK
+	for (i = 0; i < NUM_EXCEPTION_VECTORS; i++)
 		set_intr_gate(i, &early_idt_handlers[i]);
-#else
-		set_intr_gate(i, early_idt_handler);
-#endif
-	}
 	load_idt((const struct desc_ptr *)&idt_descr);
 
 	copy_bootdata(__va(real_mode_data));

arch/x86/kernel/head_64.S

@@ -336,6 +336,7 @@ early_idt_handlers:
 	i = i + 1
 	.endr
 
+/* This is global to keep gas from relaxing the jumps */
 ENTRY(early_idt_handler)
 	cld
 
@@ -404,6 +405,7 @@ ENTRY(early_idt_handler)
 	addq $16,%rsp		# drop vector number and error code
 	decl early_recursion_flag(%rip)
 	INTERRUPT_RETURN
+ENDPROC(early_idt_handler)
 
 	__INITDATA