x86, realmode: Set real_mode permissions early

Trampoline code is executed by APs with kernel low mapping on 64bit.
We need to set trampoline code to EXEC early before we boot APs.

Found the problem after switching to #PF handler set page table,
and we do not set initial kernel low mapping with EXEC anymore in
arch/x86/kernel/head_64.S.

Change to use early_initcall instead that will make sure trampoline
will have EXEC set.

-v2: Merge two comments according to Borislav Petkov <[e-mail]>

Signed-off-by: Yinghai Lu <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/realmode/init.c

···
 }
 
 /*
- * set_real_mode_permissions() gets called very early, to guarantee the
- * availability of low memory.  This is before the proper kernel page
+ * setup_real_mode() gets called very early, to guarantee the
+ * availability of low memory. This is before the proper kernel page
  * tables are set up, so we cannot set page permissions in that
- * function.  Thus, we use an arch_initcall instead.
+ * function. Also trampoline code will be executed by APs so we
+ * need to mark it executable at do_pre_smp_initcalls() at least,
+ * thus run it as a early_initcall().
  */
 static int __init set_real_mode_permissions(void)
 {
···
 
 	return 0;
 }
-
-arch_initcall(set_real_mode_permissions);
+early_initcall(set_real_mode_permissions);