x86/asm/entry, perf: Explicitly optimize vm86 handling in code_segment_base()

There's no point in checking the VM bit on 64-bit, and, since
we're explicitly checking it, we can use user_mode_ignore_vm86()
after the check.

While we're at it, rearrange the #ifdef slightly to make the code
flow a bit clearer.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/dc1457a734feccd03a19bb3538a7648582f57cdd.1426728647.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/perf_event.c

@@ -2146,6 +2146,12 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
  */
 static unsigned long code_segment_base(struct pt_regs *regs)
 {
+	/*
+	 * For IA32 we look at the GDT/LDT segment base to convert the
+	 * effective IP to a linear address.
+	 */
+
+#ifdef CONFIG_X86_32
 	/*
 	 * If we are in VM86 mode, add the segment offset to convert to a
 	 * linear address.
@@ -2153,12 +2159,7 @@ static unsigned long code_segment_base(struct pt_regs *regs)
 	if (regs->flags & X86_VM_MASK)
 		return 0x10 * regs->cs;
 
-	/*
-	 * For IA32 we look at the GDT/LDT segment base to convert the
-	 * effective IP to a linear address.
-	 */
-#ifdef CONFIG_X86_32
-	if (user_mode(regs) && regs->cs != __USER_CS)
+	if (user_mode_ignore_vm86(regs) && regs->cs != __USER_CS)
 		return get_segment_base(regs->cs);
 #else
 	if (user_mode(regs) && !user_64bit_mode(regs) &&