x86/entry: Split and inline syscall_return_slowpath()

GCC is unable to properly optimize functions that have a very
short likely case and a longer and register-heavier cold part --
it fails to sink all of the register saving and stack frame
setup code into the unlikely part.

Help it out with syscall_return_slowpath() by splitting it into
two parts and inline the hot part.

Saves 6 cycles for compat syscalls.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/0f773a894ab15c589ac794c2d34ca6ba9b5335c9.1444091585.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/common.c

@@ -284,28 +284,14 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 	user_enter();
 }
 
-/*
- * Called with IRQs on and fully valid regs.  Returns with IRQs off in a
- * state such that we can immediately switch to user mode.
- */
-__visible void syscall_return_slowpath(struct pt_regs *regs)
+#define SYSCALL_EXIT_WORK_FLAGS				\
+	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |	\
+	 _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT)
+
+static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
 {
-	struct thread_info *ti = pt_regs_to_thread_info(regs);
-	u32 cached_flags = READ_ONCE(ti->flags);
 	bool step;
 
-	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
-
-	if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
-	    WARN(irqs_disabled(), "syscall %ld left IRQs disabled", regs->orig_ax))
-		local_irq_enable();
-
-	/*
-	 * First do one-time work.  If these work items are enabled, we
-	 * want to run them exactly once per syscall exit with IRQs on.
-	 */
-	if (cached_flags & (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |
-			    _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT)) {
 	audit_syscall_exit(regs);
 
 	if (cached_flags & _TIF_SYSCALL_TRACEPOINT)
@@ -322,7 +308,29 @@ __visible void syscall_return_slowpath(struct pt_regs *regs)
 		== _TIF_SINGLESTEP);
 	if (step || cached_flags & _TIF_SYSCALL_TRACE)
 		tracehook_report_syscall_exit(regs, step);
-	}
+}
+
+/*
+ * Called with IRQs on and fully valid regs.  Returns with IRQs off in a
+ * state such that we can immediately switch to user mode.
+ */
+__visible inline void syscall_return_slowpath(struct pt_regs *regs)
+{
+	struct thread_info *ti = pt_regs_to_thread_info(regs);
+	u32 cached_flags = READ_ONCE(ti->flags);
+
+	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
+
+	if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
+	    WARN(irqs_disabled(), "syscall %ld left IRQs disabled", regs->orig_ax))
+		local_irq_enable();
+
+	/*
+	 * First do one-time work.  If these work items are enabled, we
+	 * want to run them exactly once per syscall exit with IRQs on.
+	 */
+	if (unlikely(cached_flags & SYSCALL_EXIT_WORK_FLAGS))
+		syscall_slow_exit_work(regs, cached_flags);
 
 #ifdef CONFIG_COMPAT
 	/*