x86/entry: Split and inline prepare_exit_to_usermode()

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 prepare_exit_to_usermode() by splitting it into
two parts and inline the hot part.

Saves 6-8 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/9fc53eda4a5b924070952f12fa4ae3e477640a07.1444091585.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 28 insertions, 15 deletions

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index d0874210d5b5..66ccbd664d4c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -218,14 +218,12 @@ long syscall_trace_enter(struct pt_regs *regs)
                 return syscall_trace_enter_phase2(regs, arch, phase1_result);
 }
 
-/* Called with IRQs disabled. */
-__visible void prepare_exit_to_usermode(struct pt_regs *regs)
-{
-        if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
-                local_irq_disable();
-
-        lockdep_sys_exit();
+#define EXIT_TO_USERMODE_LOOP_FLAGS                             \
+        (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |   \
+         _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY)
 
+static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
+{
         /*
          * In order to return to user mode, we need to have IRQs off with
          * none of _TIF_SIGPENDING, _TIF_NOTIFY_RESUME, _TIF_USER_RETURN_NOTIFY,
@@ -235,14 +233,6 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
          * work to clear some of the flags can sleep.
          */
         while (true) {
-                u32 cached_flags =
-                        READ_ONCE(pt_regs_to_thread_info(regs)->flags);
-
-                if (!(cached_flags & (_TIF_SIGPENDING | _TIF_NOTIFY_RESUME |
-                                      _TIF_UPROBE | _TIF_NEED_RESCHED |
-                                      _TIF_USER_RETURN_NOTIFY)))
-                        break;
-
                 /* We have work to do. */
                 local_irq_enable();
 
@@ -266,7 +256,30 @@ __visible void prepare_exit_to_usermode(struct pt_regs *regs)
 
                 /* Disable IRQs and retry */
                 local_irq_disable();
+
+                cached_flags = READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+                if (!(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+                        break;
+
         }
+}
+
+/* Called with IRQs disabled. */
+__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
+{
+        u32 cached_flags;
+
+        if (IS_ENABLED(CONFIG_PROVE_LOCKING) && WARN_ON(!irqs_disabled()))
+                local_irq_disable();
+
+        lockdep_sys_exit();
+
+        cached_flags =
+                READ_ONCE(pt_regs_to_thread_info(regs)->flags);
+
+        if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
+                exit_to_usermode_loop(regs, cached_flags);
 
         user_enter();
 }