linux-kernel - [PATCH v2 36/36] x86/entry: Split and inline syscall_return

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Message-Id: <0f773a894ab15c589ac794c2d34ca6ba9b5335c9.1444091585.git.luto@kernel.org>
Date:	Mon,  5 Oct 2015 17:48:24 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org, linux-kernel@...r.kernel.org
Cc:	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 36/36] 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@...nel.org>
---
 arch/x86/entry/common.c | 50 ++++++++++++++++++++++++++++---------------------
 1 file changed, 29 insertions(+), 21 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index d6b8f4fd8274..3d64a739e607 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -284,15 +284,40 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 	user_enter();
 }
 
+#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)
+{
+	bool step;
+
+	audit_syscall_exit(regs);
+
+	if (cached_flags & _TIF_SYSCALL_TRACEPOINT)
+		trace_sys_exit(regs, regs->ax);
+
+	/*
+	 * If TIF_SYSCALL_EMU is set, we only get here because of
+	 * TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP).
+	 * We already reported this syscall instruction in
+	 * syscall_trace_enter().
+	 */
+	step = unlikely(
+		(cached_flags & (_TIF_SINGLESTEP | _TIF_SYSCALL_EMU))
+		== _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 void syscall_return_slowpath(struct pt_regs *regs)
+__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);
-	bool step;
 
 	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
 
@@ -304,25 +329,8 @@ __visible void syscall_return_slowpath(struct pt_regs *regs)
 	 * 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)
-			trace_sys_exit(regs, regs->ax);
-
-		/*
-		 * If TIF_SYSCALL_EMU is set, we only get here because of
-		 * TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP).
-		 * We already reported this syscall instruction in
-		 * syscall_trace_enter().
-		 */
-		step = unlikely(
-			(cached_flags & (_TIF_SINGLESTEP | _TIF_SYSCALL_EMU))
-			== _TIF_SINGLESTEP);
-		if (step || cached_flags & _TIF_SYSCALL_TRACE)
-			tracehook_report_syscall_exit(regs, step);
-	}
+	if (unlikely(cached_flags & SYSCALL_EXIT_WORK_FLAGS))
+		syscall_slow_exit_work(regs, cached_flags);
 
 #ifdef CONFIG_COMPAT
 	/*
-- 
2.4.3

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