linux-kernel - [PATCH v1 1/3] x86/entry: Test ti_work for zero before processing individual bits

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Message-ID: <20240807054722.682375-2-xin@zytor.com>
Date: Tue,  6 Aug 2024 22:47:20 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        peterz@...radead.org, andrew.cooper3@...rix.com, seanjc@...gle.com
Subject: [PATCH v1 1/3] x86/entry: Test ti_work for zero before processing individual bits

In most cases, ti_work values passed to arch_exit_to_user_mode_prepare()
are zeros, e.g., 99% in kernel build tests.  So an obvious optimization
is to test ti_work for zero before processing individual bits in it.

In addition, Intel 0day tests find no perf regression with this change.

Suggested-by: H. Peter Anvin (Intel) <hpa@...or.com>
Signed-off-by: Xin Li (Intel) <xin@...or.com>
---
 arch/x86/include/asm/entry-common.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index fb2809b20b0a..4c78b99060b5 100644
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -47,15 +47,17 @@ static __always_inline void arch_enter_from_user_mode(struct pt_regs *regs)
 static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 						  unsigned long ti_work)
 {
-	if (ti_work & _TIF_USER_RETURN_NOTIFY)
-		fire_user_return_notifiers();
+	if (unlikely(ti_work)) {
+		if (ti_work & _TIF_USER_RETURN_NOTIFY)
+			fire_user_return_notifiers();
 
-	if (unlikely(ti_work & _TIF_IO_BITMAP))
-		tss_update_io_bitmap();
+		if (unlikely(ti_work & _TIF_IO_BITMAP))
+			tss_update_io_bitmap();
 
-	fpregs_assert_state_consistent();
-	if (unlikely(ti_work & _TIF_NEED_FPU_LOAD))
-		switch_fpu_return();
+		fpregs_assert_state_consistent();
+		if (unlikely(ti_work & _TIF_NEED_FPU_LOAD))
+			switch_fpu_return();
+	}
 
 #ifdef CONFIG_COMPAT
 	/*
-- 
2.45.2