linux-kernel - [RFC 30/30] x86/entry: Micro-optimize compat fast syscall arg fetch

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Message-Id: <561a519cdb34f43c16670ebf3c3f0e6db8d53c89.1441146105.git.luto@kernel.org>
Date:	Tue,  1 Sep 2015 15:41:30 -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: [RFC 30/30] x86/entry: Micro-optimize compat fast syscall arg fetch

we're following a 32-bit pointer, and the uaccess code isn't smart
enough to figure out that the access_ok check isn't needed.

This saves about three cycles on a cache-hot fast syscall.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/common.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index a3c4e7aa573b..ba469579c41a 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -390,8 +390,20 @@ __visible long do_fast_syscall_32(struct pt_regs *regs)
 	 * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
 	 */
 	local_irq_enable();
-	if (get_user(*(u32 *)&regs->cx,
-		     (u32 __user __force *)(unsigned long)(u32)regs->sp)) {
+	if (
+#ifdef CONFIG_X86_64
+		/*
+		 * Micro-optimization: the pointer we're following is explicitly
+		 * 32 bits, so it can't be out of range.
+		 */
+		__get_user(*(u32 *)&regs->cx,
+			    (u32 __user __force *)(unsigned long)(u32)regs->sp)
+#else
+		get_user(*(u32 *)&regs->cx,
+			 (u32 __user __force *)(unsigned long)(u32)regs->sp)
+#endif
+		) {
+
 		/* User code screwed up. */
 		local_irq_disable();
 		regs->ax = -EFAULT;
-- 
2.4.3

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