[tip:x86/asm] x86/entry: Micro-optimize compat fast syscall arg fetch

[tip-bot for Andy Lutomirski <tipbot@xxxxxxxxx>]

Commit-ID:  c68ca6787bdd6d2df37cf950135aa11e71af358a
Gitweb:     http://git.kernel.org/tip/c68ca6787bdd6d2df37cf950135aa11e71af358a
Author:     Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 5 Oct 2015 17:48:20 -0700
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 9 Oct 2015 09:41:12 +0200

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@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/bdff034e2f23c5eb974c760cf494cb5bddce8f29.1444091585.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 d5eee85..08a945d 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -394,8 +394,20 @@ __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;
--
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