Commit c68ca678 authored Oct 05, 2015 by Andy Lutomirski Committed by Ingo Molnar Oct 09, 2015

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@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/bdff034e2f23c5eb974c760cf494cb5bddce8f29.1444091585.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 33c52129

arch/x86/entry/common.c

+14 −2

Original line number	Diff line number	Diff line
		@@ -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;