[PATCH v2 32/36] x86/entry: Micro-optimize compat fast syscall arg fetch

From: Andy Lutomirski
Date: Mon Oct 05 2015 - 20:51:19 EST


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>
---
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 6e1ea6a401f2..3ae6191ad1a2 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;
--
2.4.3

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