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

[Andy Lutomirski]

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 <l...@kernel.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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