[ARM] lower overhead with alternative copy_to_user for small copies

Because the alternate copy_to_user implementation has a higher setup cost
than the standard implementation, the size of the memory area to copy
is tested and the standard implementation invoked instead when that size
is too small.  Still, that test is made after the processor has preserved
a bunch of registers on the stack which have to be reloaded right away
needlessly in that case, causing a measurable performance regression
compared to plain usage of the standard implementation only.

To make the size test overhead negligible, let's factorize it out of
the alternate copy_to_user function where it is clear to the compiler
that no stack frame is needed.  Thanks to CONFIG_ARM_UNWIND allowing
for frame pointers to be disabled and tail call optimization to kick in,
the overhead in the small copy case becomes only 3 assembly instructions.

A similar trick is applied to clear_user as well.
Signed-off-by: Nicolas Pitre <nico@marvell.com>

arch/arm/lib/uaccess_with_memcpy.c

@@ -49,14 +49,11 @@ pin_page_for_write(const void __user *_addr, pte_t **ptep, spinlock_t **ptlp)
 	return 1;
 }
 
-unsigned long
-__copy_to_user(void __user *to, const void *from, unsigned long n)
+static unsigned long noinline
+__copy_to_user_memcpy(void __user *to, const void *from, unsigned long n)
 {
 	int atomic;
 
-	if (n < 1024)
-		return __copy_to_user_std(to, from, n);
-
 	if (unlikely(segment_eq(get_fs(), KERNEL_DS))) {
 		memcpy((void *)to, from, n);
 		return 0;
@@ -99,11 +96,24 @@ out:
 	return n;
 }
 
-unsigned long __clear_user(void __user *addr, unsigned long n)
+unsigned long
+__copy_to_user(void __user *to, const void *from, unsigned long n)
+{
+	/*
+	 * This test is stubbed out of the main function above to keep
+	 * the overhead for small copies low by avoiding a large
+	 * register dump on the stack just to reload them right away.
+	 * With frame pointer disabled, tail call optimization kicks in
+	 * as well making this test almost invisible.
+	 */
+	if (n < 1024)
+		return __copy_to_user_std(to, from, n);
+	return __copy_to_user_memcpy(to, from, n);
+}
+	
+static unsigned long noinline
+__clear_user_memset(void __user *addr, unsigned long n)
 {
-	if (n < 256)
-		return __clear_user_std(addr, n);
-
 	if (unlikely(segment_eq(get_fs(), KERNEL_DS))) {
 		memset((void *)addr, 0, n);
 		return 0;
@@ -137,3 +147,11 @@ unsigned long __clear_user(void __user *addr, unsigned long n)
 out:
 	return n;
 }
+
+unsigned long __clear_user(void __user *addr, unsigned long n)
+{
+	/* See rational for this in __copy_to_user() above. */
+	if (n < 256)
+		return __clear_user_std(addr, n);
+	return __clear_user_memset(addr, n);
+}