Patch "x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-uaccess-optimize-copy_user_enhanced_fast_string-for-short-strings.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paolo Abeni <[email protected]>
Date: Thu, 29 Jun 2017 15:55:58 +0200
Subject: x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings

From: Paolo Abeni <[email protected]>

commit 236222d39347e0e486010f10c1493e83dbbdfba8 upstream.

According to the Intel datasheet, the REP MOVSB instruction exposes a pretty heavy setup cost (50 ticks), which hurts short string copy operations.

This change tries to avoid this cost by calling the explicit loop available in the unrolled code for strings shorter than 64 bytes.

The 64 bytes cutoff value is arbitrary from the code logic point of view - it has been selected based on measurements, as the largest value that still ensures a measurable gain.

Micro benchmarks of the __copy_from_user() function with lengths in the [0-63] range show this performance gain (shorter the string, larger the gain):

- in the [55%-4%] range on Intel Xeon(R) CPU E5-2690 v4
- in the [72%-9%] range on Intel Core i7-4810MQ

Other tested CPUs - namely Intel Atom S1260 and AMD Opteron 8216 - show no difference, because they do not expose the ERMS feature bit.

---
 arch/x86/lib/copy_user_64.S |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

@@ -84,7 +84,7 @@ ENTRY(copy_user_generic_unrolled)
 	movl %edx,%ecx
 	andl $63,%edx
 	shrl $6,%ecx
-	jz 17f
+	jz .L_copy_short_string
 1:	movq (%rsi),%r8
 2:	movq 1*8(%rsi),%r9
 3:	movq 2*8(%rsi),%r10

@@ -105,7 +105,8 @@ ENTRY(copy_user_generic_unrolled)
 	leaq 64(%rdi),%rdi
 	decl %ecx
 	jnz 1b
-17:	movl %edx,%ecx
+.L_copy_short_string:
+	movl %edx,%ecx
 	andl $7,%edx
 	shrl $3,%ecx
 	jz 20f

@@ -221,6 +222,8 @@ EXPORT_SYMBOL(copy_user_generic_string)
  */
 ENTRY(copy_user_enhanced_fast_string)
 	ASM_STAC
+	cmpl $64,%edx
+	jb .L_copy_short_string	/* less then 64 bytes, avoid the costly 'rep' */
 	movl %edx,%ecx
 1:	rep
 	movsb

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/x86-uaccess-optimize-copy_user_enhanced_fast_string-for-short-strings.patch