Patch "x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings" has been added to the 4.4-stable tree

[<gregkh@linuxfoundation.org>]

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 4.4-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-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 236222d39347e0e486010f10c1493e83dbbdfba8 Mon Sep 17 00:00:00 2001
From: Paolo Abeni <pabeni@redhat.com>
Date: Thu, 29 Jun 2017 15:55:58 +0200
Subject: x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings

From: Paolo Abeni <pabeni@redhat.com>

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.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
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: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/4533a1d101fd460f80e21329a34928fad521c1d4.1498744345.git.pabeni@redhat.com
[ Clarified the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

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

--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -80,7 +80,7 @@ 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
@@ -101,7 +101,8 @@ 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
@@ -215,6 +216,8 @@ ENDPROC(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 pabeni@redhat.com are

queue-4.4/x86-uaccess-optimize-copy_user_enhanced_fast_string-for-short-strings.patch