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[PATCH 4.11 33/36] x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings

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4.11-stable review patch.  If anyone has any objections, please let me know.

------------------

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
@@ -37,7 +37,7 @@ 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
@@ -58,7 +58,8 @@ 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
@@ -174,6 +175,8 @@ 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

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[PATCH 4.11 00/36] 4.11.10-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 08/36] usb: dwc3: replace %p with %pK Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 18/36] pinctrl: sh-pfc: r8a7795: Fix hscif2_clk_b and hscif4_ctrl Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 04/36] RDMA/uverbs: Check port number supplied by user verbs cmds Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 03/36] driver core: platform: fix race condition with driver_override Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 09/36] USB: serial: cp210x: add ID for CEL EM3588 USB ZigBee stick Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 28/36] xhci: Limit USB2 port wake support for AMD Promontory hosts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 15/36] pinctrl: sh-pfc: r8a7794: Swap ATA signals Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 14/36] arm: remove wrong CONFIG_PROC_SYSCTL ifdef Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 16/36] pinctrl: sh-pfc: r8a7791: Fix SCIF2 pinmux data Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  [PATCH 4.11 33/36] x86/uaccess: Optimize copy_user_enhanced_fast_string() for short strings Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-07-10 19:30 +0200
  Re: [PATCH 4.11 00/36] 4.11.10-stable review Guenter Roeck <linux@roeck-us.net> - 2017-07-11 02:50 +0200

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