[PATCH] x86/cpufeatures: Add support for fast short rep mov

[Tony Luck]

>From the Intel Optimization Reference Manual:

3.7.6.1 Fast Short REP MOVSB
Beginning with processors based on Ice Lake Client microarchitecture,
REP MOVSB performance of short operations is enhanced. The enhancement
applies to string lengths between 1 and 128 bytes long.  Support for
fast-short REP MOVSB is enumerated by the CPUID feature flag: CPUID
[EAX=7H, ECX=0H).EDX.FAST_SHORT_REP_MOVSB[bit 4] = 1. There is no change
in the REP STOS performance.

Add an X86_FEATURE_FSRM flag for this.

memmove() avoids REP MOVSB for short (< 32 byte) copies. Fix it
to check FSRM and use REP MOVSB for short copies on systems that
support it.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>

---

Time (cycles) for memmove() sizes 1..31 with neither source nor
destination in cache.

  1800 +-+-------+--------+---------+---------+---------+--------+-------+-+
       +         +        +         +         +         +        +         +
  1600 +-+                                          'memmove-fsrm' *******-+
       |   ######                                   'memmove-orig' ####### |
  1400 +-+ #     #####################                                   +-+
       |   #                          ############                         |
  1200 +-+#                                       ##################     +-+
       |  #                                                                |
  1000 +-+#                                                              +-+
       |  #                                                                |
       | #                                                                 |
   800 +-#                                                               +-+
       | #                                                                 |
   600 +-***********************                                         +-+
       |                        *****************************              |
   400 +-+                                                   *******     +-+
       |                                                                   |
   200 +-+                                                               +-+
       +         +        +         +         +         +        +         +
     0 +-+-------+--------+---------+---------+---------+--------+-------+-+
       0         5        10        15        20        25       30        35
---
 arch/x86/include/asm/cpufeatures.h | 1 +
 arch/x86/lib/memmove_64.S          | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index e9b62498fe75..98c60fa31ced 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -357,6 +357,7 @@
 /* Intel-defined CPU features, CPUID level 0x00000007:0 (EDX), word 18 */
 #define X86_FEATURE_AVX512_4VNNIW	(18*32+ 2) /* AVX-512 Neural Network Instructions */
 #define X86_FEATURE_AVX512_4FMAPS	(18*32+ 3) /* AVX-512 Multiply Accumulation Single precision */
+#define X86_FEATURE_FSRM		(18*32+ 4) /* Fast Short Rep Mov */
 #define X86_FEATURE_AVX512_VP2INTERSECT (18*32+ 8) /* AVX-512 Intersect for D/Q */
 #define X86_FEATURE_MD_CLEAR		(18*32+10) /* VERW clears CPU buffers */
 #define X86_FEATURE_TSX_FORCE_ABORT	(18*32+13) /* "" TSX_FORCE_ABORT */
diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
index 337830d7a59c..4a23086806e6 100644
--- a/arch/x86/lib/memmove_64.S
+++ b/arch/x86/lib/memmove_64.S
@@ -29,10 +29,7 @@
 SYM_FUNC_START_ALIAS(memmove)
 SYM_FUNC_START(__memmove)
 
-	/* Handle more 32 bytes in loop */
 	mov %rdi, %rax
-	cmp $0x20, %rdx
-	jb	1f
 
 	/* Decide forward/backward copy mode */
 	cmp %rdi, %rsi
@@ -43,6 +40,7 @@ SYM_FUNC_START(__memmove)
 	jg 2f
 
 .Lmemmove_begin_forward:
+	ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM
 	ALTERNATIVE "", "movq %rdx, %rcx; rep movsb; retq", X86_FEATURE_ERMS
 
 	/*
@@ -114,6 +112,8 @@ SYM_FUNC_START(__memmove)
 	 */
 	.p2align 4
 2:
+	cmp $0x20, %rdx
+	jb	1f
 	cmp $680, %rdx
 	jb 6f
 	cmp %dil, %sil
-- 
2.20.1