linux-kernel - [PATCH] x86/cpufeatures: Add feature flag for fast short rep movsb

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Date:   Thu, 12 Dec 2019 13:49:08 -0800
From:   Tony Luck <tony.luck@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Tony Luck <tony.luck@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] x86/cpufeatures: Add feature flag for fast short rep movsb

>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.

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

Net effect of this patch is just to make "fsrm" appear in the
flags section of /proc/cpuinfo. Maybe someone can look into whether
we should make copy routines that use "rep movsb" check for this
flag to optimize copies on older CPUs that don't have it?

 arch/x86/include/asm/cpufeatures.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 0652d3eed9bd..ab441b15d582 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -356,6 +356,7 @@
 /* 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 */
-- 
2.20.1