From a3cf06e691b827b393a858346a922de59bb318f1 Mon Sep 17 00:00:00 2001
From: Dave Martin <Dave.Martin@arm.com>
Date: Mon, 14 May 2018 18:51:09 +0100
Subject: [PATCH] arm64/sve: Write ZCR_EL1 on context switch only if changed

Git-commit: 159fd7b8d3d12b27593d4fe3f6ae1d8e14ea9d0b
Patch-mainline: v4.18-rc1
References: fate#325866

Writes to ZCR_EL1 are self-synchronising, and so may be expensive
in typical implementations.

This patch adopts the approach used for costly system register
writes elsewhere in the kernel: the system register write is
suppressed if it would not change the stored value.

Since the common case will be that of switching between tasks that
use the same vector length as one another, prediction hit rates on
the conditional branch should be reasonably good, with lower
expected amortised cost than the unconditional execution of a
heavyweight self-synchronising instruction.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@suse.com>
---
 arch/arm64/include/asm/fpsimdmacros.h | 12 +++++++-----
 arch/arm64/kernel/entry-fpsimd.S      |  2 +-
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimdmacros.h b/arch/arm64/include/asm/fpsimdmacros.h
index e050d765ca9e..46843515d77b 100644
--- a/arch/arm64/include/asm/fpsimdmacros.h
+++ b/arch/arm64/include/asm/fpsimdmacros.h
@@ -207,12 +207,14 @@
 		str		w\nxtmp, [\xpfpsr, #4]
 .endm
 
-.macro sve_load nxbase, xpfpsr, xvqminus1, nxtmp
+.macro sve_load nxbase, xpfpsr, xvqminus1, nxtmp, xtmp2
 		mrs_s		x\nxtmp, SYS_ZCR_EL1
-		bic		x\nxtmp, x\nxtmp, ZCR_ELx_LEN_MASK
-		orr		x\nxtmp, x\nxtmp, \xvqminus1
-		msr_s		SYS_ZCR_EL1, x\nxtmp	// self-synchronising
-
+		bic		\xtmp2, x\nxtmp, ZCR_ELx_LEN_MASK
+		orr		\xtmp2, \xtmp2, \xvqminus1
+		cmp		\xtmp2, x\nxtmp
+		b.eq		921f
+		msr_s		SYS_ZCR_EL1, \xtmp2	// self-synchronising
+921:
  _for n, 0, 31,	_sve_ldr_v	\n, \nxbase, \n - 34
 		_sve_ldr_p	0, \nxbase
 		_sve_wrffr	0
diff --git a/arch/arm64/kernel/entry-fpsimd.S b/arch/arm64/kernel/entry-fpsimd.S
index 73f17bffcd23..12d4958e6429 100644
--- a/arch/arm64/kernel/entry-fpsimd.S
+++ b/arch/arm64/kernel/entry-fpsimd.S
@@ -49,7 +49,7 @@ ENTRY(sve_save_state)
 ENDPROC(sve_save_state)
 
 ENTRY(sve_load_state)
-	sve_load 0, x1, x2, 3
+	sve_load 0, x1, x2, 3, x4
 	ret
 ENDPROC(sve_load_state)
 
-- 
2.11.0