[PATCH 1/3] arm64: Update copy_from_user()

From: Robin Murphy <robin.murphy@arm.com>
To: will@kernel.org, catalin.marinas@arm.com
Cc: linux-arm-kernel@lists.infradead.org, mark.rutland@arm.com,
	kristina.martsenko@arm.com
Subject: [PATCH 1/3] arm64: Update copy_from_user()
Date: Wed, 28 Sep 2022 12:58:51 +0100	[thread overview]
Message-ID: <e8f95274572627c5b3f56df4c9fa7e66f1afb5c0.1664363162.git.robin.murphy@arm.com> (raw)
In-Reply-To: <cover.1664363162.git.robin.murphy@arm.com>

Replace the old mangled-beyond-hope copy_from_user() routine with a
shiny new one based on our newest memcpy() implementation. When plumbed
into the standalone memcpy benchmark from the Arm Optimized Routines
library, this routine outperforms the old one by up to ~1.7x for small
copies, with comparable gains across a range of microarchitectures,
levelling off once sizes get up to ~2KB and general load/store
throughput starts to dominate.

Some of this is paid for by pushing more complexity into the fixup
handlers, but much of that could be recovered again with cleverer
exception records that can give more information about the original
access to the handler itself. For now though, the label sleds are at
least entertaining.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
---
 arch/arm64/lib/copy_from_user.S | 274 ++++++++++++++++++++++++++------
 1 file changed, 223 insertions(+), 51 deletions(-)

diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
index 34e317907524..a4b9bd73a5a8 100644
--- a/arch/arm64/lib/copy_from_user.S
+++ b/arch/arm64/lib/copy_from_user.S
@@ -1,73 +1,245 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 /*
- * Copyright (C) 2012 ARM Ltd.
+ * Copyright (c) 2012-2022, Arm Limited.
  */
 
 #include <linux/linkage.h>
 
 #include <asm/asm-uaccess.h>
 #include <asm/assembler.h>
-#include <asm/cache.h>
 
-/*
- * Copy from user space to a kernel buffer (alignment handled by the hardware)
+/* Assumptions:
+ *
+ * ARMv8-a, AArch64, unaligned accesses.
  *
- * Parameters:
- *	x0 - to
- *	x1 - from
- *	x2 - n
- * Returns:
- *	x0 - bytes not copied
  */
 
-	.macro ldrb1 reg, ptr, val
-	user_ldst 9998f, ldtrb, \reg, \ptr, \val
-	.endm
+#define L(label) .L ## label
 
-	.macro strb1 reg, ptr, val
-	strb \reg, [\ptr], \val
-	.endm
+#define dstin	x0
+#define src	x1
+#define count	x2
+#define dst	x3
+#define srcend	x4
+#define dstend	x5
+#define A_l	x6
+#define A_lw	w6
+#define A_h	x7
+#define B_l	x8
+#define B_lw	w8
+#define B_h	x9
+#define C_l	x10
+#define C_lw	w10
+#define C_h	x11
+#define D_l	x12
+#define D_h	x13
+#define E_l	x14
+#define E_h	x15
+#define F_l	x16
+#define F_h	x17
+#define tmp1	x14
 
-	.macro ldrh1 reg, ptr, val
-	user_ldst 9997f, ldtrh, \reg, \ptr, \val
-	.endm
+/*
+ * Derived from memcpy with various adjustments:
+ *
+ * - memmove parts are removed since user and kernel pointers won't overlap.
+ * - The main loop is scaled down to 48 bytes per iteration since the increase
+ *   in load ops changes the balance; little cores barely notice the difference,
+ *   so big cores can benefit from keeping the loop relatively short.
+ * - Similarly, preferring source rather than destination alignment works out
+ *   better on average.
+ * - The 33-128 byte cases are reworked to better balance the stores with the
+ *   doubled-up load ops, and keep a more consistent access pattern.
+ * - The 0-3 byte sequence is replaced with the one borrowed from clear_user,
+ *   since LDTRB lacks a register-offset addressing mode.
+ */
 
-	.macro strh1 reg, ptr, val
-	strh \reg, [\ptr], \val
-	.endm
+#define U_pre(x...)	USER(L(fixup_pre), x)
+#define U_dst(x...)	USER(L(fixup_dst), x)
+#define U_S1(x...)	USER(L(fixup_s1), x)
+#define U_M16(x...)	USER(L(fixup_m16), x)
+#define U_M32(x...)	USER(L(fixup_m32), x)
+#define U_M64(x...)	USER(L(fixup_m64), x)
+#define U_L32(x...)	USER(L(fixup_l32), x)
+#define U_L48(x...)	USER(L(fixup_l48), x)
+#define U_L64(x...)	USER(L(fixup_l64), x)
 
-	.macro ldr1 reg, ptr, val
-	user_ldst 9997f, ldtr, \reg, \ptr, \val
-	.endm
-
-	.macro str1 reg, ptr, val
-	str \reg, [\ptr], \val
-	.endm
-
-	.macro ldp1 reg1, reg2, ptr, val
-	user_ldp 9997f, \reg1, \reg2, \ptr, \val
-	.endm
-
-	.macro stp1 reg1, reg2, ptr, val
-	stp \reg1, \reg2, [\ptr], \val
-	.endm
-
-end	.req	x5
-srcin	.req	x15
 SYM_FUNC_START(__arch_copy_from_user)
-	add	end, x0, x2
-	mov	srcin, x1
-#include "copy_template.S"
-	mov	x0, #0				// Nothing to copy
+	add	srcend, src, count
+	add	dstend, dstin, count
+	cmp	count, 128
+	b.hi	L(copy_long)
+	cmp	count, 32
+	b.hi	L(copy32_128)
+
+	/* Small copies: 0..32 bytes.  */
+	cmp	count, 16
+	b.lo	L(copy16)
+U_pre(	ldtr	A_l, [src])
+U_pre(	ldtr	A_h, [src, 8])
+U_pre(	ldtr	D_l, [srcend, -16])
+U_pre(	ldtr	D_h, [srcend, -8])
+	stp	A_l, A_h, [dstin]
+	stp	D_l, D_h, [dstend, -16]
+	mov	x0, #0
 	ret
 
-	// Exception fixups
-9997:	cmp	dst, dstin
-	b.ne	9998f
-	// Before being absolutely sure we couldn't copy anything, try harder
-USER(9998f, ldtrb tmp1w, [srcin])
-	strb	tmp1w, [dst], #1
-9998:	sub	x0, end, dst			// bytes not copied
+	/* Copy 8-15 bytes.  */
+L(copy16):
+	tbz	count, 3, L(copy8)
+U_pre(	ldtr	A_l, [src])
+U_pre(	ldtr	A_h, [srcend, -8])
+	str	A_l, [dstin]
+	str	A_h, [dstend, -8]
+	mov	x0, #0
 	ret
+
+	.p2align 3
+	/* Copy 4-7 bytes.  */
+L(copy8):
+	tbz	count, 2, L(copy4)
+U_pre(	ldtr	A_lw, [src])
+U_pre(	ldtr	B_lw, [srcend, -4])
+	str	A_lw, [dstin]
+	str	B_lw, [dstend, -4]
+	mov	x0, #0
+	ret
+
+	/* Copy 0..3 bytes.  */
+L(copy4):
+	tbz	count, #1, L(copy1)
+U_pre(	ldtrh	A_lw, [src])
+	strh	A_lw, [dstin]
+L(copy1):
+	tbz	count, #0, L(copy0)
+U_S1(	ldtrb	A_lw, [srcend, -1])
+	strb	A_lw, [dstend, -1]
+L(copy0):
+	mov	x0, #0
+	ret
+
+	.p2align 4
+	/* Medium copies: 33..128 bytes.  */
+L(copy32_128):
+U_pre(	ldtr	A_l, [src])
+U_pre(	ldtr	A_h, [src, 8])
+U_pre(	ldtr	B_l, [src, 16])
+U_pre(	ldtr	B_h, [src, 24])
+	stp	A_l, A_h, [dstin]
+	stp	B_l, B_h, [dstin, 16]
+U_M32(	ldtr	C_l, [srcend, -32])
+U_M32(	ldtr	C_h, [srcend, -24])
+U_M32(	ldtr	D_l, [srcend, -16])
+U_M32(	ldtr	D_h, [srcend, -8])
+	cmp	count, 64
+	b.ls	L(copy64)
+U_M32(	ldtr	E_l, [src, 32])
+U_M32(	ldtr	E_h, [src, 40])
+U_M32(	ldtr	F_l, [src, 48])
+U_M32(	ldtr	F_h, [src, 56])
+	stp	E_l, E_h, [dstin, 32]
+	stp	F_l, F_h, [dstin, 48]
+U_M64(	ldtr	A_l, [srcend, -64])
+U_M64(	ldtr	A_h, [srcend, -56])
+U_M64(	ldtr	B_l, [srcend, -48])
+U_M64(	ldtr	B_h, [srcend, -40])
+	stp	A_l, A_h, [dstend, -64]
+	stp	B_l, B_h, [dstend, -48]
+L(copy64):
+	stp	C_l, C_h, [dstend, -32]
+	stp	D_l, D_h, [dstend, -16]
+	mov	x0, #0
+	ret
+
+	.p2align 4
+	/* Copy more than 128 bytes.  */
+L(copy_long):
+	/* Copy 16 bytes and then align src to 16-byte alignment.  */
+
+U_pre(	ldtr	D_l, [src])
+U_pre(	ldtr	D_h, [src, 8])
+	and	tmp1, src, 15
+	bic	src, src, 15
+	sub	dst, dstin, tmp1
+	add	count, count, tmp1	/* Count is now 16 too large.  */
+U_pre(	ldtr	A_l, [src, 16])
+U_pre(	ldtr	A_h, [src, 24])
+	stp	D_l, D_h, [dstin]
+U_M16(	ldtr	B_l, [src, 32])
+U_M16(	ldtr	B_h, [src, 40])
+U_M16(	ldtr	C_l, [src, 48])
+U_M16(	ldtr	C_h, [src, 56])
+	add	src, src, #48
+	subs	count, count, 96 + 16	/* Test and readjust count.  */
+	b.ls	L(copy48_from_end)
+
+L(loop48):
+	stp	A_l, A_h, [dst, 16]
+U_L32(	ldtr	A_l, [src, 16])
+U_L32(	ldtr	A_h, [src, 24])
+	stp	B_l, B_h, [dst, 32]
+U_L48(	ldtr	B_l, [src, 32])
+U_L48(	ldtr	B_h, [src, 40])
+	stp	C_l, C_h, [dst, 48]!
+U_dst(	ldtr	C_l, [src, 48])
+U_dst(	ldtr	C_h, [src, 56])
+	add	src, src, #48
+	subs	count, count, 48
+	b.hi	L(loop48)
+
+	/* Write the last iteration and copy 48 bytes from the end.  */
+L(copy48_from_end):
+	stp	A_l, A_h, [dst, 16]
+U_L32(	ldtr	A_l, [srcend, -48])
+U_L32(	ldtr	A_h, [srcend, -40])
+	stp	B_l, B_h, [dst, 32]
+U_L48(	ldtr	B_l, [srcend, -32])
+U_L48(	ldtr	B_h, [srcend, -24])
+	stp	C_l, C_h, [dst, 48]
+U_L64(	ldtr	C_l, [srcend, -16])
+U_L64(	ldtr	C_h, [srcend, -8])
+	stp	A_l, A_h, [dstend, -48]
+	stp	B_l, B_h, [dstend, -32]
+	stp	C_l, C_h, [dstend, -16]
+	mov	x0, #0
+	ret
+
+	/* Fixups... */
+
+	/*
+	 * Fault before anything has been written, but progress may have
+	 * been possible; realign dst and retry a single byte to confirm.
+	 */
+L(fixup_pre):
+	mov	dst, dstin
+U_dst(	ldtrb	A_lw, [src])
+	strb	A_lw, [dst], #1
+L(fixup_dst):
+	sub	x0, dstend, dst
+	ret
+
+	/* Small: Fault with 1 byte remaining, regardless of count */
+L(fixup_s1):
+	mov	x0, #1
+	ret
+
+	/* Medium: Faults after n bytes beyond dstin have been written */
+L(fixup_m64):
+	add	dstin, dstin, #32
+L(fixup_m32):
+	add	dstin, dstin, #16
+L(fixup_m16):
+	add	dst, dstin, #16
+	b	L(fixup_dst)
+
+	/* Large: Faults after n bytes beyond dst have been written */
+L(fixup_l64):
+	add	dst, dst, #16
+L(fixup_l48):
+	add	dst, dst, #16
+L(fixup_l32):
+	add	dst, dst, #32
+	b	L(fixup_dst)
+
 SYM_FUNC_END(__arch_copy_from_user)
 EXPORT_SYMBOL(__arch_copy_from_user)
-- 
2.36.1.dirty


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