linux-kernel - [PATCH 4/5] riscv: __asm_to/copy_from

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Message-ID: <767aa7e1-0cd0-1e60-3081-77b4c07d14ff@gmail.com>
Date:   Sat, 19 Jun 2021 20:37:46 +0900
From:   Akira Tsukamoto <akira.tsukamoto@...il.com>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: [PATCH 4/5] riscv: __asm_to/copy_from_user: Bulk copy while shifting


The destination address is aligned now, but often time the source address
is not in an aligned boundary.

To reduce the unaligned memory access, it reads the data from source in
aligned boundaries, which will cause the data to have an offset, and then
combines the data in the next iteration by fixing offset with shifting
before writing to destination.

The majority of the improving copy speed comes from this shift copy.

Signed-off-by: Akira Tsukamoto <akira.tsukamoto@...il.com>
---
 arch/riscv/lib/uaccess.S | 60 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
index 4906b5ca91c3..e2e57551fc76 100644
--- a/arch/riscv/lib/uaccess.S
+++ b/arch/riscv/lib/uaccess.S
@@ -56,10 +56,70 @@ ENTRY(__asm_copy_from_user)
 	bltu	a0, t1, 1b	/* t1 - start of aligned dst */
 
 .Lskip_first_bytes:
+	/*
+	 * Now dst is aligned.
+	 * Use shift-copy if src is misaligned.
+	 * Use word-copy if both src and dst are aligned because
+	 * can not use shift-copy which do not require shifting
+	 */
+	/* a1 - start of src */
+	andi	a3, a1, SZREG-1
+	bnez	a3, .Lshift_copy
 
 .Lword_copy:
 .Lshift_copy:
 
+	/*
+	 * Word copy with shifting.
+	 * For misaligned copy we still perform aligned word copy, but
+	 * we need to use the value fetched from the previous iteration and
+	 * do some shifts.
+	 * This is safe because reading less than a word size.
+	 *
+	 * a0 - start of aligned dst
+	 * a1 - start of src
+	 * a3 - a1 & mask:(SZREG-1)
+	 * t0 - end of uncopied dst
+	 * t1 - end of aligned dst
+	 */
+	/* calculating aligned word boundary for dst */
+	andi	t1, t0, ~(SZREG-1)
+	/* Converting unaligned src to aligned arc */
+	andi	a1, a1, ~(SZREG-1)
+
+	/*
+	 * Calculate shifts
+	 * t3 - prev shift
+	 * t4 - current shift
+	 */
+	slli	t3, a3, LGREG
+	li	a5, SZREG*8
+	sub	t4, a5, t3
+
+	/* Load the first word to combine with seceond word */
+	fixup REG_L   a5, 0(a1), 10f
+
+3:
+	/* Main shifting copy
+	 *
+	 * a0 - start of aligned dst
+	 * a1 - start of aligned src
+	 * t1 - end of aligned dst
+	 */
+
+	/* At least one iteration will be executed */
+	srl	a4, a5, t3
+	fixup REG_L   a5, SZREG(a1), 10f
+	addi	a1, a1, SZREG
+	sll	a2, a5, t4
+	or	a2, a2, a4
+	fixup REG_S   a2, 0(a0), 10f
+	addi	a0, a0, SZREG
+	bltu	a0, t1, 3b
+
+	/* Revert src to original unaligned value  */
+	add	a1, a1, a3
+
 .Lbyte_copy_tail:
 	/*
 	 * Byte copy anything left.
-- 
2.17.1