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Message-ID: <60c1f087-1e8b-8f22-7d25-86f5f3dcee3f@gmail.com>
Date: Wed, 23 Jun 2021 21:40:39 +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-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/1] riscv: __asm_copy_to-from_user: Optimize unaligned
memory access and pipeline stall
This patch will reduce cpu usage dramatically in kernel space especially
for application which use sys-call with large buffer size, such as network
applications. The main reason behind this is that every unaligned memory
access will raise exceptions and switch between s-mode and m-mode causing
large overhead.
First copy in bytes until reaches the first word aligned boundary in
destination memory address. This is the preparation before the bulk
aligned word copy.
The destination address is aligned now, but oftentimes 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.
In the lucky situation that the both source and destination address are on
the aligned boundary, perform load and store with register size to copy the
data. Without the unrolling, it will reduce the speed since the next store
instruction for the same register using from the load will stall the
pipeline.
At last, copying the remainder in one byte at a time.
Signed-off-by: Akira Tsukamoto <akira.tsukamoto@...il.com>
---
arch/riscv/lib/uaccess.S | 181 +++++++++++++++++++++++++++++++--------
1 file changed, 146 insertions(+), 35 deletions(-)
diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
index fceaeb18cc64..bceb0629e440 100644
--- a/arch/riscv/lib/uaccess.S
+++ b/arch/riscv/lib/uaccess.S
@@ -19,50 +19,161 @@ ENTRY(__asm_copy_from_user)
li t6, SR_SUM
csrs CSR_STATUS, t6
- add a3, a1, a2
- /* Use word-oriented copy only if low-order bits match */
- andi t0, a0, SZREG-1
- andi t1, a1, SZREG-1
- bne t0, t1, 2f
+ /* Save for return value */
+ mv t5, a2
- addi t0, a1, SZREG-1
- andi t1, a3, ~(SZREG-1)
- andi t0, t0, ~(SZREG-1)
/*
- * a3: terminal address of source region
- * t0: lowest XLEN-aligned address in source
- * t1: highest XLEN-aligned address in source
+ * Register allocation for code below:
+ * a0 - start of uncopied dst
+ * a1 - start of uncopied src
+ * a2 - size
+ * t0 - end of uncopied dst
*/
- bgeu t0, t1, 2f
- bltu a1, t0, 4f
+ add t0, a0, a2
+ bgtu a0, t0, 5f
+
+ /*
+ * Use byte copy only if too small.
+ */
+ li a3, 8*SZREG /* size must be larger than size in word_copy */
+ bltu a2, a3, .Lbyte_copy_tail
+
+ /*
+ * Copy first bytes until dst is align to word boundary.
+ * a0 - start of dst
+ * t1 - start of aligned dst
+ */
+ addi t1, a0, SZREG-1
+ andi t1, t1, ~(SZREG-1)
+ /* dst is already aligned, skip */
+ beq a0, t1, .Lskip_first_bytes
1:
- fixup REG_L, t2, (a1), 10f
- fixup REG_S, t2, (a0), 10f
- addi a1, a1, SZREG
- addi a0, a0, SZREG
- bltu a1, t1, 1b
+ /* a5 - one byte for copying data */
+ fixup lb a5, 0(a1), 10f
+ addi a1, a1, 1 /* src */
+ fixup sb a5, 0(a0), 10f
+ addi a0, a0, 1 /* dst */
+ 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:
+ /*
+ * Both src and dst are aligned, unrolled word copy
+ *
+ * a0 - start of aligned dst
+ * a1 - start of aligned src
+ * a3 - a1 & mask:(SZREG-1)
+ * t0 - end of aligned dst
+ */
+ addi t0, t0, -(8*SZREG-1) /* not to over run */
2:
- bltu a1, a3, 5f
+ fixup REG_L a4, 0(a1), 10f
+ fixup REG_L a5, SZREG(a1), 10f
+ fixup REG_L a6, 2*SZREG(a1), 10f
+ fixup REG_L a7, 3*SZREG(a1), 10f
+ fixup REG_L t1, 4*SZREG(a1), 10f
+ fixup REG_L t2, 5*SZREG(a1), 10f
+ fixup REG_L t3, 6*SZREG(a1), 10f
+ fixup REG_L t4, 7*SZREG(a1), 10f
+ fixup REG_S a4, 0(a0), 10f
+ fixup REG_S a5, SZREG(a0), 10f
+ fixup REG_S a6, 2*SZREG(a0), 10f
+ fixup REG_S a7, 3*SZREG(a0), 10f
+ fixup REG_S t1, 4*SZREG(a0), 10f
+ fixup REG_S t2, 5*SZREG(a0), 10f
+ fixup REG_S t3, 6*SZREG(a0), 10f
+ fixup REG_S t4, 7*SZREG(a0), 10f
+ addi a0, a0, 8*SZREG
+ addi a1, a1, 8*SZREG
+ bltu a0, t0, 2b
+
+ addi t0, t0, 8*SZREG-1 /* revert to original value */
+ j .Lbyte_copy_tail
+
+.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.
+ *
+ * a0 - start of remaining dst
+ * a1 - start of remaining src
+ * t0 - end of remaining dst
+ */
+ bgeu a0, t0, 5f
+4:
+ fixup lb a5, 0(a1), 10f
+ addi a1, a1, 1 /* src */
+ fixup sb a5, 0(a0), 10f
+ addi a0, a0, 1 /* dst */
+ bltu a0, t0, 4b /* t0 - end of dst */
+
+5:
/* Disable access to user memory */
csrc CSR_STATUS, t6
- li a0, 0
+ li a0, 0
ret
-4: /* Edge case: unalignment */
- fixup lbu, t2, (a1), 10f
- fixup sb, t2, (a0), 10f
- addi a1, a1, 1
- addi a0, a0, 1
- bltu a1, t0, 4b
- j 1b
-5: /* Edge case: remainder */
- fixup lbu, t2, (a1), 10f
- fixup sb, t2, (a0), 10f
- addi a1, a1, 1
- addi a0, a0, 1
- bltu a1, a3, 5b
- j 3b
ENDPROC(__asm_copy_to_user)
ENDPROC(__asm_copy_from_user)
EXPORT_SYMBOL(__asm_copy_to_user)
@@ -117,7 +228,7 @@ EXPORT_SYMBOL(__clear_user)
10:
/* Disable access to user memory */
csrs CSR_STATUS, t6
- mv a0, a2
+ mv a0, t5
ret
11:
csrs CSR_STATUS, t6
--
2.17.1
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