[ARM] 2947/1: copy template with new memcpy/memmove
Patch from Nicolas Pitre
This patch provides a new implementation for optimized memory copy
functions on ARM. It is made of two levels: a template that consists of
the core copy code and separate files that define macros to be used with
the core code depending on the type of copy needed. This allows for best
performances while sharing the same core for implementing memcpy(),
copy_from_user() and copy_to_user() for instance.
Two reasons for this work:
1) the current copy_to_user/copy_from_user implementation assumes no
task switch will ever occur in the middle of each copied page making
it completely unsafe with CONFIG_PREEMPT=y.
2) current copy implementations are measurably suboptimal and optimizing
different implementations separately is a pain and more opportunities
for bugs.
The reason for (1) is the fact that copy inside user pages are performed
with the ldm instruction which has no mean for testing user protections
and could possibly race with process preemption bypassing the COW mechanism
for example. This is a longstanding issue that we said ought to be fixed
for about two years now. The solution is to substitute those ldm insns
with a series of ldrt or strt insns to enforce user memory protection.
At least on StrongARM and XScale cores the ldm is not faster than the
equivalent ldr/str insns with a warm i-cache so there is no measurable
performance degradation with that change. The fact that the copy code is
a template makes it pretty easy to reuse the same core code as for memcpy
and benefit from the same performance optimizations.
Now (2) is best demonstrated with actual throughput measurements.
First, here is a summary of memcopy tests performed on a StrongARM core:
PTR alignment buffer size kernel version this version
------------------------------------------------------------
aligned 32 59.73 107.43
unaligned 32 61.31 74.72
aligned 100 132.47 136.15
unaligned 100 103.84 123.76
aligned 4096 130.67 130.80
unaligned 4096 130.68 130.64
aligned 1048576 68.03 68.18
unaligned 1048576 68.03 68.18
The buffer size is in bytes and the measured speed in MB/s. The copy
was performed repeatedly with given buffer and throughput averaged over
3 seconds.
Here we can see that the current kernel version has a higher entry cost
that shows up with small buffers. As buffer size grows both implementation
converge to the same throughput.
Now here's the exact same test performed on an XScale core (PXA255):
PTR alignment buffer size kernel version this version
------------------------------------------------------------
aligned 32 46.99 77.58
unaligned 32 53.61 59.59
aligned 100 107.19 136.59
unaligned 100 83.61 97.58
aligned 4096 129.13 129.98
unaligned 4096 128.36 128.53
aligned 1048576 53.76 59.41
unaligned 1048576 33.67 56.96
Again we can see the entry setup cost being higher for the current kernel
before getting to the main copy loop. Then throughput results converge
as long as the buffer remains in the cache. Then the 1MB case shows more
differences probably due to better pld placement and/or less instruction
interlocks in this proposed implementation.
Disclaimer: The PXA system was running with slower clocks than the
StrongARM system so trying to infer any conclusion by comparing those
separate sets of results side by side would be completely inappropriate.
So... What this patch does is to replace both memcpy and memmove with
an implementation based on the provided copy code template. The memmove
code is kept separate since it is used only if the memory areas involved
do overlap in which case the code is a transposition of the template but
with the copy occurring in the opposite direction (trying to fit that
mode into the template turned it into a mess not worth it for memmove
alone). And obviously both memcpy and memmove were tested with all kinds
of pointer alignments and buffer sizes to exercise all code paths for
correctness.
The next patch will provide the now trivial replacement implementation
copy_to_user and copy_from_user.
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/lib/copy_template.S b/arch/arm/lib/copy_template.S
new file mode 100644
index 0000000..838e435
--- /dev/null
+++ b/arch/arm/lib/copy_template.S
@@ -0,0 +1,255 @@
+/*
+ * linux/arch/arm/lib/copy_template.s
+ *
+ * Code template for optimized memory copy functions
+ *
+ * Author: Nicolas Pitre
+ * Created: Sep 28, 2005
+ * Copyright: MontaVista Software, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+/*
+ * This can be used to enable code to cacheline align the source pointer.
+ * Experiments on tested architectures (StrongARM and XScale) didn't show
+ * this a worthwhile thing to do. That might be different in the future.
+ */
+//#define CALGN(code...) code
+#define CALGN(code...)
+
+/*
+ * Theory of operation
+ * -------------------
+ *
+ * This file provides the core code for a forward memory copy used in
+ * the implementation of memcopy(), copy_to_user() and copy_from_user().
+ *
+ * The including file must define the following accessor macros
+ * according to the need of the given function:
+ *
+ * ldr1w ptr reg abort
+ *
+ * This loads one word from 'ptr', stores it in 'reg' and increments
+ * 'ptr' to the next word. The 'abort' argument is used for fixup tables.
+ *
+ * ldr4w ptr reg1 reg2 reg3 reg4 abort
+ * ldr8w ptr, reg1 reg2 reg3 reg4 reg5 reg6 reg7 reg8 abort
+ *
+ * This loads four or eight words starting from 'ptr', stores them
+ * in provided registers and increments 'ptr' past those words.
+ * The'abort' argument is used for fixup tables.
+ *
+ * ldr1b ptr reg cond abort
+ *
+ * Similar to ldr1w, but it loads a byte and increments 'ptr' one byte.
+ * It also must apply the condition code if provided, otherwise the
+ * "al" condition is assumed by default.
+ *
+ * str1w ptr reg abort
+ * str8w ptr reg1 reg2 reg3 reg4 reg5 reg6 reg7 reg8 abort
+ * str1b ptr reg cond abort
+ *
+ * Same as their ldr* counterparts, but data is stored to 'ptr' location
+ * rather than being loaded.
+ *
+ * enter reg1 reg2
+ *
+ * Preserve the provided registers on the stack plus any additional
+ * data as needed by the implementation including this code. Called
+ * upon code entry.
+ *
+ * exit reg1 reg2
+ *
+ * Restore registers with the values previously saved with the
+ * 'preserv' macro. Called upon code termination.
+ */
+
+
+ enter r4, lr
+
+ subs r2, r2, #4
+ blt 8f
+ ands ip, r0, #3
+ PLD( pld [r1, #0] )
+ bne 9f
+ ands ip, r1, #3
+ bne 10f
+
+1: subs r2, r2, #(28)
+ stmfd sp!, {r5 - r8}
+ blt 5f
+
+ CALGN( ands ip, r1, #31 )
+ CALGN( rsb r3, ip, #32 )
+ CALGN( sbcnes r4, r3, r2 ) @ C is always set here
+ CALGN( bcs 2f )
+ CALGN( adr r4, 6f )
+ CALGN( subs r2, r2, r3 ) @ C gets set
+ CALGN( add pc, r4, ip )
+
+ PLD( pld [r1, #0] )
+2: PLD( subs r2, r2, #96 )
+ PLD( pld [r1, #28] )
+ PLD( blt 4f )
+ PLD( pld [r1, #60] )
+ PLD( pld [r1, #92] )
+
+3: PLD( pld [r1, #124] )
+4: ldr8w r1, r3, r4, r5, r6, r7, r8, ip, lr, abort=20f
+ subs r2, r2, #32
+ str8w r0, r3, r4, r5, r6, r7, r8, ip, lr, abort=20f
+ bge 3b
+ PLD( cmn r2, #96 )
+ PLD( bge 4b )
+
+5: ands ip, r2, #28
+ rsb ip, ip, #32
+ addne pc, pc, ip @ C is always clear here
+ b 7f
+6: nop
+ ldr1w r1, r3, abort=20f
+ ldr1w r1, r4, abort=20f
+ ldr1w r1, r5, abort=20f
+ ldr1w r1, r6, abort=20f
+ ldr1w r1, r7, abort=20f
+ ldr1w r1, r8, abort=20f
+ ldr1w r1, lr, abort=20f
+
+ add pc, pc, ip
+ nop
+ nop
+ str1w r0, r3, abort=20f
+ str1w r0, r4, abort=20f
+ str1w r0, r5, abort=20f
+ str1w r0, r6, abort=20f
+ str1w r0, r7, abort=20f
+ str1w r0, r8, abort=20f
+ str1w r0, lr, abort=20f
+
+ CALGN( bcs 2b )
+
+7: ldmfd sp!, {r5 - r8}
+
+8: movs r2, r2, lsl #31
+ ldr1b r1, r3, ne, abort=21f
+ ldr1b r1, r4, cs, abort=21f
+ ldr1b r1, ip, cs, abort=21f
+ str1b r0, r3, ne, abort=21f
+ str1b r0, r4, cs, abort=21f
+ str1b r0, ip, cs, abort=21f
+
+ exit r4, pc
+
+9: rsb ip, ip, #4
+ cmp ip, #2
+ ldr1b r1, r3, gt, abort=21f
+ ldr1b r1, r4, ge, abort=21f
+ ldr1b r1, lr, abort=21f
+ str1b r0, r3, gt, abort=21f
+ str1b r0, r4, ge, abort=21f
+ subs r2, r2, ip
+ str1b r0, lr, abort=21f
+ blt 8b
+ ands ip, r1, #3
+ beq 1b
+
+10: bic r1, r1, #3
+ cmp ip, #2
+ ldr1w r1, lr, abort=21f
+ beq 17f
+ bgt 18f
+
+
+ .macro forward_copy_shift pull push
+
+ subs r2, r2, #28
+ blt 14f
+
+ CALGN( ands ip, r1, #31 )
+ CALGN( rsb ip, ip, #32 )
+ CALGN( sbcnes r4, ip, r2 ) @ C is always set here
+ CALGN( subcc r2, r2, ip )
+ CALGN( bcc 15f )
+
+11: stmfd sp!, {r5 - r9}
+
+ PLD( pld [r1, #0] )
+ PLD( subs r2, r2, #96 )
+ PLD( pld [r1, #28] )
+ PLD( blt 13f )
+ PLD( pld [r1, #60] )
+ PLD( pld [r1, #92] )
+
+12: PLD( pld [r1, #124] )
+13: ldr4w r1, r4, r5, r6, r7, abort=19f
+ mov r3, lr, pull #\pull
+ subs r2, r2, #32
+ ldr4w r1, r8, r9, ip, lr, abort=19f
+ orr r3, r3, r4, push #\push
+ mov r4, r4, pull #\pull
+ orr r4, r4, r5, push #\push
+ mov r5, r5, pull #\pull
+ orr r5, r5, r6, push #\push
+ mov r6, r6, pull #\pull
+ orr r6, r6, r7, push #\push
+ mov r7, r7, pull #\pull
+ orr r7, r7, r8, push #\push
+ mov r8, r8, pull #\pull
+ orr r8, r8, r9, push #\push
+ mov r9, r9, pull #\pull
+ orr r9, r9, ip, push #\push
+ mov ip, ip, pull #\pull
+ orr ip, ip, lr, push #\push
+ str8w r0, r3, r4, r5, r6, r7, r8, r9, ip, , abort=19f
+ bge 12b
+ PLD( cmn r2, #96 )
+ PLD( bge 13b )
+
+ ldmfd sp!, {r5 - r9}
+
+14: ands ip, r2, #28
+ beq 16f
+
+15: mov r3, lr, pull #\pull
+ ldr1w r1, lr, abort=21f
+ subs ip, ip, #4
+ orr r3, r3, lr, push #\push
+ str1w r0, r3, abort=21f
+ bgt 15b
+ CALGN( cmp r2, #0 )
+ CALGN( bge 11b )
+
+16: sub r1, r1, #(\push / 8)
+ b 8b
+
+ .endm
+
+
+ forward_copy_shift pull=8 push=24
+
+17: forward_copy_shift pull=16 push=16
+
+18: forward_copy_shift pull=24 push=8
+
+
+/*
+ * Abort preanble and completion macros.
+ * If a fixup handler is required then those macros must surround it.
+ * It is assumed that the fixup code will handle the private part of
+ * the exit macro.
+ */
+
+ .macro copy_abort_preamble
+19: ldmfd sp!, {r5 - r9}
+ b 21f
+20: ldmfd sp!, {r5 - r8}
+21:
+ .endm
+
+ .macro copy_abort_end
+ ldmfd sp!, {r4, pc}
+ .endm
+