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sys_linux_arm.s
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// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
#include "textflag.h"
// Linux/ARM atomic operations.
// Because there is so much variation in ARM devices,
// the Linux kernel provides an appropriate compare-and-swap
// implementation at address 0xffff0fc0. Caller sets:
// R0 = old value
// R1 = new value
// R2 = addr
// LR = return address
// The function returns with CS true if the swap happened.
// http://lxr.linux.no/linux+v2.6.37.2/arch/arm/kernel/entry-armv.S#L850
//
// https://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=b49c0f24cf6744a3f4fd09289fe7cade349dead5
//
TEXT cas<>(SB),NOSPLIT,$0
MOVW $0xffff0fc0, R15 // R15 is hardware PC.
TEXT ·Cas(SB),NOSPLIT|NOFRAME,$0
MOVB runtime·goarm(SB), R11
CMP $7, R11
BLT 2(PC)
JMP ·armcas(SB)
JMP kernelcas<>(SB)
TEXT kernelcas<>(SB),NOSPLIT,$0
MOVW ptr+0(FP), R2
// trigger potential paging fault here,
// because we don't know how to traceback through __kuser_cmpxchg
MOVW (R2), R0
MOVW old+4(FP), R0
MOVW new+8(FP), R1
BL cas<>(SB)
BCC ret0
MOVW $1, R0
MOVB R0, ret+12(FP)
RET
ret0:
MOVW $0, R0
MOVB R0, ret+12(FP)
RET
// As for cas, memory barriers are complicated on ARM, but the kernel
// provides a user helper. ARMv5 does not support SMP and has no
// memory barrier instruction at all. ARMv6 added SMP support and has
// a memory barrier, but it requires writing to a coprocessor
// register. ARMv7 introduced the DMB instruction, but it's expensive
// even on single-core devices. The kernel helper takes care of all of
// this for us.
// Use kernel helper version of memory_barrier, when compiled with GOARM < 7.
TEXT memory_barrier<>(SB),NOSPLIT|NOFRAME,$0
MOVW $0xffff0fa0, R15 // R15 is hardware PC.
TEXT ·Load(SB),NOSPLIT,$0-8
MOVW addr+0(FP), R0
MOVW (R0), R1
MOVB runtime·goarm(SB), R11
CMP $7, R11
BGE native_barrier
BL memory_barrier<>(SB)
B end
native_barrier:
DMB MB_ISH
end:
MOVW R1, ret+4(FP)
RET
TEXT ·Store(SB),NOSPLIT,$0-8
MOVW addr+0(FP), R1
MOVW v+4(FP), R2
MOVB runtime·goarm(SB), R8
CMP $7, R8
BGE native_barrier
BL memory_barrier<>(SB)
B store
native_barrier:
DMB MB_ISH
store:
MOVW R2, (R1)
CMP $7, R8
BGE native_barrier2
BL memory_barrier<>(SB)
RET
native_barrier2:
DMB MB_ISH
RET
TEXT ·Load8(SB),NOSPLIT,$0-5
MOVW addr+0(FP), R0
MOVB (R0), R1
MOVB runtime·goarm(SB), R11
CMP $7, R11
BGE native_barrier
BL memory_barrier<>(SB)
B end
native_barrier:
DMB MB_ISH
end:
MOVB R1, ret+4(FP)
RET
TEXT ·Store8(SB),NOSPLIT,$0-5
MOVW addr+0(FP), R1
MOVB v+4(FP), R2
MOVB runtime·goarm(SB), R8
CMP $7, R8
BGE native_barrier
BL memory_barrier<>(SB)
B store
native_barrier:
DMB MB_ISH
store:
MOVB R2, (R1)
CMP $7, R8
BGE native_barrier2
BL memory_barrier<>(SB)
RET
native_barrier2:
DMB MB_ISH
RET