/*
* Copyright (c) 2003-2007 Apple Inc. All rights reserved.
*
* @APPLE_OSREFERENCE_LICENSE_HEADER_START@
*
* This file contains Original Code and/or Modifications of Original Code
* as defined in and that are subject to the Apple Public Source License
* Version 2.0 (the 'License'). You may not use this file except in
* compliance with the License. The rights granted to you under the License
* may not be used to create, or enable the creation or redistribution of,
* unlawful or unlicensed copies of an Apple operating system, or to
* circumvent, violate, or enable the circumvention or violation of, any
* terms of an Apple operating system software license agreement.
*
* Please obtain a copy of the License at
* http://www.opensource.apple.com/apsl/ and read it before using this file.
*
* The Original Code and all software distributed under the License are
* distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
* EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
* INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
* Please see the License for the specific language governing rights and
* limitations under the License.
*
* @APPLE_OSREFERENCE_LICENSE_HEADER_END@
*/
#include <sys/appleapiopts.h>
#include <machine/cpu_capabilities.h>
#if defined(__arm__)
.text
.align 2
.globl _mach_approximate_time
_mach_approximate_time:
movw r0, #((_COMM_PAGE_APPROX_TIME_SUPPORTED>>0)&0x0FFFF)
movt r0, #((_COMM_PAGE_APPROX_TIME_SUPPORTED>>16)&0x0FFFF)
ldrb r0, [r0] // load COMM_PAGE_APPROX_TIME_SUPPORTED
cmp r0, #1 // check if approx time is supported
bne _mach_absolute_time // if not supported, fall through to
// absolute_time
movw r2, #((_COMM_PAGE_APPROX_TIME>>0)&0x0FFFF)
movt r2, #((_COMM_PAGE_APPROX_TIME>>16)&0x0FFFF)
// at this point, r2->COMM_PAGE_APPROX_TIME, which is a 64-bit value.
// Since this is a 32-bit architecture, and the commpage is readonly,
// there is no "guaranteed" atomic way to read all 64-bits with
// hardware protection. Even the ldrd instruction is not guaranteed to
// complete atomically. The solution is to use a 32-bit high/low/high
// read with a consistency check on the high bits. To further
// complicate things, reading the same location in memory back to back
// could trigger a predictive read, which would defeat the purpose of
// doing the consistency check so we insert a data memory barrier to
// prevent this.
_consistency_check:
ldr r1, [r2,#4] // load high
ldr r0, [r2] // load low
dsb // avoid predictive reads that could
// be invalid if interrupted
ldr r3, [r2,#4] // load high again
cmp r1, r3 // check that high1 == high2
bne _consistency_check // try again if not
bx lr
#elif defined(__i386__)
.text
.align 4, 0x90
.globl _mach_approximate_time
_mach_approximate_time:
pushl %ebp // push a frame
movl %esp, %ebp
cmpb $0, _COMM_PAGE_APPROX_TIME_SUPPORTED // check commpage
// "is supported" flag.
je _approx_time_not_supported // if zero, fall through to
// absolute_time
// note the comment above for 32-bit ARM applies equally to 32-bit i386.
.align 4, 0x90
_approx_time_consistency_check:
movl _COMM_PAGE_APPROX_TIME+4, %edx // load high
movl _COMM_PAGE_APPROX_TIME, %eax // load low
lfence // avoid predictive reads that
// could be invalid if
// interrupted
cmpl _COMM_PAGE_APPROX_TIME+4, %edx // load high and check if equal
// to the first read
jne _approx_time_consistency_check // if not, try again.
popl %ebp
ret
_approx_time_not_supported:
popl %ebp
jmp _mach_absolute_time
#endif