1/* 2 * Copyright (c) 2003-2007 Apple Inc. All rights reserved. 3 * 4 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@ 5 * 6 * This file contains Original Code and/or Modifications of Original Code 7 * as defined in and that are subject to the Apple Public Source License 8 * Version 2.0 (the 'License'). You may not use this file except in 9 * compliance with the License. The rights granted to you under the License 10 * may not be used to create, or enable the creation or redistribution of, 11 * unlawful or unlicensed copies of an Apple operating system, or to 12 * circumvent, violate, or enable the circumvention or violation of, any 13 * terms of an Apple operating system software license agreement. 14 * 15 * Please obtain a copy of the License at 16 * http://www.opensource.apple.com/apsl/ and read it before using this file. 17 * 18 * The Original Code and all software distributed under the License are 19 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER 20 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES, 21 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, 22 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT. 23 * Please see the License for the specific language governing rights and 24 * limitations under the License. 25 * 26 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@ 27 */ 28 29#include <sys/appleapiopts.h> 30#include <machine/cpu_capabilities.h> 31 32#if defined(__arm__) 33 34 .text 35 .align 2 36 .globl _mach_approximate_time 37_mach_approximate_time: 38 39 movw r0, #((_COMM_PAGE_APPROX_TIME_SUPPORTED>>0)&0x0FFFF) 40 movt r0, #((_COMM_PAGE_APPROX_TIME_SUPPORTED>>16)&0x0FFFF) 41 ldrb r0, [r0] // load COMM_PAGE_APPROX_TIME_SUPPORTED 42 cmp r0, #1 // check if approx time is supported 43 44 bne _mach_absolute_time // if not supported, fall through to 45 // absolute_time 46 47 movw r2, #((_COMM_PAGE_APPROX_TIME>>0)&0x0FFFF) 48 movt r2, #((_COMM_PAGE_APPROX_TIME>>16)&0x0FFFF) 49 50 // at this point, r2->COMM_PAGE_APPROX_TIME, which is a 64-bit value. 51 // Since this is a 32-bit architecture, and the commpage is readonly, 52 // there is no "guaranteed" atomic way to read all 64-bits with 53 // hardware protection. Even the ldrd instruction is not guaranteed to 54 // complete atomically. The solution is to use a 32-bit high/low/high 55 // read with a consistency check on the high bits. To further 56 // complicate things, reading the same location in memory back to back 57 // could trigger a predictive read, which would defeat the purpose of 58 // doing the consistency check so we insert a data memory barrier to 59 // prevent this. 60_consistency_check: 61 ldr r1, [r2,#4] // load high 62 ldr r0, [r2] // load low 63 dsb // avoid predictive reads that could 64 // be invalid if interrupted 65 ldr r3, [r2,#4] // load high again 66 cmp r1, r3 // check that high1 == high2 67 bne _consistency_check // try again if not 68 bx lr 69 70#elif defined(__i386__) 71 72 .text 73 .align 4, 0x90 74 .globl _mach_approximate_time 75_mach_approximate_time: 76 pushl %ebp // push a frame 77 movl %esp, %ebp 78 cmpb $0, _COMM_PAGE_APPROX_TIME_SUPPORTED // check commpage 79 // "is supported" flag. 80 je _approx_time_not_supported // if zero, fall through to 81 // absolute_time 82 // note the comment above for 32-bit ARM applies equally to 32-bit i386. 83 .align 4, 0x90 84_approx_time_consistency_check: 85 movl _COMM_PAGE_APPROX_TIME+4, %edx // load high 86 movl _COMM_PAGE_APPROX_TIME, %eax // load low 87 lfence // avoid predictive reads that 88 // could be invalid if 89 // interrupted 90 cmpl _COMM_PAGE_APPROX_TIME+4, %edx // load high and check if equal 91 // to the first read 92 jne _approx_time_consistency_check // if not, try again. 93 popl %ebp 94 ret 95 96_approx_time_not_supported: 97 popl %ebp 98 jmp _mach_absolute_time 99 100#endif 101