/*
* 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>
/*
* 64-bit version _mach_absolute_time. We return the 64-bit nanotime in %rax.
*
* The algorithm we use is:
*
* ns = ((((rdtsc - rnt_tsc_base)<<rnt_shift)*rnt_tsc_scale) / 2**32) + rnt_ns_base * rnt_shift, a constant computed during initialization, is the smallest value for which:
*
* tscFreq << rnt_shift) > SLOW_TSC_THRESHOLD
*
* Where SLOW_TSC_THRESHOLD is about 10e9. Since most processor's tscFreqs are greater
* than 1GHz, rnt_shift is usually 0. rnt_tsc_scale is also a 32-bit constant:
*
* rnt_tsc_scale = (10e9 * 2**32) / (tscFreq << rnt_shift) */
.globl _mach_absolute_time
_mach_absolute_time:
pushq %rbp // set up a frame for backtraces
movq %rsp,%rbp
movq $(_COMM_PAGE_TIME_DATA_START),%rsi
1:
movl _NT_GENERATION(%rsi),%r8d // get generation
testl %r8d,%r8d // if 0, data is being changed...
jz 1b // ...so loop until stable
lfence
rdtsc // edx:eax := tsc
lfence
shlq $32,%rdx // rax := ((edx << 32) | eax), ie 64-bit tsc
orq %rdx,%rax
/*
* Prior to supporting "slow" processors, xnu always set _NT_SHIFT to 32.
* Now it defaults to 0, unless the processor is slow. In order to maintain
* compatibility with both old and new versions of xnu, we mask the shift
* down to 0x1F, which maps the old default (32) into the new default (0).
*/
movl _NT_SHIFT(%rsi),%ecx
andl $0x1F,%ecx // *** remove this line once 10.9 is GM ***
subq _NT_TSC_BASE(%rsi), %rax // rax := (tsc - base_tsc)
shlq %cl,%rax // rax := (tsc - base_tsc) << NT_SHIFT
movl _NT_SCALE(%rsi),%ecx
mulq %rcx // rdx:rax := ((tsc - base_tsc)<<shift) * scale
shrdq $32,%rdx,%rax // divide by 2**32
addq _NT_NS_BASE(%rsi),%rax // (((tsc - base_tsc) * scale) >> 32) + ns_base
cmpl _NT_GENERATION(%rsi),%r8d // did the data change during computation?
jne 1b
popq %rbp
ret