mach_continuous_time.c   [plain text]

/*
 * Copyright (c) 2015 Apple Inc. All rights reserved.
 *
 * @APPLE_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. 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_LICENSE_HEADER_END@
 */
#include <stddef.h>
#include <stdbool.h>
#include <sys/types.h>
#include <machine/cpu_capabilities.h>
#include <mach/mach_time.h>

__attribute__((visibility("hidden")))
uint64_t
_mach_continuous_time_base(void)
{
#if !defined(__x86_64__) && !defined(__arm64__)
	// Deal with the lack of 64-bit loads on arm32 (see mach_approximate_time.s)
	while(1) {
		volatile uint64_t *base_ptr = (volatile uint64_t*)_COMM_PAGE_CONT_TIMEBASE;
		uint64_t read1, read2;
		read1 = *base_ptr;
#if   defined(__i386__)
		__asm__ volatile("lfence" ::: "memory");
#else
#error "unsupported arch"
#endif
		read2 = *base_ptr;

		if(__builtin_expect((read1 == read2), 1))
			return read1;
	}
#else // 64-bit
	return *(volatile uint64_t*)_COMM_PAGE_CONT_TIMEBASE;
#endif // 64-bit
}


__attribute__((visibility("hidden")))
kern_return_t
_mach_continuous_time(uint64_t* absolute_time, uint64_t* cont_time)
{
	volatile uint64_t *base_ptr = (volatile uint64_t*)_COMM_PAGE_CONT_TIMEBASE;
	volatile uint64_t read1, read2;
	volatile uint64_t absolute;

	do {
        read1 = *base_ptr;
        absolute = mach_absolute_time();
		read2 = *base_ptr;
	} while (__builtin_expect((read1 != read2), 0));

	if (absolute_time) *absolute_time = absolute;
	if (cont_time) *cont_time = absolute + read1;

	return KERN_SUCCESS;
}

uint64_t
mach_continuous_time(void)
{
	uint64_t cont_time;
    _mach_continuous_time(NULL, &cont_time);
	return cont_time;
}

uint64_t
mach_continuous_approximate_time(void)
{
    /*
     * No retry loop here because if we use a slightly too old timebase that's
     * okay, we are approximate time anyway.
     */
    volatile register uint64_t time_base = _mach_continuous_time_base();
    return time_base + mach_approximate_time();
}