arch/x86/entry/vdso/vclock_gettime.c - linux - Git at Google

 /*
  * Copyright 2006 Andi Kleen, SUSE Labs.
  * Subject to the GNU Public License, v.2
  *
  * Fast user context implementation of clock_gettime, gettimeofday, and time.
  *
  * 32 Bit compat layer by Stefani Seibold <stefani@seibold.net>
  *  sponsored by Rohde & Schwarz GmbH & Co. KG Munich/Germany
  *
  * The code should have no internal unresolved relocations.
  * Check with readelf after changing.
  */

 #include <uapi/linux/time.h>
 #include <asm/vgtod.h>
 #include <asm/vvar.h>
 #include <asm/unistd.h>
 #include <asm/msr.h>
 #include <asm/pvclock.h>
 #include <asm/mshyperv.h>
 #include <linux/math64.h>
 #include <linux/time.h>
 #include <linux/kernel.h>

 #define gtod (&VVAR(vsyscall_gtod_data))

 extern int __vdso_clock_gettime(clockid_t clock, struct timespec *ts);
 extern int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz);
 extern time_t __vdso_time(time_t *t);

 #ifdef CONFIG_PARAVIRT_CLOCK
 extern u8 pvclock_page[PAGE_SIZE]
 	__attribute__((visibility("hidden")));
 #endif

 #ifdef CONFIG_HYPERV_TSCPAGE
 extern u8 hvclock_page[PAGE_SIZE]
 	__attribute__((visibility("hidden")));
 #endif

 #ifndef BUILD_VDSO32

 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 {
 	long ret;
 	asm ("syscall" : "=a" (ret), "=m" (*ts) :
 	     "0" (__NR_clock_gettime), "D" (clock), "S" (ts) :
 	     "memory", "rcx", "r11");
 	return ret;
 }

 notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
 {
 	long ret;

 	asm ("syscall" : "=a" (ret), "=m" (*tv), "=m" (*tz) :
 	     "0" (__NR_gettimeofday), "D" (tv), "S" (tz) :
 	     "memory", "rcx", "r11");
 	return ret;
 }


 #else

 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 {
 	long ret;

 	asm (
 		"mov %%ebx, %%edx \n"
 		"mov %[clock], %%ebx \n"
 		"call __kernel_vsyscall \n"
 		"mov %%edx, %%ebx \n"
 		: "=a" (ret), "=m" (*ts)
 		: "0" (__NR_clock_gettime), [clock] "g" (clock), "c" (ts)
 		: "memory", "edx");
 	return ret;
 }

 notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
 {
 	long ret;

 	asm (
 		"mov %%ebx, %%edx \n"
 		"mov %[tv], %%ebx \n"
 		"call __kernel_vsyscall \n"
 		"mov %%edx, %%ebx \n"
 		: "=a" (ret), "=m" (*tv), "=m" (*tz)
 		: "0" (__NR_gettimeofday), [tv] "g" (tv), "c" (tz)
 		: "memory", "edx");
 	return ret;
 }

 #endif

 #ifdef CONFIG_PARAVIRT_CLOCK
 static notrace const struct pvclock_vsyscall_time_info *get_pvti0(void)
 {
 	return (const struct pvclock_vsyscall_time_info *)&pvclock_page;
 }

 static notrace u64 vread_pvclock(int *mode)
 {
 	const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
 	u64 ret;
 	u64 last;
 	u32 version;

 	/*
 	 * Note: The kernel and hypervisor must guarantee that cpu ID
 	 * number maps 1:1 to per-CPU pvclock time info.
 	 *
 	 * Because the hypervisor is entirely unaware of guest userspace
 	 * preemption, it cannot guarantee that per-CPU pvclock time
 	 * info is updated if the underlying CPU changes or that that
 	 * version is increased whenever underlying CPU changes.
 	 *
 	 * On KVM, we are guaranteed that pvti updates for any vCPU are
 	 * atomic as seen by *all* vCPUs.  This is an even stronger
 	 * guarantee than we get with a normal seqlock.
 	 *
 	 * On Xen, we don't appear to have that guarantee, but Xen still
 	 * supplies a valid seqlock using the version field.
 	 *
 	 * We only do pvclock vdso timing at all if
 	 * PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
 	 * mean that all vCPUs have matching pvti and that the TSC is
 	 * synced, so we can just look at vCPU 0's pvti.
 	 */

 	do {
 		version = pvclock_read_begin(pvti);

 		if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
 			*mode = VCLOCK_NONE;
 			return 0;
 		}

 		ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
 	} while (pvclock_read_retry(pvti, version));

 	/* refer to vread_tsc() comment for rationale */
 	last = gtod->cycle_last;

 	if (likely(ret >= last))
 		return ret;

 	return last;
 }
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
 static notrace u64 vread_hvclock(int *mode)
 {
 	const struct ms_hyperv_tsc_page *tsc_pg =
 		(const struct ms_hyperv_tsc_page *)&hvclock_page;
 	u64 current_tick = hv_read_tsc_page(tsc_pg);

 	if (current_tick != U64_MAX)
 		return current_tick;

 	*mode = VCLOCK_NONE;
 	return 0;
 }
 #endif

 notrace static u64 vread_tsc(void)
 {
 	u64 ret = (u64)rdtsc_ordered();
 	u64 last = gtod->cycle_last;

 	if (likely(ret >= last))
 		return ret;

 	/*
 	 * GCC likes to generate cmov here, but this branch is extremely
 	 * predictable (it's just a function of time and the likely is
 	 * very likely) and there's a data dependence, so force GCC
 	 * to generate a branch instead.  I don't barrier() because
 	 * we don't actually need a barrier, and if this function
 	 * ever gets inlined it will generate worse code.
 	 */
 	asm volatile ("");
 	return last;
 }

 notrace static inline u64 vgetsns(int *mode)
 {
 	u64 v;
 	cycles_t cycles;

 	if (gtod->vclock_mode == VCLOCK_TSC)
 		cycles = vread_tsc();

 	/*
 	 * For any memory-mapped vclock type, we need to make sure that gcc
 	 * doesn't cleverly hoist a load before the mode check.  Otherwise we
 	 * might end up touching the memory-mapped page even if the vclock in
 	 * question isn't enabled, which will segfault.  Hence the barriers.
 	 */
 #ifdef CONFIG_PARAVIRT_CLOCK
 	else if (gtod->vclock_mode == VCLOCK_PVCLOCK) {
 		barrier();
 		cycles = vread_pvclock(mode);
 	}
 #endif
 #ifdef CONFIG_HYPERV_TSCPAGE
 	else if (gtod->vclock_mode == VCLOCK_HVCLOCK) {
 		barrier();
 		cycles = vread_hvclock(mode);
 	}
 #endif
 	else
 		return 0;
 	v = (cycles - gtod->cycle_last) & gtod->mask;
 	return v * gtod->mult;
 }

 /* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
 notrace static int __always_inline do_realtime(struct timespec *ts)
 {
 	unsigned long seq;
 	u64 ns;
 	int mode;

 	do {
 		seq = gtod_read_begin(gtod);
 		mode = gtod->vclock_mode;
 		ts->tv_sec = gtod->wall_time_sec;
 		ns = gtod->wall_time_snsec;
 		ns += vgetsns(&mode);
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));

 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
 	ts->tv_nsec = ns;

 	return mode;
 }

 notrace static int __always_inline do_monotonic(struct timespec *ts)
 {
 	unsigned long seq;
 	u64 ns;
 	int mode;

 	do {
 		seq = gtod_read_begin(gtod);
 		mode = gtod->vclock_mode;
 		ts->tv_sec = gtod->monotonic_time_sec;
 		ns = gtod->monotonic_time_snsec;
 		ns += vgetsns(&mode);
 		ns >>= gtod->shift;
 	} while (unlikely(gtod_read_retry(gtod, seq)));

 	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
 	ts->tv_nsec = ns;

 	return mode;
 }

 notrace static void do_realtime_coarse(struct timespec *ts)
 {
 	unsigned long seq;
 	do {
 		seq = gtod_read_begin(gtod);
 		ts->tv_sec = gtod->wall_time_coarse_sec;
 		ts->tv_nsec = gtod->wall_time_coarse_nsec;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }

 notrace static void do_monotonic_coarse(struct timespec *ts)
 {
 	unsigned long seq;
 	do {
 		seq = gtod_read_begin(gtod);
 		ts->tv_sec = gtod->monotonic_time_coarse_sec;
 		ts->tv_nsec = gtod->monotonic_time_coarse_nsec;
 	} while (unlikely(gtod_read_retry(gtod, seq)));
 }

 notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 {
 	switch (clock) {
 	case CLOCK_REALTIME:
 		if (do_realtime(ts) == VCLOCK_NONE)
 			goto fallback;
 		break;
 	case CLOCK_MONOTONIC:
 		if (do_monotonic(ts) == VCLOCK_NONE)
 			goto fallback;
 		break;
 	case CLOCK_REALTIME_COARSE:
 		do_realtime_coarse(ts);
 		break;
 	case CLOCK_MONOTONIC_COARSE:
 		do_monotonic_coarse(ts);
 		break;
 	default:
 		goto fallback;
 	}

 	return 0;
 fallback:
 	return vdso_fallback_gettime(clock, ts);
 }
 int clock_gettime(clockid_t, struct timespec *)
 	__attribute__((weak, alias("__vdso_clock_gettime")));

 notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
 	if (likely(tv != NULL)) {
 		if (unlikely(do_realtime((struct timespec *)tv) == VCLOCK_NONE))
 			return vdso_fallback_gtod(tv, tz);
 		tv->tv_usec /= 1000;
 	}
 	if (unlikely(tz != NULL)) {
 		tz->tz_minuteswest = gtod->tz_minuteswest;
 		tz->tz_dsttime = gtod->tz_dsttime;
 	}

 	return 0;
 }
 int gettimeofday(struct timeval *, struct timezone *)
 	__attribute__((weak, alias("__vdso_gettimeofday")));

 /*
  * This will break when the xtime seconds get inaccurate, but that is
  * unlikely
  */
 notrace time_t __vdso_time(time_t *t)
 {
 	/* This is atomic on x86 so we don't need any locks. */
 	time_t result = READ_ONCE(gtod->wall_time_sec);

 	if (t)
 		*t = result;
 	return result;
 }
 time_t time(time_t *t)
 	__attribute__((weak, alias("__vdso_time")));
	/*
	* Copyright 2006 Andi Kleen, SUSE Labs.
	* Subject to the GNU Public License, v.2
	*
	* Fast user context implementation of clock_gettime, gettimeofday, and time.
	*
	* 32 Bit compat layer by Stefani Seibold <stefani@seibold.net>
	* sponsored by Rohde & Schwarz GmbH & Co. KG Munich/Germany
	*
	* The code should have no internal unresolved relocations.
	* Check with readelf after changing.
	*/

	#include <uapi/linux/time.h>
	#include <asm/vgtod.h>
	#include <asm/vvar.h>
	#include <asm/unistd.h>
	#include <asm/msr.h>
	#include <asm/pvclock.h>
	#include <asm/mshyperv.h>
	#include <linux/math64.h>
	#include <linux/time.h>
	#include <linux/kernel.h>

	#define gtod (&VVAR(vsyscall_gtod_data))

	extern int __vdso_clock_gettime(clockid_t clock, struct timespec *ts);
	extern int __vdso_gettimeofday(struct timeval tv, struct timezone tz);
	extern time_t __vdso_time(time_t *t);

	#ifdef CONFIG_PARAVIRT_CLOCK
	extern u8 pvclock_page[PAGE_SIZE]
	__attribute__((visibility("hidden")));
	#endif

	#ifdef CONFIG_HYPERV_TSCPAGE
	extern u8 hvclock_page[PAGE_SIZE]
	__attribute__((visibility("hidden")));
	#endif

	#ifndef BUILD_VDSO32

	notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
	{
	long ret;
	asm ("syscall" : "=a" (ret), "=m" (*ts) :
	"0" (__NR_clock_gettime), "D" (clock), "S" (ts) :
	"memory", "rcx", "r11");
	return ret;
	}

	notrace static long vdso_fallback_gtod(struct timeval tv, struct timezone tz)
	{
	long ret;

	asm ("syscall" : "=a" (ret), "=m" (tv), "=m" (tz) :
	"0" (__NR_gettimeofday), "D" (tv), "S" (tz) :
	"memory", "rcx", "r11");
	return ret;
	}


	#else

	notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
	{
	long ret;

	asm (
	"mov %%ebx, %%edx \n"
	"mov %[clock], %%ebx \n"
	"call __kernel_vsyscall \n"
	"mov %%edx, %%ebx \n"
	: "=a" (ret), "=m" (*ts)
	: "0" (__NR_clock_gettime), [clock] "g" (clock), "c" (ts)
	: "memory", "edx");
	return ret;
	}

	notrace static long vdso_fallback_gtod(struct timeval tv, struct timezone tz)
	{
	long ret;

	asm (
	"mov %%ebx, %%edx \n"
	"mov %[tv], %%ebx \n"
	"call __kernel_vsyscall \n"
	"mov %%edx, %%ebx \n"
	: "=a" (ret), "=m" (tv), "=m" (tz)
	: "0" (__NR_gettimeofday), [tv] "g" (tv), "c" (tz)
	: "memory", "edx");
	return ret;
	}

	#endif

	#ifdef CONFIG_PARAVIRT_CLOCK
	static notrace const struct pvclock_vsyscall_time_info *get_pvti0(void)
	{
	return (const struct pvclock_vsyscall_time_info *)&pvclock_page;
	}

	static notrace u64 vread_pvclock(int *mode)
	{
	const struct pvclock_vcpu_time_info *pvti = &get_pvti0()->pvti;
	u64 ret;
	u64 last;
	u32 version;

	/*
	* Note: The kernel and hypervisor must guarantee that cpu ID
	* number maps 1:1 to per-CPU pvclock time info.
	*
	* Because the hypervisor is entirely unaware of guest userspace
	* preemption, it cannot guarantee that per-CPU pvclock time
	* info is updated if the underlying CPU changes or that that
	* version is increased whenever underlying CPU changes.
	*
	* On KVM, we are guaranteed that pvti updates for any vCPU are
	* atomic as seen by all vCPUs. This is an even stronger
	* guarantee than we get with a normal seqlock.
	*
	* On Xen, we don't appear to have that guarantee, but Xen still
	* supplies a valid seqlock using the version field.
	*
	* We only do pvclock vdso timing at all if
	* PVCLOCK_TSC_STABLE_BIT is set, and we interpret that bit to
	* mean that all vCPUs have matching pvti and that the TSC is
	* synced, so we can just look at vCPU 0's pvti.
	*/

	do {
	version = pvclock_read_begin(pvti);

	if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
	*mode = VCLOCK_NONE;
	return 0;
	}

	ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
	} while (pvclock_read_retry(pvti, version));

	/* refer to vread_tsc() comment for rationale */
	last = gtod->cycle_last;

	if (likely(ret >= last))
	return ret;

	return last;
	}
	#endif
	#ifdef CONFIG_HYPERV_TSCPAGE
	static notrace u64 vread_hvclock(int *mode)
	{
	const struct ms_hyperv_tsc_page *tsc_pg =
	(const struct ms_hyperv_tsc_page *)&hvclock_page;
	u64 current_tick = hv_read_tsc_page(tsc_pg);

	if (current_tick != U64_MAX)
	return current_tick;

	*mode = VCLOCK_NONE;
	return 0;
	}
	#endif

	notrace static u64 vread_tsc(void)
	{
	u64 ret = (u64)rdtsc_ordered();
	u64 last = gtod->cycle_last;

	if (likely(ret >= last))
	return ret;

	/*
	* GCC likes to generate cmov here, but this branch is extremely
	* predictable (it's just a function of time and the likely is
	* very likely) and there's a data dependence, so force GCC
	* to generate a branch instead. I don't barrier() because
	* we don't actually need a barrier, and if this function
	* ever gets inlined it will generate worse code.
	*/
	asm volatile ("");
	return last;
	}

	notrace static inline u64 vgetsns(int *mode)
	{
	u64 v;
	cycles_t cycles;

	if (gtod->vclock_mode == VCLOCK_TSC)
	cycles = vread_tsc();

	/*
	* For any memory-mapped vclock type, we need to make sure that gcc
	* doesn't cleverly hoist a load before the mode check. Otherwise we
	* might end up touching the memory-mapped page even if the vclock in
	* question isn't enabled, which will segfault. Hence the barriers.
	*/
	#ifdef CONFIG_PARAVIRT_CLOCK
	else if (gtod->vclock_mode == VCLOCK_PVCLOCK) {
	barrier();
	cycles = vread_pvclock(mode);
	}
	#endif
	#ifdef CONFIG_HYPERV_TSCPAGE
	else if (gtod->vclock_mode == VCLOCK_HVCLOCK) {
	barrier();
	cycles = vread_hvclock(mode);
	}
	#endif
	else
	return 0;
	v = (cycles - gtod->cycle_last) & gtod->mask;
	return v * gtod->mult;
	}

	/* Code size doesn't matter (vdso is 4k anyway) and this is faster. */
	notrace static int __always_inline do_realtime(struct timespec *ts)
	{
	unsigned long seq;
	u64 ns;
	int mode;

	do {
	seq = gtod_read_begin(gtod);
	mode = gtod->vclock_mode;
	ts->tv_sec = gtod->wall_time_sec;
	ns = gtod->wall_time_snsec;
	ns += vgetsns(&mode);
	ns >>= gtod->shift;
	} while (unlikely(gtod_read_retry(gtod, seq)));

	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
	ts->tv_nsec = ns;

	return mode;
	}

	notrace static int __always_inline do_monotonic(struct timespec *ts)
	{
	unsigned long seq;
	u64 ns;
	int mode;

	do {
	seq = gtod_read_begin(gtod);
	mode = gtod->vclock_mode;
	ts->tv_sec = gtod->monotonic_time_sec;
	ns = gtod->monotonic_time_snsec;
	ns += vgetsns(&mode);
	ns >>= gtod->shift;
	} while (unlikely(gtod_read_retry(gtod, seq)));

	ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
	ts->tv_nsec = ns;

	return mode;
	}

	notrace static void do_realtime_coarse(struct timespec *ts)
	{
	unsigned long seq;
	do {
	seq = gtod_read_begin(gtod);
	ts->tv_sec = gtod->wall_time_coarse_sec;
	ts->tv_nsec = gtod->wall_time_coarse_nsec;
	} while (unlikely(gtod_read_retry(gtod, seq)));
	}

	notrace static void do_monotonic_coarse(struct timespec *ts)
	{
	unsigned long seq;
	do {
	seq = gtod_read_begin(gtod);
	ts->tv_sec = gtod->monotonic_time_coarse_sec;
	ts->tv_nsec = gtod->monotonic_time_coarse_nsec;
	} while (unlikely(gtod_read_retry(gtod, seq)));
	}

	notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
	{
	switch (clock) {
	case CLOCK_REALTIME:
	if (do_realtime(ts) == VCLOCK_NONE)
	goto fallback;
	break;
	case CLOCK_MONOTONIC:
	if (do_monotonic(ts) == VCLOCK_NONE)
	goto fallback;
	break;
	case CLOCK_REALTIME_COARSE:
	do_realtime_coarse(ts);
	break;
	case CLOCK_MONOTONIC_COARSE:
	do_monotonic_coarse(ts);
	break;
	default:
	goto fallback;
	}

	return 0;
	fallback:
	return vdso_fallback_gettime(clock, ts);
	}
	int clock_gettime(clockid_t, struct timespec *)
	__attribute__((weak, alias("__vdso_clock_gettime")));

	notrace int __vdso_gettimeofday(struct timeval tv, struct timezone tz)
	{
	if (likely(tv != NULL)) {
	if (unlikely(do_realtime((struct timespec *)tv) == VCLOCK_NONE))
	return vdso_fallback_gtod(tv, tz);
	tv->tv_usec /= 1000;
	}
	if (unlikely(tz != NULL)) {
	tz->tz_minuteswest = gtod->tz_minuteswest;
	tz->tz_dsttime = gtod->tz_dsttime;
	}

	return 0;
	}
	int gettimeofday(struct timeval , struct timezone )
	__attribute__((weak, alias("__vdso_gettimeofday")));

	/*
	* This will break when the xtime seconds get inaccurate, but that is
	* unlikely
	*/
	notrace time_t __vdso_time(time_t *t)
	{
	/* This is atomic on x86 so we don't need any locks. */
	time_t result = READ_ONCE(gtod->wall_time_sec);

	if (t)
	*t = result;
	return result;
	}
	time_t time(time_t *t)
	__attribute__((weak, alias("__vdso_time")));