arch/x86/kernel/tsc_sync.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * check TSC synchronization.
  *
  * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
  *
  * We check whether all boot CPUs have their TSC's synchronized,
  * print a warning if not and turn off the TSC clock-source.
  *
  * The warp-check is point-to-point between two CPUs, the CPU
  * initiating the bootup is the 'source CPU', the freshly booting
  * CPU is the 'target CPU'.
  *
  * Only two CPUs may participate - they can enter in any order.
  * ( The serial nature of the boot logic and the CPU hotplug lock
  *   protects against more than 2 CPUs entering this code. )
  */
 #include <linux/topology.h>
 #include <linux/spinlock.h>
 #include <linux/kernel.h>
 #include <linux/smp.h>
 #include <linux/nmi.h>
 #include <asm/tsc.h>

 struct tsc_adjust {
 	s64		bootval;
 	s64		adjusted;
 	unsigned long	nextcheck;
 	bool		warned;
 };

 static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust);

 /*
  * TSC's on different sockets may be reset asynchronously.
  * This may cause the TSC ADJUST value on socket 0 to be NOT 0.
  */
 bool __read_mostly tsc_async_resets;

 void mark_tsc_async_resets(char *reason)
 {
 	if (tsc_async_resets)
 		return;
 	tsc_async_resets = true;
 	pr_info("tsc: Marking TSC async resets true due to %s\n", reason);
 }

 void tsc_verify_tsc_adjust(bool resume)
 {
 	struct tsc_adjust *adj = this_cpu_ptr(&tsc_adjust);
 	s64 curval;

 	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
 		return;

 	/* Skip unnecessary error messages if TSC already unstable */
 	if (check_tsc_unstable())
 		return;

 	/* Rate limit the MSR check */
 	if (!resume && time_before(jiffies, adj->nextcheck))
 		return;

 	adj->nextcheck = jiffies + HZ;

 	rdmsrl(MSR_IA32_TSC_ADJUST, curval);
 	if (adj->adjusted == curval)
 		return;

 	/* Restore the original value */
 	wrmsrl(MSR_IA32_TSC_ADJUST, adj->adjusted);

 	if (!adj->warned || resume) {
 		pr_warn(FW_BUG "TSC ADJUST differs: CPU%u %lld --> %lld. Restoring\n",
 			smp_processor_id(), adj->adjusted, curval);
 		adj->warned = true;
 	}
 }

 static void tsc_sanitize_first_cpu(struct tsc_adjust *cur, s64 bootval,
 				   unsigned int cpu, bool bootcpu)
 {
 	/*
 	 * First online CPU in a package stores the boot value in the
 	 * adjustment value. This value might change later via the sync
 	 * mechanism. If that fails we still can yell about boot values not
 	 * being consistent.
 	 *
 	 * On the boot cpu we just force set the ADJUST value to 0 if it's
 	 * non zero. We don't do that on non boot cpus because physical
 	 * hotplug should have set the ADJUST register to a value > 0 so
 	 * the TSC is in sync with the already running cpus.
 	 *
 	 * Also don't force the ADJUST value to zero if that is a valid value
 	 * for socket 0 as determined by the system arch.  This is required
 	 * when multiple sockets are reset asynchronously with each other
 	 * and socket 0 may not have an TSC ADJUST value of 0.
 	 */
 	if (bootcpu && bootval != 0) {
 		if (likely(!tsc_async_resets)) {
 			pr_warn(FW_BUG "TSC ADJUST: CPU%u: %lld force to 0\n",
 				cpu, bootval);
 			wrmsrl(MSR_IA32_TSC_ADJUST, 0);
 			bootval = 0;
 		} else {
 			pr_info("TSC ADJUST: CPU%u: %lld NOT forced to 0\n",
 				cpu, bootval);
 		}
 	}
 	cur->adjusted = bootval;
 }

 #ifndef CONFIG_SMP
 bool __init tsc_store_and_check_tsc_adjust(bool bootcpu)
 {
 	struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
 	s64 bootval;

 	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
 		return false;

 	/* Skip unnecessary error messages if TSC already unstable */
 	if (check_tsc_unstable())
 		return false;

 	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
 	cur->bootval = bootval;
 	cur->nextcheck = jiffies + HZ;
 	tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(), bootcpu);
 	return false;
 }

 #else /* !CONFIG_SMP */

 /*
  * Store and check the TSC ADJUST MSR if available
  */
 bool tsc_store_and_check_tsc_adjust(bool bootcpu)
 {
 	struct tsc_adjust *ref, *cur = this_cpu_ptr(&tsc_adjust);
 	unsigned int refcpu, cpu = smp_processor_id();
 	struct cpumask *mask;
 	s64 bootval;

 	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
 		return false;

 	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
 	cur->bootval = bootval;
 	cur->nextcheck = jiffies + HZ;
 	cur->warned = false;

 	/*
 	 * If a non-zero TSC value for socket 0 may be valid then the default
 	 * adjusted value cannot assumed to be zero either.
 	 */
 	if (tsc_async_resets)
 		cur->adjusted = bootval;

 	/*
 	 * Check whether this CPU is the first in a package to come up. In
 	 * this case do not check the boot value against another package
 	 * because the new package might have been physically hotplugged,
 	 * where TSC_ADJUST is expected to be different. When called on the
 	 * boot CPU topology_core_cpumask() might not be available yet.
 	 */
 	mask = topology_core_cpumask(cpu);
 	refcpu = mask ? cpumask_any_but(mask, cpu) : nr_cpu_ids;

 	if (refcpu >= nr_cpu_ids) {
 		tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(),
 				       bootcpu);
 		return false;
 	}

 	ref = per_cpu_ptr(&tsc_adjust, refcpu);
 	/*
 	 * Compare the boot value and complain if it differs in the
 	 * package.
 	 */
 	if (bootval != ref->bootval)
 		printk_once(FW_BUG "TSC ADJUST differs within socket(s), fixing all errors\n");

 	/*
 	 * The TSC_ADJUST values in a package must be the same. If the boot
 	 * value on this newly upcoming CPU differs from the adjustment
 	 * value of the already online CPU in this package, set it to that
 	 * adjusted value.
 	 */
 	if (bootval != ref->adjusted) {
 		cur->adjusted = ref->adjusted;
 		wrmsrl(MSR_IA32_TSC_ADJUST, ref->adjusted);
 	}
 	/*
 	 * We have the TSCs forced to be in sync on this package. Skip sync
 	 * test:
 	 */
 	return true;
 }

 /*
  * Entry/exit counters that make sure that both CPUs
  * run the measurement code at once:
  */
 static atomic_t start_count;
 static atomic_t stop_count;
 static atomic_t skip_test;
 static atomic_t test_runs;

 /*
  * We use a raw spinlock in this exceptional case, because
  * we want to have the fastest, inlined, non-debug version
  * of a critical section, to be able to prove TSC time-warps:
  */
 static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;

 static cycles_t last_tsc;
 static cycles_t max_warp;
 static int nr_warps;
 static int random_warps;

 /*
  * TSC-warp measurement loop running on both CPUs.  This is not called
  * if there is no TSC.
  */
 static cycles_t check_tsc_warp(unsigned int timeout)
 {
 	cycles_t start, now, prev, end, cur_max_warp = 0;
 	int i, cur_warps = 0;

 	start = rdtsc_ordered();
 	/*
 	 * The measurement runs for 'timeout' msecs:
 	 */
 	end = start + (cycles_t) tsc_khz * timeout;

 	for (i = 0; ; i++) {
 		/*
 		 * We take the global lock, measure TSC, save the
 		 * previous TSC that was measured (possibly on
 		 * another CPU) and update the previous TSC timestamp.
 		 */
 		arch_spin_lock(&sync_lock);
 		prev = last_tsc;
 		now = rdtsc_ordered();
 		last_tsc = now;
 		arch_spin_unlock(&sync_lock);

 		/*
 		 * Be nice every now and then (and also check whether
 		 * measurement is done [we also insert a 10 million
 		 * loops safety exit, so we dont lock up in case the
 		 * TSC readout is totally broken]):
 		 */
 		if (unlikely(!(i & 7))) {
 			if (now > end || i > 10000000)
 				break;
 			cpu_relax();
 			touch_nmi_watchdog();
 		}
 		/*
 		 * Outside the critical section we can now see whether
 		 * we saw a time-warp of the TSC going backwards:
 		 */
 		if (unlikely(prev > now)) {
 			arch_spin_lock(&sync_lock);
 			max_warp = max(max_warp, prev - now);
 			cur_max_warp = max_warp;
 			/*
 			 * Check whether this bounces back and forth. Only
 			 * one CPU should observe time going backwards.
 			 */
 			if (cur_warps != nr_warps)
 				random_warps++;
 			nr_warps++;
 			cur_warps = nr_warps;
 			arch_spin_unlock(&sync_lock);
 		}
 	}
 	WARN(!(now-start),
 		"Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
 			now-start, end-start);
 	return cur_max_warp;
 }

 /*
  * If the target CPU coming online doesn't have any of its core-siblings
  * online, a timeout of 20msec will be used for the TSC-warp measurement
  * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
  * information about this socket already (and this information grows as we
  * have more and more logical-siblings in that socket).
  *
  * Ideally we should be able to skip the TSC sync check on the other
  * core-siblings, if the first logical CPU in a socket passed the sync test.
  * But as the TSC is per-logical CPU and can potentially be modified wrongly
  * by the bios, TSC sync test for smaller duration should be able
  * to catch such errors. Also this will catch the condition where all the
  * cores in the socket doesn't get reset at the same time.
  */
 static inline unsigned int loop_timeout(int cpu)
 {
 	return (cpumask_weight(topology_core_cpumask(cpu)) > 1) ? 2 : 20;
 }

 /*
  * Source CPU calls into this - it waits for the freshly booted
  * target CPU to arrive and then starts the measurement:
  */
 void check_tsc_sync_source(int cpu)
 {
 	int cpus = 2;

 	/*
 	 * No need to check if we already know that the TSC is not
 	 * synchronized or if we have no TSC.
 	 */
 	if (unsynchronized_tsc())
 		return;

 	/*
 	 * Set the maximum number of test runs to
 	 *  1 if the CPU does not provide the TSC_ADJUST MSR
 	 *  3 if the MSR is available, so the target can try to adjust
 	 */
 	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
 		atomic_set(&test_runs, 1);
 	else
 		atomic_set(&test_runs, 3);
 retry:
 	/*
 	 * Wait for the target to start or to skip the test:
 	 */
 	while (atomic_read(&start_count) != cpus - 1) {
 		if (atomic_read(&skip_test) > 0) {
 			atomic_set(&skip_test, 0);
 			return;
 		}
 		cpu_relax();
 	}

 	/*
 	 * Trigger the target to continue into the measurement too:
 	 */
 	atomic_inc(&start_count);

 	check_tsc_warp(loop_timeout(cpu));

 	while (atomic_read(&stop_count) != cpus-1)
 		cpu_relax();

 	/*
 	 * If the test was successful set the number of runs to zero and
 	 * stop. If not, decrement the number of runs an check if we can
 	 * retry. In case of random warps no retry is attempted.
 	 */
 	if (!nr_warps) {
 		atomic_set(&test_runs, 0);

 		pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
 			smp_processor_id(), cpu);

 	} else if (atomic_dec_and_test(&test_runs) || random_warps) {
 		/* Force it to 0 if random warps brought us here */
 		atomic_set(&test_runs, 0);

 		pr_warn("TSC synchronization [CPU#%d -> CPU#%d]:\n",
 			smp_processor_id(), cpu);
 		pr_warn("Measured %Ld cycles TSC warp between CPUs, "
 			"turning off TSC clock.\n", max_warp);
 		if (random_warps)
 			pr_warn("TSC warped randomly between CPUs\n");
 		mark_tsc_unstable("check_tsc_sync_source failed");
 	}

 	/*
 	 * Reset it - just in case we boot another CPU later:
 	 */
 	atomic_set(&start_count, 0);
 	random_warps = 0;
 	nr_warps = 0;
 	max_warp = 0;
 	last_tsc = 0;

 	/*
 	 * Let the target continue with the bootup:
 	 */
 	atomic_inc(&stop_count);

 	/*
 	 * Retry, if there is a chance to do so.
 	 */
 	if (atomic_read(&test_runs) > 0)
 		goto retry;
 }

 /*
  * Freshly booted CPUs call into this:
  */
 void check_tsc_sync_target(void)
 {
 	struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
 	unsigned int cpu = smp_processor_id();
 	cycles_t cur_max_warp, gbl_max_warp;
 	int cpus = 2;

 	/* Also aborts if there is no TSC. */
 	if (unsynchronized_tsc())
 		return;

 	/*
 	 * Store, verify and sanitize the TSC adjust register. If
 	 * successful skip the test.
 	 *
 	 * The test is also skipped when the TSC is marked reliable. This
 	 * is true for SoCs which have no fallback clocksource. On these
 	 * SoCs the TSC is frequency synchronized, but still the TSC ADJUST
 	 * register might have been wreckaged by the BIOS..
 	 */
 	if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) {
 		atomic_inc(&skip_test);
 		return;
 	}

 retry:
 	/*
 	 * Register this CPU's participation and wait for the
 	 * source CPU to start the measurement:
 	 */
 	atomic_inc(&start_count);
 	while (atomic_read(&start_count) != cpus)
 		cpu_relax();

 	cur_max_warp = check_tsc_warp(loop_timeout(cpu));

 	/*
 	 * Store the maximum observed warp value for a potential retry:
 	 */
 	gbl_max_warp = max_warp;

 	/*
 	 * Ok, we are done:
 	 */
 	atomic_inc(&stop_count);

 	/*
 	 * Wait for the source CPU to print stuff:
 	 */
 	while (atomic_read(&stop_count) != cpus)
 		cpu_relax();

 	/*
 	 * Reset it for the next sync test:
 	 */
 	atomic_set(&stop_count, 0);

 	/*
 	 * Check the number of remaining test runs. If not zero, the test
 	 * failed and a retry with adjusted TSC is possible. If zero the
 	 * test was either successful or failed terminally.
 	 */
 	if (!atomic_read(&test_runs))
 		return;

 	/*
 	 * If the warp value of this CPU is 0, then the other CPU
 	 * observed time going backwards so this TSC was ahead and
 	 * needs to move backwards.
 	 */
 	if (!cur_max_warp)
 		cur_max_warp = -gbl_max_warp;

 	/*
 	 * Add the result to the previous adjustment value.
 	 *
 	 * The adjustement value is slightly off by the overhead of the
 	 * sync mechanism (observed values are ~200 TSC cycles), but this
 	 * really depends on CPU, node distance and frequency. So
 	 * compensating for this is hard to get right. Experiments show
 	 * that the warp is not longer detectable when the observed warp
 	 * value is used. In the worst case the adjustment needs to go
 	 * through a 3rd run for fine tuning.
 	 */
 	cur->adjusted += cur_max_warp;

 	pr_warn("TSC ADJUST compensate: CPU%u observed %lld warp. Adjust: %lld\n",
 		cpu, cur_max_warp, cur->adjusted);

 	wrmsrl(MSR_IA32_TSC_ADJUST, cur->adjusted);
 	goto retry;

 }

 #endif /* CONFIG_SMP */
	// SPDX-License-Identifier: GPL-2.0
	/*
	* check TSC synchronization.
	*
	* Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
	*
	* We check whether all boot CPUs have their TSC's synchronized,
	* print a warning if not and turn off the TSC clock-source.
	*
	* The warp-check is point-to-point between two CPUs, the CPU
	* initiating the bootup is the 'source CPU', the freshly booting
	* CPU is the 'target CPU'.
	*
	* Only two CPUs may participate - they can enter in any order.
	* ( The serial nature of the boot logic and the CPU hotplug lock
	* protects against more than 2 CPUs entering this code. )
	*/
	#include <linux/topology.h>
	#include <linux/spinlock.h>
	#include <linux/kernel.h>
	#include <linux/smp.h>
	#include <linux/nmi.h>
	#include <asm/tsc.h>

	struct tsc_adjust {
	s64 bootval;
	s64 adjusted;
	unsigned long nextcheck;
	bool warned;
	};

	static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust);

	/*
	* TSC's on different sockets may be reset asynchronously.
	* This may cause the TSC ADJUST value on socket 0 to be NOT 0.
	*/
	bool __read_mostly tsc_async_resets;

	void mark_tsc_async_resets(char *reason)
	{
	if (tsc_async_resets)
	return;
	tsc_async_resets = true;
	pr_info("tsc: Marking TSC async resets true due to %s\n", reason);
	}

	void tsc_verify_tsc_adjust(bool resume)
	{
	struct tsc_adjust *adj = this_cpu_ptr(&tsc_adjust);
	s64 curval;

	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
	return;

	/* Skip unnecessary error messages if TSC already unstable */
	if (check_tsc_unstable())
	return;

	/* Rate limit the MSR check */
	if (!resume && time_before(jiffies, adj->nextcheck))
	return;

	adj->nextcheck = jiffies + HZ;

	rdmsrl(MSR_IA32_TSC_ADJUST, curval);
	if (adj->adjusted == curval)
	return;

	/* Restore the original value */
	wrmsrl(MSR_IA32_TSC_ADJUST, adj->adjusted);

	if (!adj->warned \|\| resume) {
	pr_warn(FW_BUG "TSC ADJUST differs: CPU%u %lld --> %lld. Restoring\n",
	smp_processor_id(), adj->adjusted, curval);
	adj->warned = true;
	}
	}

	static void tsc_sanitize_first_cpu(struct tsc_adjust *cur, s64 bootval,
	unsigned int cpu, bool bootcpu)
	{
	/*
	* First online CPU in a package stores the boot value in the
	* adjustment value. This value might change later via the sync
	* mechanism. If that fails we still can yell about boot values not
	* being consistent.
	*
	* On the boot cpu we just force set the ADJUST value to 0 if it's
	* non zero. We don't do that on non boot cpus because physical
	* hotplug should have set the ADJUST register to a value > 0 so
	* the TSC is in sync with the already running cpus.
	*
	* Also don't force the ADJUST value to zero if that is a valid value
	* for socket 0 as determined by the system arch. This is required
	* when multiple sockets are reset asynchronously with each other
	* and socket 0 may not have an TSC ADJUST value of 0.
	*/
	if (bootcpu && bootval != 0) {
	if (likely(!tsc_async_resets)) {
	pr_warn(FW_BUG "TSC ADJUST: CPU%u: %lld force to 0\n",
	cpu, bootval);
	wrmsrl(MSR_IA32_TSC_ADJUST, 0);
	bootval = 0;
	} else {
	pr_info("TSC ADJUST: CPU%u: %lld NOT forced to 0\n",
	cpu, bootval);
	}
	}
	cur->adjusted = bootval;
	}

	#ifndef CONFIG_SMP
	bool __init tsc_store_and_check_tsc_adjust(bool bootcpu)
	{
	struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
	s64 bootval;

	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
	return false;

	/* Skip unnecessary error messages if TSC already unstable */
	if (check_tsc_unstable())
	return false;

	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
	cur->bootval = bootval;
	cur->nextcheck = jiffies + HZ;
	tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(), bootcpu);
	return false;
	}

	#else /* !CONFIG_SMP */

	/*
	* Store and check the TSC ADJUST MSR if available
	*/
	bool tsc_store_and_check_tsc_adjust(bool bootcpu)
	{
	struct tsc_adjust ref, cur = this_cpu_ptr(&tsc_adjust);
	unsigned int refcpu, cpu = smp_processor_id();
	struct cpumask *mask;
	s64 bootval;

	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
	return false;

	rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
	cur->bootval = bootval;
	cur->nextcheck = jiffies + HZ;
	cur->warned = false;

	/*
	* If a non-zero TSC value for socket 0 may be valid then the default
	* adjusted value cannot assumed to be zero either.
	*/
	if (tsc_async_resets)
	cur->adjusted = bootval;

	/*
	* Check whether this CPU is the first in a package to come up. In
	* this case do not check the boot value against another package
	* because the new package might have been physically hotplugged,
	* where TSC_ADJUST is expected to be different. When called on the
	* boot CPU topology_core_cpumask() might not be available yet.
	*/
	mask = topology_core_cpumask(cpu);
	refcpu = mask ? cpumask_any_but(mask, cpu) : nr_cpu_ids;

	if (refcpu >= nr_cpu_ids) {
	tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(),
	bootcpu);
	return false;
	}

	ref = per_cpu_ptr(&tsc_adjust, refcpu);
	/*
	* Compare the boot value and complain if it differs in the
	* package.
	*/
	if (bootval != ref->bootval)
	printk_once(FW_BUG "TSC ADJUST differs within socket(s), fixing all errors\n");

	/*
	* The TSC_ADJUST values in a package must be the same. If the boot
	* value on this newly upcoming CPU differs from the adjustment
	* value of the already online CPU in this package, set it to that
	* adjusted value.
	*/
	if (bootval != ref->adjusted) {
	cur->adjusted = ref->adjusted;
	wrmsrl(MSR_IA32_TSC_ADJUST, ref->adjusted);
	}
	/*
	* We have the TSCs forced to be in sync on this package. Skip sync
	* test:
	*/
	return true;
	}

	/*
	* Entry/exit counters that make sure that both CPUs
	* run the measurement code at once:
	*/
	static atomic_t start_count;
	static atomic_t stop_count;
	static atomic_t skip_test;
	static atomic_t test_runs;

	/*
	* We use a raw spinlock in this exceptional case, because
	* we want to have the fastest, inlined, non-debug version
	* of a critical section, to be able to prove TSC time-warps:
	*/
	static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;

	static cycles_t last_tsc;
	static cycles_t max_warp;
	static int nr_warps;
	static int random_warps;

	/*
	* TSC-warp measurement loop running on both CPUs. This is not called
	* if there is no TSC.
	*/
	static cycles_t check_tsc_warp(unsigned int timeout)
	{
	cycles_t start, now, prev, end, cur_max_warp = 0;
	int i, cur_warps = 0;

	start = rdtsc_ordered();
	/*
	* The measurement runs for 'timeout' msecs:
	*/
	end = start + (cycles_t) tsc_khz * timeout;

	for (i = 0; ; i++) {
	/*
	* We take the global lock, measure TSC, save the
	* previous TSC that was measured (possibly on
	* another CPU) and update the previous TSC timestamp.
	*/
	arch_spin_lock(&sync_lock);
	prev = last_tsc;
	now = rdtsc_ordered();
	last_tsc = now;
	arch_spin_unlock(&sync_lock);

	/*
	* Be nice every now and then (and also check whether
	* measurement is done [we also insert a 10 million
	* loops safety exit, so we dont lock up in case the
	* TSC readout is totally broken]):
	*/
	if (unlikely(!(i & 7))) {
	if (now > end \|\| i > 10000000)
	break;
	cpu_relax();
	touch_nmi_watchdog();
	}
	/*
	* Outside the critical section we can now see whether
	* we saw a time-warp of the TSC going backwards:
	*/
	if (unlikely(prev > now)) {
	arch_spin_lock(&sync_lock);
	max_warp = max(max_warp, prev - now);
	cur_max_warp = max_warp;
	/*
	* Check whether this bounces back and forth. Only
	* one CPU should observe time going backwards.
	*/
	if (cur_warps != nr_warps)
	random_warps++;
	nr_warps++;
	cur_warps = nr_warps;
	arch_spin_unlock(&sync_lock);
	}
	}
	WARN(!(now-start),
	"Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
	now-start, end-start);
	return cur_max_warp;
	}

	/*
	* If the target CPU coming online doesn't have any of its core-siblings
	* online, a timeout of 20msec will be used for the TSC-warp measurement
	* loop. Otherwise a smaller timeout of 2msec will be used, as we have some
	* information about this socket already (and this information grows as we
	* have more and more logical-siblings in that socket).
	*
	* Ideally we should be able to skip the TSC sync check on the other
	* core-siblings, if the first logical CPU in a socket passed the sync test.
	* But as the TSC is per-logical CPU and can potentially be modified wrongly
	* by the bios, TSC sync test for smaller duration should be able
	* to catch such errors. Also this will catch the condition where all the
	* cores in the socket doesn't get reset at the same time.
	*/
	static inline unsigned int loop_timeout(int cpu)
	{
	return (cpumask_weight(topology_core_cpumask(cpu)) > 1) ? 2 : 20;
	}

	/*
	* Source CPU calls into this - it waits for the freshly booted
	* target CPU to arrive and then starts the measurement:
	*/
	void check_tsc_sync_source(int cpu)
	{
	int cpus = 2;

	/*
	* No need to check if we already know that the TSC is not
	* synchronized or if we have no TSC.
	*/
	if (unsynchronized_tsc())
	return;

	/*
	* Set the maximum number of test runs to
	* 1 if the CPU does not provide the TSC_ADJUST MSR
	* 3 if the MSR is available, so the target can try to adjust
	*/
	if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
	atomic_set(&test_runs, 1);
	else
	atomic_set(&test_runs, 3);
	retry:
	/*
	* Wait for the target to start or to skip the test:
	*/
	while (atomic_read(&start_count) != cpus - 1) {
	if (atomic_read(&skip_test) > 0) {
	atomic_set(&skip_test, 0);
	return;
	}
	cpu_relax();
	}

	/*
	* Trigger the target to continue into the measurement too:
	*/
	atomic_inc(&start_count);

	check_tsc_warp(loop_timeout(cpu));

	while (atomic_read(&stop_count) != cpus-1)
	cpu_relax();

	/*
	* If the test was successful set the number of runs to zero and
	* stop. If not, decrement the number of runs an check if we can
	* retry. In case of random warps no retry is attempted.
	*/
	if (!nr_warps) {
	atomic_set(&test_runs, 0);

	pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
	smp_processor_id(), cpu);

	} else if (atomic_dec_and_test(&test_runs) \|\| random_warps) {
	/* Force it to 0 if random warps brought us here */
	atomic_set(&test_runs, 0);

	pr_warn("TSC synchronization [CPU#%d -> CPU#%d]:\n",
	smp_processor_id(), cpu);
	pr_warn("Measured %Ld cycles TSC warp between CPUs, "
	"turning off TSC clock.\n", max_warp);
	if (random_warps)
	pr_warn("TSC warped randomly between CPUs\n");
	mark_tsc_unstable("check_tsc_sync_source failed");
	}

	/*
	* Reset it - just in case we boot another CPU later:
	*/
	atomic_set(&start_count, 0);
	random_warps = 0;
	nr_warps = 0;
	max_warp = 0;
	last_tsc = 0;

	/*
	* Let the target continue with the bootup:
	*/
	atomic_inc(&stop_count);

	/*
	* Retry, if there is a chance to do so.
	*/
	if (atomic_read(&test_runs) > 0)
	goto retry;
	}

	/*
	* Freshly booted CPUs call into this:
	*/
	void check_tsc_sync_target(void)
	{
	struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
	unsigned int cpu = smp_processor_id();
	cycles_t cur_max_warp, gbl_max_warp;
	int cpus = 2;

	/* Also aborts if there is no TSC. */
	if (unsynchronized_tsc())
	return;

	/*
	* Store, verify and sanitize the TSC adjust register. If
	* successful skip the test.
	*
	* The test is also skipped when the TSC is marked reliable. This
	* is true for SoCs which have no fallback clocksource. On these
	* SoCs the TSC is frequency synchronized, but still the TSC ADJUST
	* register might have been wreckaged by the BIOS..
	*/
	if (tsc_store_and_check_tsc_adjust(false) \|\| tsc_clocksource_reliable) {
	atomic_inc(&skip_test);
	return;
	}

	retry:
	/*
	* Register this CPU's participation and wait for the
	* source CPU to start the measurement:
	*/
	atomic_inc(&start_count);
	while (atomic_read(&start_count) != cpus)
	cpu_relax();

	cur_max_warp = check_tsc_warp(loop_timeout(cpu));

	/*
	* Store the maximum observed warp value for a potential retry:
	*/
	gbl_max_warp = max_warp;

	/*
	* Ok, we are done:
	*/
	atomic_inc(&stop_count);

	/*
	* Wait for the source CPU to print stuff:
	*/
	while (atomic_read(&stop_count) != cpus)
	cpu_relax();

	/*
	* Reset it for the next sync test:
	*/
	atomic_set(&stop_count, 0);

	/*
	* Check the number of remaining test runs. If not zero, the test
	* failed and a retry with adjusted TSC is possible. If zero the
	* test was either successful or failed terminally.
	*/
	if (!atomic_read(&test_runs))
	return;

	/*
	* If the warp value of this CPU is 0, then the other CPU
	* observed time going backwards so this TSC was ahead and
	* needs to move backwards.
	*/
	if (!cur_max_warp)
	cur_max_warp = -gbl_max_warp;

	/*
	* Add the result to the previous adjustment value.
	*
	* The adjustement value is slightly off by the overhead of the
	* sync mechanism (observed values are ~200 TSC cycles), but this
	* really depends on CPU, node distance and frequency. So
	* compensating for this is hard to get right. Experiments show
	* that the warp is not longer detectable when the observed warp
	* value is used. In the worst case the adjustment needs to go
	* through a 3rd run for fine tuning.
	*/
	cur->adjusted += cur_max_warp;

	pr_warn("TSC ADJUST compensate: CPU%u observed %lld warp. Adjust: %lld\n",
	cpu, cur_max_warp, cur->adjusted);

	wrmsrl(MSR_IA32_TSC_ADJUST, cur->adjusted);
	goto retry;

	}

	#endif /* CONFIG_SMP */