1/*
2 * check TSC synchronization.
3 *
4 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
5 *
6 * We check whether all boot CPUs have their TSC's synchronized,
7 * print a warning if not and turn off the TSC clock-source.
8 *
9 * The warp-check is point-to-point between two CPUs, the CPU
10 * initiating the bootup is the 'source CPU', the freshly booting
11 * CPU is the 'target CPU'.
12 *
13 * Only two CPUs may participate - they can enter in any order.
14 * ( The serial nature of the boot logic and the CPU hotplug lock
15 *   protects against more than 2 CPUs entering this code. )
16 */
17#include <linux/spinlock.h>
18#include <linux/kernel.h>
19#include <linux/smp.h>
20#include <linux/nmi.h>
21#include <asm/tsc.h>
22
23/*
24 * Entry/exit counters that make sure that both CPUs
25 * run the measurement code at once:
26 */
27static atomic_t start_count;
28static atomic_t stop_count;
29
30/*
31 * We use a raw spinlock in this exceptional case, because
32 * we want to have the fastest, inlined, non-debug version
33 * of a critical section, to be able to prove TSC time-warps:
34 */
35static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
36
37static cycles_t last_tsc;
38static cycles_t max_warp;
39static int nr_warps;
40
41/*
42 * TSC-warp measurement loop running on both CPUs:
43 */
44static void check_tsc_warp(unsigned int timeout)
45{
46	cycles_t start, now, prev, end;
47	int i;
48
49	rdtsc_barrier();
50	start = get_cycles();
51	rdtsc_barrier();
52	/*
53	 * The measurement runs for 'timeout' msecs:
54	 */
55	end = start + (cycles_t) tsc_khz * timeout;
56	now = start;
57
58	for (i = 0; ; i++) {
59		/*
60		 * We take the global lock, measure TSC, save the
61		 * previous TSC that was measured (possibly on
62		 * another CPU) and update the previous TSC timestamp.
63		 */
64		arch_spin_lock(&sync_lock);
65		prev = last_tsc;
66		rdtsc_barrier();
67		now = get_cycles();
68		rdtsc_barrier();
69		last_tsc = now;
70		arch_spin_unlock(&sync_lock);
71
72		/*
73		 * Be nice every now and then (and also check whether
74		 * measurement is done [we also insert a 10 million
75		 * loops safety exit, so we dont lock up in case the
76		 * TSC readout is totally broken]):
77		 */
78		if (unlikely(!(i & 7))) {
79			if (now > end || i > 10000000)
80				break;
81			cpu_relax();
82			touch_nmi_watchdog();
83		}
84		/*
85		 * Outside the critical section we can now see whether
86		 * we saw a time-warp of the TSC going backwards:
87		 */
88		if (unlikely(prev > now)) {
89			arch_spin_lock(&sync_lock);
90			max_warp = max(max_warp, prev - now);
91			nr_warps++;
92			arch_spin_unlock(&sync_lock);
93		}
94	}
95	WARN(!(now-start),
96		"Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
97			now-start, end-start);
98}
99
100/*
101 * If the target CPU coming online doesn't have any of its core-siblings
102 * online, a timeout of 20msec will be used for the TSC-warp measurement
103 * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
104 * information about this socket already (and this information grows as we
105 * have more and more logical-siblings in that socket).
106 *
107 * Ideally we should be able to skip the TSC sync check on the other
108 * core-siblings, if the first logical CPU in a socket passed the sync test.
109 * But as the TSC is per-logical CPU and can potentially be modified wrongly
110 * by the bios, TSC sync test for smaller duration should be able
111 * to catch such errors. Also this will catch the condition where all the
112 * cores in the socket doesn't get reset at the same time.
113 */
114static inline unsigned int loop_timeout(int cpu)
115{
116	return (cpumask_weight(cpu_core_mask(cpu)) > 1) ? 2 : 20;
117}
118
119/*
120 * Source CPU calls into this - it waits for the freshly booted
121 * target CPU to arrive and then starts the measurement:
122 */
123void check_tsc_sync_source(int cpu)
124{
125	int cpus = 2;
126
127	/*
128	 * No need to check if we already know that the TSC is not
129	 * synchronized:
130	 */
131	if (unsynchronized_tsc())
132		return;
133
134	if (tsc_clocksource_reliable) {
135		if (cpu == (nr_cpu_ids-1) || system_state != SYSTEM_BOOTING)
136			pr_info(
137			"Skipped synchronization checks as TSC is reliable.\n");
138		return;
139	}
140
141	/*
142	 * Reset it - in case this is a second bootup:
143	 */
144	atomic_set(&stop_count, 0);
145
146	/*
147	 * Wait for the target to arrive:
148	 */
149	while (atomic_read(&start_count) != cpus-1)
150		cpu_relax();
151	/*
152	 * Trigger the target to continue into the measurement too:
153	 */
154	atomic_inc(&start_count);
155
156	check_tsc_warp(loop_timeout(cpu));
157
158	while (atomic_read(&stop_count) != cpus-1)
159		cpu_relax();
160
161	if (nr_warps) {
162		pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
163			smp_processor_id(), cpu);
164		pr_warning("Measured %Ld cycles TSC warp between CPUs, "
165			   "turning off TSC clock.\n", max_warp);
166		mark_tsc_unstable("check_tsc_sync_source failed");
167	} else {
168		pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
169			smp_processor_id(), cpu);
170	}
171
172	/*
173	 * Reset it - just in case we boot another CPU later:
174	 */
175	atomic_set(&start_count, 0);
176	nr_warps = 0;
177	max_warp = 0;
178	last_tsc = 0;
179
180	/*
181	 * Let the target continue with the bootup:
182	 */
183	atomic_inc(&stop_count);
184}
185
186/*
187 * Freshly booted CPUs call into this:
188 */
189void check_tsc_sync_target(void)
190{
191	int cpus = 2;
192
193	if (unsynchronized_tsc() || tsc_clocksource_reliable)
194		return;
195
196	/*
197	 * Register this CPU's participation and wait for the
198	 * source CPU to start the measurement:
199	 */
200	atomic_inc(&start_count);
201	while (atomic_read(&start_count) != cpus)
202		cpu_relax();
203
204	check_tsc_warp(loop_timeout(smp_processor_id()));
205
206	/*
207	 * Ok, we are done:
208	 */
209	atomic_inc(&stop_count);
210
211	/*
212	 * Wait for the source CPU to print stuff:
213	 */
214	while (atomic_read(&stop_count) != cpus)
215		cpu_relax();
216}
217