1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * check TSC synchronization.
4 *
5 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
6 *
7 * We check whether all boot CPUs have their TSC's synchronized,
8 * print a warning if not and turn off the TSC clock-source.
9 *
10 * The warp-check is point-to-point between two CPUs, the CPU
11 * initiating the bootup is the 'source CPU', the freshly booting
12 * CPU is the 'target CPU'.
13 *
14 * Only two CPUs may participate - they can enter in any order.
15 * ( The serial nature of the boot logic and the CPU hotplug lock
16 * protects against more than 2 CPUs entering this code. )
17 */
18 #include <linux/topology.h>
19 #include <linux/spinlock.h>
20 #include <linux/kernel.h>
21 #include <linux/smp.h>
22 #include <linux/nmi.h>
23 #include <asm/tsc.h>
24
25 struct tsc_adjust {
26 s64 bootval;
27 s64 adjusted;
28 unsigned long nextcheck;
29 bool warned;
30 };
31
32 static DEFINE_PER_CPU(struct tsc_adjust, tsc_adjust);
33
34 /*
35 * TSC's on different sockets may be reset asynchronously.
36 * This may cause the TSC ADJUST value on socket 0 to be NOT 0.
37 */
38 bool __read_mostly tsc_async_resets;
39
40 void mark_tsc_async_resets(char *reason)
41 {
42 if (tsc_async_resets)
43 return;
44 tsc_async_resets = true;
45 pr_info("tsc: Marking TSC async resets true due to %s\n", reason);
46 }
47
48 void tsc_verify_tsc_adjust(bool resume)
49 {
50 struct tsc_adjust *adj = this_cpu_ptr(&tsc_adjust);
51 s64 curval;
52
53 if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
54 return;
55
56 /* Skip unnecessary error messages if TSC already unstable */
57 if (check_tsc_unstable())
58 return;
59
60 /* Rate limit the MSR check */
61 if (!resume && time_before(jiffies, adj->nextcheck))
62 return;
63
64 adj->nextcheck = jiffies + HZ;
65
66 rdmsrl(MSR_IA32_TSC_ADJUST, curval);
67 if (adj->adjusted == curval)
68 return;
69
70 /* Restore the original value */
71 wrmsrl(MSR_IA32_TSC_ADJUST, adj->adjusted);
72
73 if (!adj->warned || resume) {
74 pr_warn(FW_BUG "TSC ADJUST differs: CPU%u %lld --> %lld. Restoring\n",
75 smp_processor_id(), adj->adjusted, curval);
76 adj->warned = true;
77 }
78 }
79
80 static void tsc_sanitize_first_cpu(struct tsc_adjust *cur, s64 bootval,
81 unsigned int cpu, bool bootcpu)
82 {
83 /*
84 * First online CPU in a package stores the boot value in the
85 * adjustment value. This value might change later via the sync
86 * mechanism. If that fails we still can yell about boot values not
87 * being consistent.
88 *
89 * On the boot cpu we just force set the ADJUST value to 0 if it's
90 * non zero. We don't do that on non boot cpus because physical
91 * hotplug should have set the ADJUST register to a value > 0 so
92 * the TSC is in sync with the already running cpus.
93 *
94 * Also don't force the ADJUST value to zero if that is a valid value
95 * for socket 0 as determined by the system arch. This is required
96 * when multiple sockets are reset asynchronously with each other
97 * and socket 0 may not have an TSC ADJUST value of 0.
98 */
99 if (bootcpu && bootval != 0) {
100 if (likely(!tsc_async_resets)) {
101 pr_warn(FW_BUG "TSC ADJUST: CPU%u: %lld force to 0\n",
102 cpu, bootval);
103 wrmsrl(MSR_IA32_TSC_ADJUST, 0);
104 bootval = 0;
105 } else {
106 pr_info("TSC ADJUST: CPU%u: %lld NOT forced to 0\n",
107 cpu, bootval);
108 }
109 }
110 cur->adjusted = bootval;
111 }
112
113 #ifndef CONFIG_SMP
114 bool __init tsc_store_and_check_tsc_adjust(bool bootcpu)
115 {
116 struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
117 s64 bootval;
118
119 if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
120 return false;
121
122 /* Skip unnecessary error messages if TSC already unstable */
123 if (check_tsc_unstable())
124 return false;
125
126 rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
127 cur->bootval = bootval;
128 cur->nextcheck = jiffies + HZ;
129 tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(), bootcpu);
130 return false;
131 }
132
133 #else /* !CONFIG_SMP */
134
135 /*
136 * Store and check the TSC ADJUST MSR if available
137 */
138 bool tsc_store_and_check_tsc_adjust(bool bootcpu)
139 {
140 struct tsc_adjust *ref, *cur = this_cpu_ptr(&tsc_adjust);
141 unsigned int refcpu, cpu = smp_processor_id();
142 struct cpumask *mask;
143 s64 bootval;
144
145 if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
146 return false;
147
148 rdmsrl(MSR_IA32_TSC_ADJUST, bootval);
149 cur->bootval = bootval;
150 cur->nextcheck = jiffies + HZ;
151 cur->warned = false;
152
153 /*
154 * If a non-zero TSC value for socket 0 may be valid then the default
155 * adjusted value cannot assumed to be zero either.
156 */
157 if (tsc_async_resets)
158 cur->adjusted = bootval;
159
160 /*
161 * Check whether this CPU is the first in a package to come up. In
162 * this case do not check the boot value against another package
163 * because the new package might have been physically hotplugged,
164 * where TSC_ADJUST is expected to be different. When called on the
165 * boot CPU topology_core_cpumask() might not be available yet.
166 */
167 mask = topology_core_cpumask(cpu);
168 refcpu = mask ? cpumask_any_but(mask, cpu) : nr_cpu_ids;
169
170 if (refcpu >= nr_cpu_ids) {
171 tsc_sanitize_first_cpu(cur, bootval, smp_processor_id(),
172 bootcpu);
173 return false;
174 }
175
176 ref = per_cpu_ptr(&tsc_adjust, refcpu);
177 /*
178 * Compare the boot value and complain if it differs in the
179 * package.
180 */
181 if (bootval != ref->bootval)
182 printk_once(FW_BUG "TSC ADJUST differs within socket(s), fixing all errors\n");
183
184 /*
185 * The TSC_ADJUST values in a package must be the same. If the boot
186 * value on this newly upcoming CPU differs from the adjustment
187 * value of the already online CPU in this package, set it to that
188 * adjusted value.
189 */
190 if (bootval != ref->adjusted) {
191 cur->adjusted = ref->adjusted;
192 wrmsrl(MSR_IA32_TSC_ADJUST, ref->adjusted);
193 }
194 /*
195 * We have the TSCs forced to be in sync on this package. Skip sync
196 * test:
197 */
198 return true;
199 }
200
201 /*
202 * Entry/exit counters that make sure that both CPUs
203 * run the measurement code at once:
204 */
205 static atomic_t start_count;
206 static atomic_t stop_count;
207 static atomic_t skip_test;
208 static atomic_t test_runs;
209
210 /*
211 * We use a raw spinlock in this exceptional case, because
212 * we want to have the fastest, inlined, non-debug version
213 * of a critical section, to be able to prove TSC time-warps:
214 */
215 static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
216
217 static cycles_t last_tsc;
218 static cycles_t max_warp;
219 static int nr_warps;
220 static int random_warps;
221
222 /*
223 * TSC-warp measurement loop running on both CPUs. This is not called
224 * if there is no TSC.
225 */
226 static cycles_t check_tsc_warp(unsigned int timeout)
227 {
228 cycles_t start, now, prev, end, cur_max_warp = 0;
229 int i, cur_warps = 0;
230
231 start = rdtsc_ordered();
232 /*
233 * The measurement runs for 'timeout' msecs:
234 */
235 end = start + (cycles_t) tsc_khz * timeout;
236 now = start;
237
238 for (i = 0; ; i++) {
239 /*
240 * We take the global lock, measure TSC, save the
241 * previous TSC that was measured (possibly on
242 * another CPU) and update the previous TSC timestamp.
243 */
244 arch_spin_lock(&sync_lock);
245 prev = last_tsc;
246 now = rdtsc_ordered();
247 last_tsc = now;
248 arch_spin_unlock(&sync_lock);
249
250 /*
251 * Be nice every now and then (and also check whether
252 * measurement is done [we also insert a 10 million
253 * loops safety exit, so we dont lock up in case the
254 * TSC readout is totally broken]):
255 */
256 if (unlikely(!(i & 7))) {
257 if (now > end || i > 10000000)
258 break;
259 cpu_relax();
260 touch_nmi_watchdog();
261 }
262 /*
263 * Outside the critical section we can now see whether
264 * we saw a time-warp of the TSC going backwards:
265 */
266 if (unlikely(prev > now)) {
267 arch_spin_lock(&sync_lock);
268 max_warp = max(max_warp, prev - now);
269 cur_max_warp = max_warp;
270 /*
271 * Check whether this bounces back and forth. Only
272 * one CPU should observe time going backwards.
273 */
274 if (cur_warps != nr_warps)
275 random_warps++;
276 nr_warps++;
277 cur_warps = nr_warps;
278 arch_spin_unlock(&sync_lock);
279 }
280 }
281 WARN(!(now-start),
282 "Warning: zero tsc calibration delta: %Ld [max: %Ld]\n",
283 now-start, end-start);
284 return cur_max_warp;
285 }
286
287 /*
288 * If the target CPU coming online doesn't have any of its core-siblings
289 * online, a timeout of 20msec will be used for the TSC-warp measurement
290 * loop. Otherwise a smaller timeout of 2msec will be used, as we have some
291 * information about this socket already (and this information grows as we
292 * have more and more logical-siblings in that socket).
293 *
294 * Ideally we should be able to skip the TSC sync check on the other
295 * core-siblings, if the first logical CPU in a socket passed the sync test.
296 * But as the TSC is per-logical CPU and can potentially be modified wrongly
297 * by the bios, TSC sync test for smaller duration should be able
298 * to catch such errors. Also this will catch the condition where all the
299 * cores in the socket doesn't get reset at the same time.
300 */
301 static inline unsigned int loop_timeout(int cpu)
302 {
303 return (cpumask_weight(topology_core_cpumask(cpu)) > 1) ? 2 : 20;
304 }
305
306 /*
307 * Source CPU calls into this - it waits for the freshly booted
308 * target CPU to arrive and then starts the measurement:
309 */
310 void check_tsc_sync_source(int cpu)
311 {
312 int cpus = 2;
313
314 /*
315 * No need to check if we already know that the TSC is not
316 * synchronized or if we have no TSC.
317 */
318 if (unsynchronized_tsc())
319 return;
320
321 /*
322 * Set the maximum number of test runs to
323 * 1 if the CPU does not provide the TSC_ADJUST MSR
324 * 3 if the MSR is available, so the target can try to adjust
325 */
326 if (!boot_cpu_has(X86_FEATURE_TSC_ADJUST))
327 atomic_set(&test_runs, 1);
328 else
329 atomic_set(&test_runs, 3);
330 retry:
331 /*
332 * Wait for the target to start or to skip the test:
333 */
334 while (atomic_read(&start_count) != cpus - 1) {
335 if (atomic_read(&skip_test) > 0) {
336 atomic_set(&skip_test, 0);
337 return;
338 }
339 cpu_relax();
340 }
341
342 /*
343 * Trigger the target to continue into the measurement too:
344 */
345 atomic_inc(&start_count);
346
347 check_tsc_warp(loop_timeout(cpu));
348
349 while (atomic_read(&stop_count) != cpus-1)
350 cpu_relax();
351
352 /*
353 * If the test was successful set the number of runs to zero and
354 * stop. If not, decrement the number of runs an check if we can
355 * retry. In case of random warps no retry is attempted.
356 */
357 if (!nr_warps) {
358 atomic_set(&test_runs, 0);
359
360 pr_debug("TSC synchronization [CPU#%d -> CPU#%d]: passed\n",
361 smp_processor_id(), cpu);
362
363 } else if (atomic_dec_and_test(&test_runs) || random_warps) {
364 /* Force it to 0 if random warps brought us here */
365 atomic_set(&test_runs, 0);
366
367 pr_warning("TSC synchronization [CPU#%d -> CPU#%d]:\n",
368 smp_processor_id(), cpu);
369 pr_warning("Measured %Ld cycles TSC warp between CPUs, "
370 "turning off TSC clock.\n", max_warp);
371 if (random_warps)
372 pr_warning("TSC warped randomly between CPUs\n");
373 mark_tsc_unstable("check_tsc_sync_source failed");
374 }
375
376 /*
377 * Reset it - just in case we boot another CPU later:
378 */
379 atomic_set(&start_count, 0);
380 random_warps = 0;
381 nr_warps = 0;
382 max_warp = 0;
383 last_tsc = 0;
384
385 /*
386 * Let the target continue with the bootup:
387 */
388 atomic_inc(&stop_count);
389
390 /*
391 * Retry, if there is a chance to do so.
392 */
393 if (atomic_read(&test_runs) > 0)
394 goto retry;
395 }
396
397 /*
398 * Freshly booted CPUs call into this:
399 */
400 void check_tsc_sync_target(void)
401 {
402 struct tsc_adjust *cur = this_cpu_ptr(&tsc_adjust);
403 unsigned int cpu = smp_processor_id();
404 cycles_t cur_max_warp, gbl_max_warp;
405 int cpus = 2;
406
407 /* Also aborts if there is no TSC. */
408 if (unsynchronized_tsc())
409 return;
410
411 /*
412 * Store, verify and sanitize the TSC adjust register. If
413 * successful skip the test.
414 *
415 * The test is also skipped when the TSC is marked reliable. This
416 * is true for SoCs which have no fallback clocksource. On these
417 * SoCs the TSC is frequency synchronized, but still the TSC ADJUST
418 * register might have been wreckaged by the BIOS..
419 */
420 if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) {
421 atomic_inc(&skip_test);
422 return;
423 }
424
425 retry:
426 /*
427 * Register this CPU's participation and wait for the
428 * source CPU to start the measurement:
429 */
430 atomic_inc(&start_count);
431 while (atomic_read(&start_count) != cpus)
432 cpu_relax();
433
434 cur_max_warp = check_tsc_warp(loop_timeout(cpu));
435
436 /*
437 * Store the maximum observed warp value for a potential retry:
438 */
439 gbl_max_warp = max_warp;
440
441 /*
442 * Ok, we are done:
443 */
444 atomic_inc(&stop_count);
445
446 /*
447 * Wait for the source CPU to print stuff:
448 */
449 while (atomic_read(&stop_count) != cpus)
450 cpu_relax();
451
452 /*
453 * Reset it for the next sync test:
454 */
455 atomic_set(&stop_count, 0);
456
457 /*
458 * Check the number of remaining test runs. If not zero, the test
459 * failed and a retry with adjusted TSC is possible. If zero the
460 * test was either successful or failed terminally.
461 */
462 if (!atomic_read(&test_runs))
463 return;
464
465 /*
466 * If the warp value of this CPU is 0, then the other CPU
467 * observed time going backwards so this TSC was ahead and
468 * needs to move backwards.
469 */
470 if (!cur_max_warp)
471 cur_max_warp = -gbl_max_warp;
472
473 /*
474 * Add the result to the previous adjustment value.
475 *
476 * The adjustement value is slightly off by the overhead of the
477 * sync mechanism (observed values are ~200 TSC cycles), but this
478 * really depends on CPU, node distance and frequency. So
479 * compensating for this is hard to get right. Experiments show
480 * that the warp is not longer detectable when the observed warp
481 * value is used. In the worst case the adjustment needs to go
482 * through a 3rd run for fine tuning.
483 */
484 cur->adjusted += cur_max_warp;
485
486 pr_warn("TSC ADJUST compensate: CPU%u observed %lld warp. Adjust: %lld\n",
487 cpu, cur_max_warp, cur->adjusted);
488
489 wrmsrl(MSR_IA32_TSC_ADJUST, cur->adjusted);
490 goto retry;
491
492 }
493
494 #endif /* CONFIG_SMP */