1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Copyright 2014, Michael Ellerman, IBM Corp.
4 */
5
6 #include <sched.h>
7 #include <signal.h>
8 #include <stdio.h>
9 #include <stdlib.h>
10 #include <sys/mman.h>
11 #include <time.h>
12
13 #include "ebb.h"
14
15
16 /*
17 * Test that tries to trigger CPU_FTR_PMAO_BUG. Which is a hardware defect
18 * where an exception triggers but we context switch before it is delivered and
19 * lose the exception.
20 */
21
test_body(void)22 static int test_body(void)
23 {
24 int i, orig_period, max_period;
25 struct event event;
26 struct timespec ts = { .tv_sec = 0, .tv_nsec = 1 };
27
28 SKIP_IF(!ebb_is_supported());
29
30 /* We use PMC4 to make sure the kernel switches all counters correctly */
31 event_init_named(&event, 0x40002, "instructions");
32 event_leader_ebb_init(&event);
33
34 event.attr.exclude_kernel = 1;
35 event.attr.exclude_hv = 1;
36 event.attr.exclude_idle = 1;
37
38 FAIL_IF(event_open(&event));
39
40 ebb_enable_pmc_counting(4);
41 setup_ebb_handler(standard_ebb_callee);
42 ebb_global_enable();
43 FAIL_IF(ebb_event_enable(&event));
44
45 /*
46 * We want a low sample period, but we also want to get out of the EBB
47 * handler without tripping up again.
48 *
49 * This value picked after much experimentation.
50 */
51 orig_period = max_period = sample_period = 400;
52
53 mtspr(SPRN_PMC4, pmc_sample_period(sample_period));
54
55 while (ebb_state.stats.ebb_count < 1000000) {
56 /*
57 * We are trying to get the EBB exception to race exactly with
58 * us entering the kernel to do the syscall. We then need the
59 * kernel to decide our timeslice is up and context switch to
60 * the other thread. When we come back our EBB will have been
61 * lost and we'll spin in this while loop forever.
62 *
63 * Use nanosleep(0) instead of sched_yield() to guarantee a
64 * context switch to the eat_cpu child regardless of the
65 * eligibility state. sched_yield() via yield_task_fair() may
66 * become a no-op when the task is ineligible (vruntime ahead
67 * of avg_vruntime), preventing the required context switch.
68 */
69 for (i = 0; i < 100000; i++)
70 nanosleep(&ts, NULL);
71
72 /* Change the sample period slightly to try and hit the race */
73 if (sample_period >= (orig_period + 200))
74 sample_period = orig_period;
75 else
76 sample_period++;
77
78 if (sample_period > max_period)
79 max_period = sample_period;
80 }
81
82 ebb_freeze_pmcs();
83 ebb_global_disable();
84
85 mtspr(SPRN_PMC4, 0xdead);
86
87 dump_summary_ebb_state();
88 dump_ebb_hw_state();
89
90 event_close(&event);
91
92 FAIL_IF(ebb_state.stats.ebb_count == 0);
93
94 /* We vary our sample period so we need extra fudge here */
95 FAIL_IF(!ebb_check_count(4, orig_period, 2 * (max_period - orig_period)));
96
97 return 0;
98 }
99
lost_exception(void)100 static int lost_exception(void)
101 {
102 return eat_cpu(test_body);
103 }
104
main(void)105 int main(void)
106 {
107 test_harness_set_timeout(300);
108 return test_harness(lost_exception, "lost_exception");
109 }
110