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 
12 #include "ebb.h"
13 
14 
15 /*
16  * Test that tries to trigger CPU_FTR_PMAO_BUG. Which is a hardware defect
17  * where an exception triggers but we context switch before it is delivered and
18  * lose the exception.
19  */
20 
21 static int test_body(void)
22 {
23 	int i, orig_period, max_period;
24 	struct event event;
25 
26 	SKIP_IF(!ebb_is_supported());
27 
28 	/* We use PMC4 to make sure the kernel switches all counters correctly */
29 	event_init_named(&event, 0x40002, "instructions");
30 	event_leader_ebb_init(&event);
31 
32 	event.attr.exclude_kernel = 1;
33 	event.attr.exclude_hv = 1;
34 	event.attr.exclude_idle = 1;
35 
36 	FAIL_IF(event_open(&event));
37 
38 	ebb_enable_pmc_counting(4);
39 	setup_ebb_handler(standard_ebb_callee);
40 	ebb_global_enable();
41 	FAIL_IF(ebb_event_enable(&event));
42 
43 	/*
44 	 * We want a low sample period, but we also want to get out of the EBB
45 	 * handler without tripping up again.
46 	 *
47 	 * This value picked after much experimentation.
48 	 */
49 	orig_period = max_period = sample_period = 400;
50 
51 	mtspr(SPRN_PMC4, pmc_sample_period(sample_period));
52 
53 	while (ebb_state.stats.ebb_count < 1000000) {
54 		/*
55 		 * We are trying to get the EBB exception to race exactly with
56 		 * us entering the kernel to do the syscall. We then need the
57 		 * kernel to decide our timeslice is up and context switch to
58 		 * the other thread. When we come back our EBB will have been
59 		 * lost and we'll spin in this while loop forever.
60 		 */
61 
62 		for (i = 0; i < 100000; i++)
63 			sched_yield();
64 
65 		/* Change the sample period slightly to try and hit the race */
66 		if (sample_period >= (orig_period + 200))
67 			sample_period = orig_period;
68 		else
69 			sample_period++;
70 
71 		if (sample_period > max_period)
72 			max_period = sample_period;
73 	}
74 
75 	ebb_freeze_pmcs();
76 	ebb_global_disable();
77 
78 	mtspr(SPRN_PMC4, 0xdead);
79 
80 	dump_summary_ebb_state();
81 	dump_ebb_hw_state();
82 
83 	event_close(&event);
84 
85 	FAIL_IF(ebb_state.stats.ebb_count == 0);
86 
87 	/* We vary our sample period so we need extra fudge here */
88 	FAIL_IF(!ebb_check_count(4, orig_period, 2 * (max_period - orig_period)));
89 
90 	return 0;
91 }
92 
93 static int lost_exception(void)
94 {
95 	return eat_cpu(test_body);
96 }
97 
98 int main(void)
99 {
100 	test_harness_set_timeout(300);
101 	return test_harness(lost_exception, "lost_exception");
102 }
103