1 /*
2  * Copyright 2014, Michael Ellerman, IBM Corp.
3  * Licensed under GPLv2.
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 	/* We use PMC4 to make sure the kernel switches all counters correctly */
27 	event_init_named(&event, 0x40002, "instructions");
28 	event_leader_ebb_init(&event);
29 
30 	event.attr.exclude_kernel = 1;
31 	event.attr.exclude_hv = 1;
32 	event.attr.exclude_idle = 1;
33 
34 	FAIL_IF(event_open(&event));
35 
36 	ebb_enable_pmc_counting(4);
37 	setup_ebb_handler(standard_ebb_callee);
38 	ebb_global_enable();
39 	FAIL_IF(ebb_event_enable(&event));
40 
41 	/*
42 	 * We want a low sample period, but we also want to get out of the EBB
43 	 * handler without tripping up again.
44 	 *
45 	 * This value picked after much experimentation.
46 	 */
47 	orig_period = max_period = sample_period = 400;
48 
49 	mtspr(SPRN_PMC4, pmc_sample_period(sample_period));
50 
51 	while (ebb_state.stats.ebb_count < 1000000) {
52 		/*
53 		 * We are trying to get the EBB exception to race exactly with
54 		 * us entering the kernel to do the syscall. We then need the
55 		 * kernel to decide our timeslice is up and context switch to
56 		 * the other thread. When we come back our EBB will have been
57 		 * lost and we'll spin in this while loop forever.
58 		 */
59 
60 		for (i = 0; i < 100000; i++)
61 			sched_yield();
62 
63 		/* Change the sample period slightly to try and hit the race */
64 		if (sample_period >= (orig_period + 200))
65 			sample_period = orig_period;
66 		else
67 			sample_period++;
68 
69 		if (sample_period > max_period)
70 			max_period = sample_period;
71 	}
72 
73 	ebb_freeze_pmcs();
74 	ebb_global_disable();
75 
76 	count_pmc(4, sample_period);
77 	mtspr(SPRN_PMC4, 0xdead);
78 
79 	dump_summary_ebb_state();
80 	dump_ebb_hw_state();
81 
82 	event_close(&event);
83 
84 	FAIL_IF(ebb_state.stats.ebb_count == 0);
85 
86 	/* We vary our sample period so we need extra fudge here */
87 	FAIL_IF(!ebb_check_count(4, orig_period, 2 * (max_period - orig_period)));
88 
89 	return 0;
90 }
91 
92 static int lost_exception(void)
93 {
94 	return eat_cpu(test_body);
95 }
96 
97 int main(void)
98 {
99 	return test_harness(lost_exception, "lost_exception");
100 }
101