lost_exception_test.c

  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
 21static 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
 93static int lost_exception(void)
 94{
 95	return eat_cpu(test_body);
 96}
 97
 98int main(void)
 99{
100	test_harness_set_timeout(300);
101	return test_harness(lost_exception, "lost_exception");
102}