From: dongsheng During the execution of __measure(), VM exits (e.g., due to WRMSR/EXTERNAL_INTERRUPT) may occur. On systems affected by the instruction overcount issue, each VM-Exit/VM-Entry can erroneously increment the instruction count by one, leading to false failures in overflow tests. To address this, the patch introduces a range-based validation in place of precise instruction count checks. Additionally, overflow_preset is now statically set to 1 - LOOP_INSNS, rather than being dynamically determined via measure_for_overflow(). These changes ensure consistent and predictable behavior aligned with the intended loop instruction count, while avoiding modifications to the subsequent status and status-clear testing logic. The chosen validation range is empirically derived to maintain test reliability across hardware variations. Signed-off-by: dongsheng Signed-off-by: Dapeng Mi Tested-by: Yi Lai --- x86/pmu.c | 23 +++++++++++++++++++++-- 1 file changed, 21 insertions(+), 2 deletions(-) diff --git a/x86/pmu.c b/x86/pmu.c index 44c728a5..c54c0988 100644 --- a/x86/pmu.c +++ b/x86/pmu.c @@ -518,6 +518,21 @@ static void check_counters_many(void) static uint64_t measure_for_overflow(pmu_counter_t *cnt) { + /* + * During the execution of __measure(), VM exits (e.g., due to + * WRMSR/EXTERNAL_INTERRUPT) may occur. On systems affected by the + * instruction overcount issue, each VM-Exit/VM-Entry can erroneously + * increment the instruction count by one, leading to false failures + * in overflow tests. + * + * To mitigate this, if the overcount issue is detected, we hardcode + * the overflow preset to (1 - LOOP_INSNS) instead of calculating it + * dynamically. This ensures that an overflow will reliably occur, + * regardless of any overcounting caused by VM exits. + */ + if (intel_inst_overcount_flags & INST_RETIRED_OVERCOUNT) + return 1 - LOOP_INSNS; + __measure(cnt, 0); /* * To generate overflow, i.e. roll over to '0', the initial count just @@ -574,8 +589,12 @@ static void check_counter_overflow(void) cnt.config &= ~EVNTSEL_INT; idx = event_to_global_idx(&cnt); __measure(&cnt, cnt.count); - if (pmu.is_intel) - report(cnt.count == 1, "cntr-%d", i); + if (pmu.is_intel) { + if (intel_inst_overcount_flags & INST_RETIRED_OVERCOUNT) + report(cnt.count < 14, "cntr-%d", i); + else + report(cnt.count == 1, "cntr-%d", i); + } else report(cnt.count == 0xffffffffffff || cnt.count < 7, "cntr-%d", i); -- 2.34.1