[kvm-unit-tests patch v3 4/8] x86/pmu: Handle instruction overcount issue in overflow test

Dapeng Mi
2025-09-03T06:47:34Z

From: dongsheng <dongsheng.x.zhang@intel.com>

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 <dongsheng.x.zhang@intel.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Tested-by: Yi Lai <yi1.lai@intel.com>
---
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