[Qemu-devel] [kvm-unit-tests PATCHv3 2/3] arm: pmu: Check cycle count increases

[Christopher Covington <cov@codeaurora.org>]

Ensure that reads of the PMCCNTR_EL0 are monotonically increasing,
even for the smallest delta of two subsequent reads.

Signed-off-by: Christopher Covington <cov@codeaurora.org>
---
 arm/pmu.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/arm/pmu.c b/arm/pmu.c
index 91a3688..589e605 100644
--- a/arm/pmu.c
+++ b/arm/pmu.c
@@ -33,6 +33,8 @@ struct pmu_data {
 	};
 };
 
+static const int samples = 10;
+
 /* As a simple sanity check on the PMCR_EL0, ensure the implementer field isn't
  * null. Also print out a couple other interesting fields for diagnostic
  * purposes. For example, as of fall 2015, QEMU TCG mode doesn't implement
@@ -56,11 +58,38 @@ static bool check_pmcr(void)
 	return false;
 }
 
+/* Ensure that the cycle counter progresses between back-to-back reads.
+ */
+static bool check_cycles_increase(void)
+{
+	struct pmu_data pmcr;
+
+	pmcr.enable = 1;
+	asm volatile("msr pmcr_el0, %0" : : "r" (pmcr));
+
+	for (int i = 0; i < samples; i++) {
+		int a, b;
+
+		asm volatile(
+			"mrs %[a], pmccntr_el0\n"
+			"mrs %[b], pmccntr_el0\n"
+		: [a] "=r" (a), [b] "=r" (b));
+
+		if (a >= b) {
+			printf("Read %d then %d.\n", a, b);
+			return false;
+		}
+	}
+
+	return true;
+}
+
 int main(void)
 {
 	report_prefix_push("pmu");
 
 	report("Control register", check_pmcr());
+	report("Monotonically increasing cycle count", check_cycles_increase());
 
 	return report_summary();
 }
-- 
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