linux-kernel - Re: [PATCH v2 3/5] KVM: Selftests: Validate more arch-events in pmu_counters

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Message-ID: <aMIO5ZLNur5JkdYl@google.com>
Date: Wed, 10 Sep 2025 16:51:01 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jim Mattson <jmattson@...gle.com>, Mingwei Zhang <mizhang@...gle.com>, 
	Zide Chen <zide.chen@...el.com>, Das Sandipan <Sandipan.Das@....com>, 
	Shukla Manali <Manali.Shukla@....com>, Yi Lai <yi1.lai@...el.com>, 
	Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [PATCH v2 3/5] KVM: Selftests: Validate more arch-events in pmu_counters_test

On Fri, Jul 18, 2025, Dapeng Mi wrote:
> Clearwater Forest introduces 5 new architectural events (4 topdown
> level 1 metrics events and LBR inserts event). This patch supports
> to validate these 5 newly added events. The detailed info about these
> 5 events can be found in SDM section 21.2.7 "Pre-defined Architectural
>  Performance Events".
> 
> It becomes unrealistic to traverse all possible combinations of
> unavailable events mask (may need dozens of minutes to finish all
> possible combination validation). So only limit unavailable events mask
> traverse to the first 8 arch-events.

Split these into separate patches.  Buring a meaningful change like this in a big
patch that seemingly just adds architectural collateral is pure evil.
> @@ -612,15 +620,19 @@ static void test_intel_counters(void)
>  			pr_info("Testing arch events, PMU version %u, perf_caps = %lx\n",
>  				v, perf_caps[i]);
>  			/*
> -			 * To keep the total runtime reasonable, test every
> -			 * possible non-zero, non-reserved bitmap combination
> -			 * only with the native PMU version and the full bit
> -			 * vector length.
> +			 * To keep the total runtime reasonable, especially after
> +			 * the total number of arch-events increasing to 13, It's
> +			 * impossible to test every possible non-zero, non-reserved
> +			 * bitmap combination. Only test the first 8-bits combination
> +			 * with the native PMU version and the full bit vector length.
>  			 */
>  			if (v == pmu_version) {
> -				for (k = 1; k < (BIT(NR_INTEL_ARCH_EVENTS) - 1); k++)
> +				int max_events = min(NR_INTEL_ARCH_EVENTS, 8);

Too arbitrary, and worse, bad coverage.  And honestly, even iterating over 255
(or 512?) different values is a waste of time.  Ha!  And test_arch_events() is
buggy, it takes unavailable_mask as u8 instead of a u32.  I'll slot in a patch
to fix that.

As for the runtime, I think it's time to throw in the towel in terms of brute
forcing the validation space, and just test a handful of hopefully-interesting
values, e.g.

---
 .../selftests/kvm/x86/pmu_counters_test.c     | 38 +++++++++++--------
 1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86/pmu_counters_test.c b/tools/testing/selftests/kvm/x86/pmu_counters_test.c
index cfeed0103341..09ad68675576 100644
--- a/tools/testing/selftests/kvm/x86/pmu_counters_test.c
+++ b/tools/testing/selftests/kvm/x86/pmu_counters_test.c
@@ -577,6 +577,26 @@ static void test_intel_counters(void)
 		PMU_CAP_FW_WRITES,
 	};
 
+	/*
+	 * To keep the total runtime reasonable, test only a handful of select,
+	 * semi-arbitrary values for the mask of unavailable PMU events.  Test
+	 * 0 (all events available) and all ones (no events available) as well
+	 * as alternating bit sequencues, e.g. to detect if KVM is checking the
+	 * wrong bit(s).
+	 */
+	const uint32_t unavailable_masks[] = {
+		0x0,
+		0xffffffffu,
+		0xf0f0f0f0u,
+		0x0f0f0f0fu,
+		0xaaaaaaaau,
+		0xa0a0a0a0u,
+		0x0a0a0a0au,
+		0x55555555u,
+		0x50505050u,
+		0x05050505u,
+	};
+
 	/*
 	 * Test up to PMU v5, which is the current maximum version defined by
 	 * Intel, i.e. is the last version that is guaranteed to be backwards
@@ -614,16 +634,7 @@ static void test_intel_counters(void)
 
 			pr_info("Testing arch events, PMU version %u, perf_caps = %lx\n",
 				v, perf_caps[i]);
-			/*
-			 * To keep the total runtime reasonable, test every
-			 * possible non-zero, non-reserved bitmap combination
-			 * only with the native PMU version and the full bit
-			 * vector length.
-			 */
-			if (v == pmu_version) {
-				for (k = 1; k < (BIT(NR_INTEL_ARCH_EVENTS) - 1); k++)
-					test_arch_events(v, perf_caps[i], NR_INTEL_ARCH_EVENTS, k);
-			}
+
 			/*
 			 * Test single bits for all PMU version and lengths up
 			 * the number of events +1 (to verify KVM doesn't do
@@ -632,11 +643,8 @@ static void test_intel_counters(void)
 			 * ones i.e. all events being available and unavailable.
 			 */
 			for (j = 0; j <= NR_INTEL_ARCH_EVENTS + 1; j++) {
-				test_arch_events(v, perf_caps[i], j, 0);
-				test_arch_events(v, perf_caps[i], j, -1u);
-
-				for (k = 0; k < NR_INTEL_ARCH_EVENTS; k++)
-					test_arch_events(v, perf_caps[i], j, BIT(k));
+				for (k = 1; k < ARRAY_SIZE(unavailable_masks); k++)
+					test_arch_events(v, perf_caps[i], j, unavailable_masks[k]);
 			}
 
 			pr_info("Testing GP counters, PMU version %u, perf_caps = %lx\n",
--