linux-kernel - [PATCH 4/7] KVM: x86/pmu: Not to generate PEBS records for emulated instructions

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Date:   Wed, 13 Jul 2022 20:25:03 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Like Xu <likexu@...cent.com>
Subject: [PATCH 4/7] KVM: x86/pmu: Not to generate PEBS records for emulated instructions

From: Like Xu <likexu@...cent.com>

The KVM accumulate an enabeld counter for at least INSTRUCTIONS or
BRANCH_INSTRUCTION hw event from any KVM emulated instructions,
generating emulated overflow interrupt on counter overflow, which
in theory should also happen when the PEBS counter overflows but
it currently lacks this part of the underlying support (e.g. through
software injection of records in the irq context or a lazy approach).

In this case, KVM skips the injection of this BUFFER_OVF PMI (effectively
dropping one PEBS record) and let the overflow counter move on. The loss
of a single sample does not introduce a loss of accuracy, but is easily
noticeable for certain specific instructions.

This issue is expected to be addressed along with the issue
of PEBS cross-mapped counters with a slow-path proposal.

Fixes: 79f3e3b58386 ("KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter")
Signed-off-by: Like Xu <likexu@...cent.com>
---
 arch/x86/kvm/pmu.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 02f9e4f245bd..08ee0fed63d5 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -106,9 +106,14 @@ static inline void __kvm_perf_overflow(struct kvm_pmc *pmc, bool in_pmi)
 		return;

 	if (pmc->perf_event && pmc->perf_event->attr.precise_ip) {
-		/* Indicate PEBS overflow PMI to guest. */
-		skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
-					      (unsigned long *)&pmu->global_status);
+		if (!in_pmi) {
+			/* The emulated instructions does not generate PEBS records. */
+			skip_pmi = true;
+		} else {
+			/* Indicate PEBS overflow PMI to guest. */
+			skip_pmi = __test_and_set_bit(GLOBAL_STATUS_BUFFER_OVF_BIT,
+						      (unsigned long *)&pmu->global_status);
+		}
 	} else {
 		__set_bit(pmc->idx, (unsigned long *)&pmu->global_status);
 	}
-- 
2.37.0