linux-kernel - [PATCH] KVM: x86: never specify a sample period for virtualized in_tx

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Message-Id: <20170131014813.15220-1-robert@ocallahan.org>
Date:   Tue, 31 Jan 2017 14:48:13 +1300
From:   Robert O'Callahan <robert@...llahan.org>
To:     kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Robert O'Callahan <robert@...llahan.org>
Subject: [PATCH] KVM: x86: never specify a sample period for virtualized in_tx_cp counters

pmc_reprogram_counter() always sets a sample period based on the value of
pmc->counter. However, hsw_hw_config() rejects sample periods less than
2^31 - 1. So for example, a KVM guest doing
  perf stat -e r2005101c4 sleep 0
will count some conditional branch events, deschedule the task, reschedule
the task, try to restore the guest PMU state for the task, in the host
reach pmc_reprogram_counter() with nonzero pmc->count, trigger EOPNOTSUPP
in hsw_hw_config(), print "kvm_pmu: event creation failed" in
pmc_reprogram_counter(), and silently (from the guest's point of view) stop
counting events.

We fix event counting by forcing attr.sample_period to always be zero for
in_tx_cp counters. Sampling doesn't work, but it already didn't work and
can't be fixed without major changes to the approach in hsw_hw_config().

Signed-off-by: Robert O'Callahan <robert@...llahan.org>
---
 arch/x86/kvm/pmu.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 06ce377..af993d7 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -113,12 +113,18 @@ static void pmc_reprogram_counter(struct kvm_pmc *pmc, u32 type,
 		.config = config,
 	};

+	attr.sample_period = (-pmc->counter) & pmc_bitmask(pmc);
+
 	if (in_tx)
 		attr.config |= HSW_IN_TX;
-	if (in_tx_cp)
+	if (in_tx_cp) {
+		/* HSW_IN_TX_CHECKPOINTED is not supported with nonzero
+		 * period. Just clear the sample period so at least
+		 * allocating the counter doesn't fail.
+		 */
+		attr.sample_period = 0;
 		attr.config |= HSW_IN_TX_CHECKPOINTED;
-
-	attr.sample_period = (-pmc->counter) & pmc_bitmask(pmc);
+	}

 	event = perf_event_create_kernel_counter(&attr, -1, current,
 						 intr ? kvm_perf_overflow_intr :
-- 
2.9.3