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KVM: x86: never specify a sample period for virtualized in_tx_cp counters

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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, if a KVM guest does

    struct perf_event_attr attr;
    memset(&attr, 0, sizeof(attr));
    attr.type = PERF_TYPE_RAW;
    attr.size = sizeof(attr);
    attr.config = 0x2005101c4; // conditional branches retired IN_TXCP
    attr.sample_period = 0;
    int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
    ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

the guest kernel counts some conditional branch events, then updates the
virtual PMU register with a nonzero count. The host reaches
pmc_reprogram_counter() with nonzero pmc->counter, triggers EOPNOTSUPP
in hsw_hw_config(), prints "kvm_pmu: event creation failed" in
pmc_reprogram_counter(), and silently (from the guest's point of view) stops
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@ocallahan.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
This commit is contained in:
Robert O'Callahan 2017-02-01 17:06:11 +13:00 committed by Radim Krčmář
parent fd7e9a8834
commit bba82fd756
1 changed files with 10 additions and 3 deletions
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13
arch/x86/kvm/pmu.c
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@ -113,12 +113,19 @@ 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 :