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[PATCH v2 00/01] KVM: x86: never specify a sample period for virtualized in_tx_cp counters

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  [PATCH v2 00/01] KVM: x86: never specify a sample period for virtualized in_tx_cp counters Robert O'Callahan <robert@ocallahan.org> - 2017-02-01 05:10 +0100
    [PATCH v2 01/01] KVM: x86: never specify a sample period for virtualized in_tx_cp counters Robert O'Callahan <robert@ocallahan.org> - 2017-02-01 05:10 +0100

#1571259 — [PATCH v2 00/01] KVM: x86: never specify a sample period for virtualized in_tx_cp counters

Changes in version 2:
- Fixed block comment formatting as requested by Ingo Molnar
- Changed commit summary to refer to a testcase that more easily shows the
problem.

For reference here's a complete testcase for an Intel TSX-enabled KVM guest:

#include <linux/perf_event.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <unistd.h>
int main(void) {
    struct perf_event_attr attr;
    int fd;
    long long count;
    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;
    attr.exclude_kernel = 1;
    attr.exclude_guest = 1;
    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);
    for (int i = 0; i < 500; ++i) {
        putchar('.');
    }
    read(fd, &count, sizeof(count));
    printf("\nConditional branches: %lld\n%s\n", count,
           count < 500 ? "FAILED" : "PASSED");
    return 0;
}

Robert O'Callahan (1):
  KVM: x86: never specify a sample period for virtualized in_tx_cp
    counters

 arch/x86/kvm/pmu.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

-- 
2.9.3

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#1571260 — [PATCH v2 01/01] 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, 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>
---
 arch/x86/kvm/pmu.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index 06ce377..026db42 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -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 :
-- 
2.9.3

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