lkml.org 
[lkml]   [2022]   [Sep]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    Subject[PATCH 1/4] KVM: x86/pmu: Force reprogramming of all counters on PMU filter change
    From
    Force vCPUs to reprogram all counters on a PMU filter change to provide
    a sane ABI for userspace. Use the existing KVM_REQ_PMU to do the
    programming, and take advantage of the fact that the reprogram_pmi bitmap
    fits in a u64 to set all bits in a single atomic update. Note, setting
    the bitmap and making the request needs to be done _after_ the SRCU
    synchronization to ensure that vCPUs will reprogram using the new filter.

    KVM's current "lazy" approach is confusing and non-deterministic. It's
    confusing because, from a developer perspective, the code is buggy as it
    makes zero sense to let userspace modify the filter but then not actually
    enforce the new filter. The lazy approach is non-deterministic because
    KVM enforces the filter whenever a counter is reprogrammed, not just on
    guest WRMSRs, i.e. a guest might gain/lose access to an event at random
    times depending on what is going on in the host.

    Note, the resulting behavior is still non-determinstic while the filter
    is in flux. If userspace wants to guarantee deterministic behavior, all
    vCPUs should be paused during the filter update.

    Fixes: 66bb8a065f5a ("KVM: x86: PMU Event Filter")
    Cc: Aaron Lewis <aaronlewis@google.com>
    Jim Mattson <jmattson@google.com>
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    ---
    arch/x86/include/asm/kvm_host.h | 11 ++++++++++-
    arch/x86/kvm/pmu.c | 15 +++++++++++++--
    2 files changed, 23 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    index b3ce723efb43..462f041ede9f 100644
    --- a/arch/x86/include/asm/kvm_host.h
    +++ b/arch/x86/include/asm/kvm_host.h
    @@ -519,7 +519,16 @@ struct kvm_pmu {
    struct kvm_pmc gp_counters[INTEL_PMC_MAX_GENERIC];
    struct kvm_pmc fixed_counters[KVM_PMC_MAX_FIXED];
    struct irq_work irq_work;
    - DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
    +
    + /*
    + * Overlay the bitmap with a 64-bit atomic so that all bits can be
    + * set in a single access, e.g. to reprogram all counters when the PMU
    + * filter changes.
    + */
    + union {
    + DECLARE_BITMAP(reprogram_pmi, X86_PMC_IDX_MAX);
    + atomic64_t __reprogram_pmi;
    + };
    DECLARE_BITMAP(all_valid_pmc_idx, X86_PMC_IDX_MAX);
    DECLARE_BITMAP(pmc_in_use, X86_PMC_IDX_MAX);

    diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
    index d9b9a0f0db17..4504987cbbe2 100644
    --- a/arch/x86/kvm/pmu.c
    +++ b/arch/x86/kvm/pmu.c
    @@ -577,6 +577,8 @@ EXPORT_SYMBOL_GPL(kvm_pmu_trigger_event);
    int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
    {
    struct kvm_pmu_event_filter tmp, *filter;
    + struct kvm_vcpu *vcpu;
    + unsigned long i;
    size_t size;
    int r;

    @@ -613,9 +615,18 @@ int kvm_vm_ioctl_set_pmu_event_filter(struct kvm *kvm, void __user *argp)
    mutex_lock(&kvm->lock);
    filter = rcu_replace_pointer(kvm->arch.pmu_event_filter, filter,
    mutex_is_locked(&kvm->lock));
    - mutex_unlock(&kvm->lock);
    -
    synchronize_srcu_expedited(&kvm->srcu);
    +
    + BUILD_BUG_ON(sizeof(((struct kvm_pmu *)0)->reprogram_pmi) >
    + sizeof(((struct kvm_pmu *)0)->__reprogram_pmi));
    +
    + kvm_for_each_vcpu(i, vcpu, kvm)
    + atomic64_set(&vcpu_to_pmu(vcpu)->__reprogram_pmi, -1ull);
    +
    + kvm_make_all_cpus_request(kvm, KVM_REQ_PMU);
    +
    + mutex_unlock(&kvm->lock);
    +
    r = 0;
    cleanup:
    kfree(filter);
    --
    2.37.3.998.g577e59143f-goog
    \
     
     \ /
      Last update: 2022-09-23 02:15    [W:4.986 / U:0.128 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site