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SubjectRe: [PATCH 1/4] KVM: x86/pmu: Force reprogramming of all counters on PMU filter change
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Firstly, thanks for your comments that spewed out around vpmu.

On 23/9/2022 8:13 am, Sean Christopherson wrote:
> 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

The resolute lazy approach was introduced in patch 03, right after this change.

> 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>

miss "Cc:" ?

> 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);

The relative order of these two operations has been reversed
mutex_unlock() and synchronize_srcu_expedited()
, extending the execution window of the critical area of "kvm->lock)".
The motivation is also not explicitly stated in the commit message.

> +
> + 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);

How about:
bitmap_copy(pmu->reprogram_pmi, pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX);
to avoid further cycles on calls of
"static_call(kvm_x86_pmu_pmc_idx_to_pmc)(pmu, bit)" ?

> +
> + kvm_make_all_cpus_request(kvm, KVM_REQ_PMU);
> +
> + mutex_unlock(&kvm->lock);
> +
> r = 0;
> cleanup:
> kfree(filter);

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