Re: [PATCH v6 5/7] KVM: arm64: pmu: Make overflow handler NMI safe

[Will Deacon]

On Wed, Aug 19, 2020 at 02:34:17PM +0100, Alexandru Elisei wrote:
> From: Julien Thierry <julien.thierry@xxxxxxx>
> 
> kvm_vcpu_kick() is not NMI safe. When the overflow handler is called from
> NMI context, defer waking the vcpu to an irq_work queue.
> 
> Cc: Julien Thierry <julien.thierry.kdev@xxxxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: Suzuki K Pouloze <suzuki.poulose@xxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Cc: kvmarm@xxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
>  arch/arm64/kvm/pmu-emul.c | 25 ++++++++++++++++++++++++-
>  include/kvm/arm_pmu.h     |  1 +
>  2 files changed, 25 insertions(+), 1 deletion(-)

I'd like an Ack from the KVM side on this one, but some minor comments
inline.

> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> index f0d0312c0a55..30268397ed06 100644
> --- a/arch/arm64/kvm/pmu-emul.c
> +++ b/arch/arm64/kvm/pmu-emul.c
> @@ -433,6 +433,22 @@ void kvm_pmu_sync_hwstate(struct kvm_vcpu *vcpu)
>  	kvm_pmu_update_state(vcpu);
>  }
>  
> +/**
> + * When perf interrupt is an NMI, we cannot safely notify the vcpu corresponding
> + * to the event.
> + * This is why we need a callback to do it once outside of the NMI context.
> + */
> +static void kvm_pmu_perf_overflow_notify_vcpu(struct irq_work *work)
> +{
> +	struct kvm_vcpu *vcpu;
> +	struct kvm_pmu *pmu;
> +
> +	pmu = container_of(work, struct kvm_pmu, overflow_work);
> +	vcpu = kvm_pmc_to_vcpu(&pmu->pmc[0]);

Can you spell this kvm_pmc_to_vcpu(pmu->pmc); ?

> +
> +	kvm_vcpu_kick(vcpu);

How do we guarantee that the vCPU is still around by the time this runs?
Sorry to ask such a horrible question, but I don't see anything associating
the workqueue with the lifetime of the vCPU.

Will