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

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Date:   Mon, 21 Sep 2020 14:43:02 +0100
From:   Will Deacon <will@...nel.org>
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, maz@...nel.org, catalin.marinas@....com,
        swboyd@...omium.org, sumit.garg@...aro.org,
        Julien Thierry <julien.thierry@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v6 5/7] KVM: arm64: pmu: Make overflow handler NMI safe

On Wed, Aug 19, 2020 at 02:34:17PM +0100, Alexandru Elisei wrote:
> From: Julien Thierry <julien.thierry@....com>
> 
> 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@...il.com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: James Morse <james.morse@....com>
> Cc: Suzuki K Pouloze <suzuki.poulose@....com>
> Cc: kvm@...r.kernel.org
> Cc: kvmarm@...ts.cs.columbia.edu
> Signed-off-by: Julien Thierry <julien.thierry@....com>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@....com>
> ---
>  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