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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Will,

On 9/21/20 2:43 PM, Will Deacon wrote:
> On Wed, Aug 19, 2020 at 02:34:17PM +0100, Alexandru Elisei wrote:
>> From: Julien Thierry <julien.thierry@arm.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@gmail.com>
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: James Morse <james.morse@arm.com>
>> Cc: Suzuki K Pouloze <suzuki.poulose@arm.com>
>> Cc: kvm@vger.kernel.org
>> Cc: kvmarm@lists.cs.columbia.edu
>> Signed-off-by: Julien Thierry <julien.thierry@arm.com>
>> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.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); ?

Of course, that is much better.

>
>> +
>> +	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.

That's a very nice catch, indeed the code doesn't guarantee that the VM is still
around when the work is executed. I will add an irq_work_sync() call to
kvm_pmu_vcpu_destroy() (which is called by kvm_vcpu_destroy() ->
kvm_arch_vcpu_destroy()), and to kvm_pmu_vcpu_reset(), similar to how x86 handles it.

Thanks,
Alex