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

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:

quoted

From: Julien Thierry <redacted>

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 <redacted>
Cc: Marc Zyngier <redacted>
Cc: Will Deacon <redacted>
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 <redacted>
Signed-off-by: Alexandru Elisei <redacted>
---
 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.

quoted

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.

quoted

+
+	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

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