Re: [PATCH 5/5] KVM: arm/arm64: kvm_arch_vcpu_runnable: don't miss injected irqs

Christoffer Dall
2017-10-14T19:12:48Z

On Fri, Sep 29, 2017 at 01:30:41PM +0200, Andrew Jones wrote:
> When the vPMU is in use if a VCPU's perf event overflow handler
> were to fire after the VCPU started waiting, then the wake up
> done by the kvm_vcpu_kick() call in the handler would do nothing,
> as no "pmu overflow" state is checked in kvm_arch_vcpu_runnable().
> Fix this by checking the IRQ_PENDING VCPU request in runnable().
> Checking the request also sufficiently covers all the cases that
> kvm_vgic_vcpu_pending_irq() cover, so we can just replace that.
>
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
> virt/kvm/arm/arm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 5bc9b0d2fd0f..725527f491e4 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -423,7 +423,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
> return !vcpu_should_sleep(vcpu) &&
> (vcpu->arch.mp_state != KVM_MP_STATE_HALTED ||
> (!!vcpu->arch.irq_lines ||
> - kvm_vgic_vcpu_pending_irq(vcpu)));
> + kvm_test_request(KVM_REQ_IRQ_PENDING, vcpu)));

So what if a VCPU blocks, a device raises an IRQ, the VCPU loops around,
clears the KVM_REQ_IRQ_PENDING flag, enters the VM again, which does
another WFI (for fun), and you end up here again with a pending IRQ but
no KVM_REQ_IRQ_PENDING flag anymore. Doesn't this end up incorrectly
stalling the VCPU?

I don't think that a transient flag will work for a persistent binary
state here.

> }
>
> bool kvm_arch_vcpu_in_kernel(struct kvm_vcpu *vcpu)
> --
> 2.13.5
>

Thanks,
-Christoffer