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

Andrew Jones
2017-10-18T12:12:10Z

On Sat, Oct 14, 2017 at 09:13:35PM +0200, Christoffer Dall wrote:
> 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?

Hmm, I see what you mean. I'm sorry I missed that.

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

I think we can fix it by adding

if (kvm_vgic_vcpu_pending_irq(vcpu))
kvm_make_request(KVM_REQ_IRQ_PENDING, vcpu);

to kvm_vgic_sync_hwstate(), if the additional overhead would be
acceptable. Otherwise we need to find some other way to ensure
vPMU irqs unblock the VCPU.

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

Thanks,
drew