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

Andrew Jones
2017-10-18T13:55:01Z

On Wed, Oct 18, 2017 at 03:18:43PM +0200, Christoffer Dall wrote:
> On Wed, Oct 18, 2017 at 02:13:05PM +0200, Andrew Jones wrote:
> > 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.
> >
>
> I think you just need to check
> kvm_vgic_vcpu_pending_irq() ||
> kvm_test_request(KVM_REQ_IRQ_PENDING, vcpu)
>
> Wouldn't that fix it?

It would. I was just hoping to eliminate the lock-taking
kvm_vgic_vcpu_pending_irq() call from kvm_arch_vcpu_runnable()
though, as kvm_arch_vcpu_runnable() may be called repeatedly
during halt polling.

Thanks,
drew