Re: [PATCH 3/6] KVM: SVM: fix AVIC race of host->guest IPI delivery vs AVIC inhibition

[Maxim Levitsky <mlevitsk@redhat.com>]

On Thu, 2021-12-09 at 15:11 +0100, Paolo Bonzini wrote:
> On 12/9/21 12:54, Maxim Levitsky wrote:
> > If svm_deliver_avic_intr is called just after the target vcpu's AVIC got
> > inhibited, it might read a stale value of vcpu->arch.apicv_active
> > which can lead to the target vCPU not noticing the interrupt.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> > ---
> >   arch/x86/kvm/svm/avic.c | 16 +++++++++++++---
> >   1 file changed, 13 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > index 859ad2dc50f1..8c1b934bfa9b 100644
> > --- a/arch/x86/kvm/svm/avic.c
> > +++ b/arch/x86/kvm/svm/avic.c
> > @@ -691,6 +691,15 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
> >   	 * automatically process AVIC interrupts at VMRUN.
> >   	 */
> >   	if (vcpu->mode == IN_GUEST_MODE) {
> > +
> > +		/*
> > +		 * At this point we had read the vcpu->arch.apicv_active == true
> > +		 * and the vcpu->mode == IN_GUEST_MODE.
> > +		 * Since we have a memory barrier after setting IN_GUEST_MODE,
> > +		 * it ensures that AVIC inhibition is complete and thus
> > +		 * the target is really running with AVIC enabled.
> > +		 */
> > +
> >   		int cpu = READ_ONCE(vcpu->cpu);
> 
> I don't think it's correct.  The vCPU has apicv_active written (in 
> kvm_vcpu_update_apicv) before vcpu->mode.

I thought that we have a full memory barrier just prior to setting IN_GUEST_MODE
thus if I see vcpu->mode == IN_GUEST_MODE then I'll see correct apicv_active value.
But apparently the memory barrier is after setting vcpu->mode.


> 
> For the acquire/release pair to work properly you need to 1) read 
> apicv_active *after* vcpu->mode here 2) use store_release and 
> load_acquire for vcpu->mode, respectively in vcpu_enter_guest and here.

store_release for vcpu->mode in vcpu_enter_guest means a write barrier just before setting it,
which I expected to be there.

And yes I see now, I need a read barrier here as well. I am still learning this.

Best regards,
	Maxim Levitsky

> 
> Paolo
> 
> >   		/*
> > @@ -706,10 +715,11 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
> >   		put_cpu();
> >   	} else {
> >   		/*
> > -		 * Wake the vCPU if it was blocking.  KVM will then detect the
> > -		 * pending IRQ when checking if the vCPU has a wake event.
> > +		 * Kick the target vCPU otherwise, to make sure
> > +		 * it processes the interrupt even if its AVIC is inhibited.
> >   		 */
> > -		kvm_vcpu_wake_up(vcpu);
> > +		kvm_make_request(KVM_REQ_EVENT, vcpu);
> > +		kvm_vcpu_kick(vcpu);
> >   	}
> >   
> >   	return 0;
> >