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 15:27 +0000, Sean Christopherson wrote:
> On Thu, Dec 09, 2021, Maxim Levitsky wrote:
> > 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.
> 
> Sans barriers and comments, can't this be written as returning an "error" if the
> vCPU is not IN_GUEST_MODE?  Effectively the same thing, but a little more precise
> and it avoids duplicating the lapic.c code.

Yes, beside the fact that we already set the vIRR bit so if I return -1 here, it will be set again..
(and these are set using atomic ops)

I don't know how much that matters except the fact that while a vCPU runs a nested guest,
callers wishing to send IPI to it, will go through this code path a lot 
(even when I implement nested AVIC as it is a separate thing which is used by L2 only).

Best regards,
	Maxim Levitsky

> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 26ed5325c593..cddf7a8da3ea 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -671,7 +671,7 @@ void svm_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap)
> 
>  int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec)
>  {
> -       if (!vcpu->arch.apicv_active)
> +       if (vcpu->mode != IN_GUEST_MODE || !vcpu->arch.apicv_active)
>                 return -1;
> 
>         kvm_lapic_set_irr(vec, vcpu->arch.apic);
> @@ -706,8 +706,9 @@ 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.
> +                * Wake the vCPU if it is blocking.  If the vCPU exited the
> +                * guest since the previous vcpu->mode check, it's guaranteed
> +                * to see the event before re-enterring the guest.
>                  */
>                 kvm_vcpu_wake_up(vcpu);
>         }
>