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

Paolo Bonzini
2021-12-09T14:11:32Z

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.

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.

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;
>