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

[Paolo Bonzini <pbonzini@redhat.com>]

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;
>