Re: [PATCH v2 01/23] KVM: x86: Purge "highest ISR" cache when updating APICv state

[Paolo Bonzini]

On 9/3/22 02:22, Sean Christopherson wrote:
> Purge the "highest ISR" cache when updating APICv state on a vCPU.  The
> cache must not be used when APICv is active as hardware may emulate EOIs
> (and other operations) without exiting to KVM.
>
> This fixes a bug where KVM will effectively block IRQs in perpetuity due
> to the "highest ISR" never getting reset if APICv is activated on a vCPU
> while an IRQ is in-service.  Hardware emulates the EOI and KVM never gets
> a chance to update its cache.
>
> Fixes: b26a695a1d78 ("kvm: lapic: Introduce APICv update helper function")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
> Cc: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/lapic.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 9dda989a1cf0..38e9b8e5278c 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2429,6 +2429,7 @@ void kvm_apic_update_apicv(struct kvm_vcpu *vcpu)
>   		 */
>   		apic->isr_count = count_vectors(apic->regs + APIC_ISR);
>   	}
> +	apic->highest_isr_cache = -1;
>   }
>   EXPORT_SYMBOL_GPL(kvm_apic_update_apicv);
>   
> @@ -2485,7 +2486,6 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
>   		kvm_lapic_set_reg(apic, APIC_TMR + 0x10 * i, 0);
>   	}
>   	kvm_apic_update_apicv(vcpu);
> -	apic->highest_isr_cache = -1;
>   	update_divide_count(apic);
>   	atomic_set(&apic->lapic_timer.pending, 0);
>   
> @@ -2772,7 +2772,6 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>   	__start_apic_timer(apic, APIC_TMCCT);
>   	kvm_lapic_set_reg(apic, APIC_TMCCT, 0);
>   	kvm_apic_update_apicv(vcpu);
> -	apic->highest_isr_cache = -1;
>   	if (apic->apicv_active) {
>   		static_call_cond(kvm_x86_apicv_post_state_restore)(vcpu);
>   		static_call_cond(kvm_x86_hwapic_irr_update)(vcpu, apic_find_highest_irr(apic));

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>