Re: [PATCH v4 02/32] KVM: x86: Purge "highest ISR" cache when updating APICv state

[Maxim Levitsky]

On Sat, 2022-10-01 at 00:58 +0000, 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>
> Reviewed-by: Paolo Bonzini <pbonzini@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 05d079fc2c66..5de1c7aa1ce9 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2424,6 +2424,7 @@ 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);
>  
> @@ -2480,7 +2481,6 @@ 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);
>  
> @@ -2767,7 +2767,6 @@ 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: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky