Date:   Mon, 29 Nov 2021 23:39:05 +0000
From:   David Matlack <dmatlack@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        seanjc@google.com, stable@vger.kernel.org
Subject: Re: [PATCH 3/4] KVM: x86: check PIR even for vCPUs with disabled
 APICv
Message-ID: <YaVkmfqkau2EYuy+@google.com>
References: <20211123004311.2954158-1-pbonzini@redhat.com>
 <20211123004311.2954158-4-pbonzini@redhat.com>
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On Mon, Nov 22, 2021 at 07:43:10PM -0500, Paolo Bonzini wrote:
> The IRTE for an assigned device can trigger a POSTED_INTR_VECTOR even
> if APICv is disabled on the vCPU that receives it.  In that case, the
> interrupt will just cause a vmexit and leave the ON bit set together
> with the PIR bit corresponding to the interrupt.
> 
> Right now, the interrupt would not be delivered until APICv is re-enabled.
> However, fixing this is just a matter of always doing the PIR->IRR
> synchronization, even if the vCPU has temporarily disabled APICv.
> 
> This is not a problem for performance, or if anything it is an
> improvement.  First, in the common case where vcpu->arch.apicv_active is
> true, one fewer check has to be performed.  Second, static_call_cond will
> elide the function call if APICv is not present or disabled.  Finally,
> in the case for AMD hardware we can remove the sync_pir_to_irr callback:
> it is only needed for apic_has_interrupt_for_ppr, and that function
> already has a fallback for !APICv.
> 
> Cc: stable@vger.kernel.org
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: David Matlack <dmatlack@google.com>

> ---
>  arch/x86/kvm/lapic.c   |  2 +-
>  arch/x86/kvm/svm/svm.c |  1 -
>  arch/x86/kvm/x86.c     | 18 +++++++++---------
>  3 files changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 759952dd1222..f206fc35deff 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -707,7 +707,7 @@ static void pv_eoi_clr_pending(struct kvm_vcpu *vcpu)
>  static int apic_has_interrupt_for_ppr(struct kvm_lapic *apic, u32 ppr)
>  {
>  	int highest_irr;
> -	if (apic->vcpu->arch.apicv_active)
> +	if (kvm_x86_ops.sync_pir_to_irr)
>  		highest_irr = static_call(kvm_x86_sync_pir_to_irr)(apic->vcpu);
>  	else
>  		highest_irr = apic_find_highest_irr(apic);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 5630c241d5f6..d0f68d11ec70 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4651,7 +4651,6 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.load_eoi_exitmap = svm_load_eoi_exitmap,
>  	.hwapic_irr_update = svm_hwapic_irr_update,
>  	.hwapic_isr_update = svm_hwapic_isr_update,
> -	.sync_pir_to_irr = kvm_lapic_find_highest_irr,
>  	.apicv_post_state_restore = avic_post_state_restore,
>  
>  	.set_tss_addr = svm_set_tss_addr,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 441f4769173e..a8f12c83db4b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4448,8 +4448,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
>  				    struct kvm_lapic_state *s)
>  {
> -	if (vcpu->arch.apicv_active)
> -		static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> +	static_call_cond(kvm_x86_sync_pir_to_irr)(vcpu);
>  
>  	return kvm_apic_get_state(vcpu, s);
>  }
> @@ -9528,8 +9527,7 @@ static void vcpu_scan_ioapic(struct kvm_vcpu *vcpu)
>  	if (irqchip_split(vcpu->kvm))
>  		kvm_scan_ioapic_routes(vcpu, vcpu->arch.ioapic_handled_vectors);
>  	else {
> -		if (vcpu->arch.apicv_active)
> -			static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> +		static_call_cond(kvm_x86_sync_pir_to_irr)(vcpu);
>  		if (ioapic_in_kernel(vcpu->kvm))
>  			kvm_ioapic_scan_entry(vcpu, vcpu->arch.ioapic_handled_vectors);
>  	}
> @@ -9802,10 +9800,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>  	/*
>  	 * This handles the case where a posted interrupt was
> -	 * notified with kvm_vcpu_kick.
> +	 * notified with kvm_vcpu_kick.  Assigned devices can
> +	 * use the POSTED_INTR_VECTOR even if APICv is disabled,
> +	 * so do it even if APICv is disabled on this vCPU.
>  	 */
> -	if (kvm_lapic_enabled(vcpu) && vcpu->arch.apicv_active)
> -		static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> +	if (kvm_lapic_enabled(vcpu))
> +		static_call_cond(kvm_x86_sync_pir_to_irr)(vcpu);
>  
>  	if (kvm_vcpu_exit_request(vcpu)) {
>  		vcpu->mode = OUTSIDE_GUEST_MODE;
> @@ -9849,8 +9849,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		if (likely(exit_fastpath != EXIT_FASTPATH_REENTER_GUEST))
>  			break;
>  
> -		if (kvm_lapic_enabled(vcpu) && vcpu->arch.apicv_active)
> -			static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> +		if (kvm_lapic_enabled(vcpu))
> +			static_call_cond(kvm_x86_sync_pir_to_irr)(vcpu);
>  
>  		if (unlikely(kvm_vcpu_exit_request(vcpu))) {
>  			exit_fastpath = EXIT_FASTPATH_EXIT_HANDLED;
> -- 
> 2.27.0
> 
>