linux-kernel - Re: [PATCH v3 12/14] kvm/x86: Warning APICv inconsistency only when vcpu APIC mode is valid

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Date:   Wed, 04 May 2022 15:33:37 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v3 12/14] kvm/x86: Warning APICv inconsistency only when
 vcpu APIC mode is valid

On Wed, 2022-05-04 at 02:31 -0500, Suravee Suthikulpanit wrote:
> When launching a VM with x2APIC and specify more than 255 vCPUs,
> the guest kernel can disable x2APIC (e.g. specify nox2apic kernel option).
> The VM fallbacks to xAPIC mode, and disable the vCPU ID 255 and greater.
> 
> In this case, APICV should be deactivated for the disabled vCPUs.
> However, the current APICv consistency warning does not account for
> this case, which results in a warning.
> 
> Therefore, modify warning logic to report only when vCPU APIC mode
> is valid.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kvm/x86.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8ee8c91fa762..b14e02ea0ff6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9838,6 +9838,10 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
>  
>  	activate = kvm_vcpu_apicv_activated(vcpu);
>  
> +	/* Do not activate AVIC when APIC is disabled */
> +	if (kvm_get_apic_mode(vcpu) == LAPIC_MODE_DISABLED)
> +		activate = false;
> +
>  	if (vcpu->arch.apicv_active == activate)
>  		goto out;
>  
> @@ -10240,7 +10244,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		 * per-VM state, and responsing vCPUs must wait for the update
>  		 * to complete before servicing KVM_REQ_APICV_UPDATE.
>  		 */
> -		WARN_ON_ONCE(kvm_vcpu_apicv_activated(vcpu) != kvm_vcpu_apicv_active(vcpu));
> +		if (kvm_vcpu_apicv_activated(vcpu) != kvm_vcpu_apicv_active(vcpu))
> +			WARN_ON_ONCE(kvm_get_apic_mode(vcpu) != LAPIC_MODE_DISABLED);
>  
>  		exit_fastpath = static_call(kvm_x86_vcpu_run)(vcpu);
>  		if (likely(exit_fastpath != EXIT_FASTPATH_REENTER_GUEST))

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky