linux-kernel - Re: [RFC PATCH 12/13] KVM: SVM: Remove APICv inhibit reasone due to x2APIC

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Date:   Thu, 24 Feb 2022 22:06:34 +0200
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: [RFC PATCH 12/13] KVM: SVM: Remove APICv inhibit reasone due to
 x2APIC

On Sun, 2022-02-20 at 20:19 -0600, Suravee Suthikulpanit wrote:
> Currently, AVIC is inactive when booting a VM w/ x2APIC support.
> With x2AVIC support, the APICV_INHIBIT_REASON_X2APIC can be removed.
The commit title is a bit misleading - the inhibit reason is not removed,
but rather AVIC is not inhibited when x2avic is present.

> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kvm/svm/avic.c | 21 +++++++++++++++++++++
>  arch/x86/kvm/svm/svm.c  | 18 ++----------------
>  arch/x86/kvm/svm/svm.h  |  1 +
>  3 files changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> index 3306b74f1d8b..874c89f8fd47 100644
> --- a/arch/x86/kvm/svm/avic.c
> +++ b/arch/x86/kvm/svm/avic.c
> @@ -21,6 +21,7 @@
>  
>  #include <asm/irq_remapping.h>
>  
> +#include "cpuid.h"
>  #include "trace.h"
>  #include "lapic.h"
>  #include "x86.h"
> @@ -176,6 +177,26 @@ void avic_vm_destroy(struct kvm *kvm)
>  	spin_unlock_irqrestore(&svm_vm_data_hash_lock, flags);
>  }
>  
> +void avic_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu, int nested)
> +{
> +	/*
> +	 * If the X2APIC feature is exposed to the guest,
> +	 * disable AVIC unless X2AVIC mode is enabled.
> +	 */
> +	if (avic_mode == AVIC_MODE_X1 &&
> +	    guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
> +		kvm_request_apicv_update(vcpu->kvm, false,
> +					 APICV_INHIBIT_REASON_X2APIC);
> +
> +	/*
> +	 * Currently, AVIC does not work with nested virtualization.
> +	 * So, we disable AVIC when cpuid for SVM is set in the L1 guest.
> +	 */
> +	if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
> +		kvm_request_apicv_update(vcpu->kvm, false,
> +					 APICV_INHIBIT_REASON_NESTED);

BTW, now that I am thinking about it, it would be nice to be able to force
the AVIC_MODE_X1 even if x2avic is present, for debug purposes from a module
param. Just a suggestion.

> +}
> +
>  int avic_vm_init(struct kvm *kvm)
>  {
>  	unsigned long flags;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index afca26aa1f40..b7bc6cd74aba 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3992,23 +3992,9 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  			vcpu->arch.reserved_gpa_bits &= ~(1UL << (best->ebx & 0x3f));
>  	}
>  
> -	if (kvm_vcpu_apicv_active(vcpu)) {
> -		/*
> -		 * AVIC does not work with an x2APIC mode guest. If the X2APIC feature
> -		 * is exposed to the guest, disable AVIC.
> -		 */
> -		if (guest_cpuid_has(vcpu, X86_FEATURE_X2APIC))
> -			kvm_request_apicv_update(vcpu->kvm, false,
> -						 APICV_INHIBIT_REASON_X2APIC);
> +	if (kvm_vcpu_apicv_active(vcpu))
> +		avic_vcpu_after_set_cpuid(vcpu, nested);
>  
> -		/*
> -		 * Currently, AVIC does not work with nested virtualization.
> -		 * So, we disable AVIC when cpuid for SVM is set in the L1 guest.
> -		 */
> -		if (nested && guest_cpuid_has(vcpu, X86_FEATURE_SVM))
> -			kvm_request_apicv_update(vcpu->kvm, false,
> -						 APICV_INHIBIT_REASON_NESTED);
> -	}
>  	init_vmcb_after_set_cpuid(vcpu);
>  }
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 41514df5107e..aea80abe9186 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -578,6 +578,7 @@ void avic_update_vapic_bar(struct vcpu_svm *svm, u64 data);
>  void avic_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
>  void avic_vcpu_put(struct kvm_vcpu *vcpu);
>  void avic_post_state_restore(struct kvm_vcpu *vcpu);
> +void avic_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu, int nested);
>  void svm_set_virtual_apic_mode(struct kvm_vcpu *vcpu);
>  void svm_refresh_apicv_exec_ctrl(struct kvm_vcpu *vcpu);
>  bool svm_check_apicv_inhibit_reasons(ulong bit);

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

Best regards,
	Maxim Levitsky