linux-kernel - Re: [PATCH 4/4] KVM: x86: Skip KVM_GUESTDBG_BLOCKIRQ APICv update if APICv is disabled

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Date:   Mon, 18 Apr 2022 15:50:00 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Gaoning Pan <pgn@....edu.cn>,
        Yongkang Jia <kangel@....edu.cn>
Subject: Re: [PATCH 4/4] KVM: x86: Skip KVM_GUESTDBG_BLOCKIRQ APICv update
 if APICv is disabled

On Sat, 2022-04-16 at 03:42 +0000, Sean Christopherson wrote:
> Skip the APICv inhibit update for KVM_GUESTDBG_BLOCKIRQ if APICv is
> disabled at the module level to avoid having to acquire the mutex and
> potentially process all vCPUs. The DISABLE inhibit will (barring bugs)
> never be lifted, so piling on more inhibits is unnecessary.
> 
> Fixes: cae72dcc3b21 ("KVM: x86: inhibit APICv when KVM_GUESTDBG_BLOCKIRQ active")
> Cc: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 09a270cc1c8f..16c5fa7d165d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11048,6 +11048,9 @@ static void kvm_arch_vcpu_guestdbg_update_apicv_inhibit(struct kvm *kvm)
>  	struct kvm_vcpu *vcpu;
>  	unsigned long i;
>  
> +	if (!enable_apicv)
> +		return;
> +
>  	down_write(&kvm->arch.apicv_update_lock);
>  
>  	kvm_for_each_vcpu(i, vcpu, kvm) {

Makes sense as a precation.

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


Best regards,
	Maxim Levitsky