linux-kernel - Re: [PATCH 4/9] KVM: x86: Avoid double CPUID lookup when updating MWAIT at runtime

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Date:   Sun, 19 Nov 2023 19:33:01 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/9] KVM: x86: Avoid double CPUID lookup when updating
 MWAIT at runtime

On Fri, 2023-11-10 at 15:55 -0800, Sean Christopherson wrote:
> Move the handling of X86_FEATURE_MWAIT during CPUID runtime updates to
> utilize the lookup done for other CPUID.0x1 features.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/cpuid.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 5cf3d697ecb3..6777780be6ae 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -276,6 +276,11 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>  
>  		cpuid_entry_change(best, X86_FEATURE_APIC,
>  			   vcpu->arch.apic_base & MSR_IA32_APICBASE_ENABLE);
> +
> +		if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT))
> +			cpuid_entry_change(best, X86_FEATURE_MWAIT,
> +					   vcpu->arch.ia32_misc_enable_msr &
> +					   MSR_IA32_MISC_ENABLE_MWAIT);
>  	}
>  
>  	best = cpuid_entry2_find(entries, nent, 7, 0);
> @@ -296,14 +301,6 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>  	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
>  		(best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
>  		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> -
> -	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
> -		best = cpuid_entry2_find(entries, nent, 0x1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> -		if (best)
> -			cpuid_entry_change(best, X86_FEATURE_MWAIT,
> -					   vcpu->arch.ia32_misc_enable_msr &
> -					   MSR_IA32_MISC_ENABLE_MWAIT);
> -	}
>  }
>  
>  void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)

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

Best regards,
	Maxim Levitsky