linux-kernel - Re: [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry

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Date:   Thu, 14 Dec 2017 11:27:27 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     wanpeng.li@...mail.com
Subject: Re: [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry



On 2017/12/13 20:51, Paolo Bonzini wrote:
> xsetbv can be expensive when running on nested virtualization, try to
> avoid it.
>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/x86.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0f82e2cbf64c..daa1918031df 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -702,7 +702,8 @@ static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
>   	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
>   			!vcpu->guest_xcr0_loaded) {
>   		/* kvm_set_xcr() also depends on this */
> -		xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> +		if (vcpu->arch.xcr0 != host_xcr0)
> +			xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
>   		vcpu->guest_xcr0_loaded = 1;
>   	}
>   }

Reviewed-by: Quan Xu <quan.xu0@...il.com>