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

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Date:   Wed, 13 Dec 2017 11:03:31 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry

Reviewed-by: Jim Mattson <jmattson@...gle.com>

On Wed, Dec 13, 2017 at 4:51 AM, Paolo Bonzini <pbonzini@...hat.com> 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;
>         }
>  }
> --
> 1.8.3.1
>