linux-kernel - Re: [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

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Message-ID: <0776e1d0-f0e0-68ef-a07c-9a4884ea1498@redhat.com>
Date:   Sat, 14 Mar 2020 12:00:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing
 PMU

On 13/03/20 04:55, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> PMU is not exposed to guest by most of products from cloud providers since the 
> bad performance of PMU emulation and security concern. However, it calls 
> perf_guest_switch_get_msrs() and clear_atomic_switch_msr() unconditionally 
> even if PMU is not exposed to the guest before each vmentry. 
> 
> ~2% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my 
> SKX server.
> 
> Before patch:
> vmcall 1559
> 
> After patch:
> vmcall 1529
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v1 -> v2:
>  * move the check before atomic_switch_perf_msrs
> 
>  arch/x86/kvm/vmx/vmx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e61..b20423c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  
>  	pt_guest_enter(vmx);
>  
> -	atomic_switch_perf_msrs(vmx);
> +	if (vcpu_to_pmu(vcpu)->version)
> +		atomic_switch_perf_msrs(vmx);
>  	atomic_switch_umwait_control_msr(vmx);
>  
>  	if (enable_preemption_timer)
> 

Queued, thanks.

Paolo