linux-kernel - Re: [PATCH v4 2/7] KVM: X86: Enable fastpath when APICv is enabled

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Date:   Thu, 30 Apr 2020 15:34:06 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v4 2/7] KVM: X86: Enable fastpath when APICv is enabled

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpengli@...cent.com>
>
> We can't observe benefit from single target IPI fastpath when APICv is
> disabled, let's just enable IPI and Timer fastpath when APICv is enabled
> for now.
>
> Tested-by: Haiwei Li <lihaiwei@...cent.com>
> Cc: Haiwei Li <lihaiwei@...cent.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/svm/svm.c | 2 +-
>  arch/x86/kvm/vmx/vmx.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 8f8fc65..1e7220e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3344,7 +3344,7 @@ static void svm_cancel_injection(struct kvm_vcpu *vcpu)
>  
>  static enum exit_fastpath_completion svm_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
>  {
> -	if (!is_guest_mode(vcpu) &&
> +	if (!is_guest_mode(vcpu) && vcpu->arch.apicv_active &&
>  	    to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_MSR &&
>  	    to_svm(vcpu)->vmcb->control.exit_info_1)
>  		return handle_fastpath_set_msr_irqoff(vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9b5adb4..f207004 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6585,7 +6585,7 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
>  
>  static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
>  {
> -	if (!is_guest_mode(vcpu)) {
> +	if (!is_guest_mode(vcpu) && vcpu->arch.apicv_active) {
>  		switch (to_vmx(vcpu)->exit_reason) {
>  		case EXIT_REASON_MSR_WRITE:
>  			return handle_fastpath_set_msr_irqoff(vcpu);

I think that apicv_active checks are specific to APIC MSRs but
handle_fastpath_set_msr_irqoff() can handle any other MSR as well. I'd
suggest to move the check inside handle_fastpath_set_msr_irqoff().

Also, enabling Hyper-V SynIC leads to disabling apicv. It it still
pointless to keep fastpath enabled?

-- 
Vitaly