linux-kernel - Re: [PATCH v3 5/5] KVM: VMX: Handle preemption timer fastpath

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Message-ID: <20200427184253.GR14870@linux.intel.com>
Date:   Mon, 27 Apr 2020 11:42:53 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v3 5/5] KVM: VMX: Handle preemption timer fastpath

On Fri, Apr 24, 2020 at 02:22:44PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> This patch implements handle preemption timer fastpath, after timer fire 
> due to VMX-preemption timer counts down to zero, handle it as soon as 
> possible and vmentry immediately without checking various kvm stuff when 
> possible.
> 
> Testing on SKX Server.
> 
> cyclictest in guest(w/o mwait exposed, adaptive advance lapic timer is default -1):
> 
> 5540.5ns -> 4602ns       17%
> 
> kvm-unit-test/vmexit.flat:
> 
> w/o avanced timer:
> tscdeadline_immed: 2885    -> 2431.25  15.7%
> tscdeadline:       5668.75 -> 5188.5    8.4%
> 
> w/ adaptive advance timer default -1:
> tscdeadline_immed: 2965.25 -> 2520     15.0%
> tscdeadline:       4663.75 -> 4537      2.7%
> 
> Tested-by: Haiwei Li <lihaiwei@...cent.com>
> Cc: Haiwei Li <lihaiwei@...cent.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d21b66b..028967a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6560,12 +6560,28 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
>  	}
>  }
>  
> +static enum exit_fastpath_completion handle_fastpath_preemption_timer(struct kvm_vcpu *vcpu)

Somewhat offtopic, would it make sense to add a fastpath_t typedef?  These
enum lines are a bit long...

> +{
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +
> +	if (!vmx->req_immediate_exit &&
> +		!unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {

Bad indentation.

Also, this is is identical to handle_preemption_timer(), why not something
like:

static bool __handle_preemption_timer(struct vcpu)
{
	struct vcpu_vmx *vmx = to_vmx(vcpu);

	if (!vmx->req_immediate_exit &&
	    !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
		kvm_lapic_expired_hv_timer(vcpu);
		return true;
	}

	return false;
}

static enum exit_fastpath_completion handle_fastpath_preemption_timer(struct kvm_vcpu *vcpu)
{
	if (__handle_preemption_timer(vcpu))
		return EXIT_FASTPATH_CONT_RUN;
	return EXIT_FASTPATH_NONE;
}

static int handle_preemption_timer(struct kvm_vcpu *vcpu)
{
	__handle_preemption_timer(vcpu);
	return 1;
}

> +		kvm_lapic_expired_hv_timer(vcpu);
> +		trace_kvm_exit(EXIT_REASON_PREEMPTION_TIMER, vcpu, KVM_ISA_VMX);
> +		return EXIT_FASTPATH_CONT_RUN;
> +	}
> +
> +	return EXIT_FASTPATH_NONE;
> +}
> +
>  static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *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);
> +		case EXIT_REASON_PREEMPTION_TIMER:
> +			return handle_fastpath_preemption_timer(vcpu);
>  		default:
>  			return EXIT_FASTPATH_NONE;
>  		}
> -- 
> 2.7.4
>