linux-kernel - Re: [RFC] KVM: optimize the kvm_vcpu_on

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Date:   Mon, 31 Jul 2017 13:31:45 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Longpeng(Mike)" <longpeng2@...wei.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com
Cc:     agraf@...e.com, borntraeger@...ibm.com, cohuck@...hat.com,
        christoffer.dall@...aro.org, marc.zyngier@....com,
        james.hogan@...tec.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, weidong.huang@...wei.com,
        arei.gonglei@...wei.com, wangxinxin.wang@...wei.com,
        longpeng.mike@...il.com
Subject: Re: [RFC] KVM: optimize the kvm_vcpu_on_spin

[no idea if this change makes sense (and especially if it has any bad
side effects), do you have performance numbers? I'll just have a look at
the general structure of the patch in the meanwhile]

> +bool kvm_arch_vcpu_spin_kernmode(struct kvm_vcpu *vcpu)

kvm_arch_vcpu_in_kernel() ?

> +{
> +	return kvm_x86_ops->get_cpl(vcpu) == 0;
> +}
> +
>  int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  {
>  	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 648b34c..f8f0d74 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -272,6 +272,9 @@ struct kvm_vcpu {
>  	} spin_loop;
>  #endif
>  	bool preempted;
> +	/* If vcpu is in kernel-mode when preempted */
> +	bool in_kernmode;
> +

Why do you have to store that ...

[...]> +	me->in_kernmode = kvm_arch_vcpu_spin_kernmode(me);
>  	kvm_vcpu_set_in_spin_loop(me, true);
>  	/*
>  	 * We boost the priority of a VCPU that is runnable but not
> @@ -2351,6 +2353,8 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  				continue;
>  			if (swait_active(&vcpu->wq) && !kvm_arch_vcpu_runnable(vcpu))
>  				continue;
> +			if (me->in_kernmode && !vcpu->in_kernmode)

Wouldn't it be easier to simply have

in_kernel = kvm_arch_vcpu_in_kernel(me);
...
if (in_kernel && !kvm_arch_vcpu_in_kernel(vcpu))
...

> +				continue;
>  			if (!kvm_vcpu_eligible_for_directed_yield(vcpu))
>  				continue;
>  
> @@ -4009,8 +4013,11 @@ static void kvm_sched_out(struct preempt_notifier *pn,
>  {
>  	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
>  
> -	if (current->state == TASK_RUNNING)
> +	if (current->state == TASK_RUNNING) {
>  		vcpu->preempted = true;
> +		vcpu->in_kernmode = kvm_arch_vcpu_spin_kernmode(vcpu);
> +	}
> +

so you don't have to do this change, too.

>  	kvm_arch_vcpu_put(vcpu);
>  }
>  
> 


-- 

Thanks,

David