Re: [PATCH 4/5] KVM: Lazify fpu activation and deactivation

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Dec 30, 2009 at 06:25:41PM +0200, Avi Kivity wrote:
> Defer fpu deactivation as much as possible - if the guest fpu is loaded, keep
> it loaded until the next heavyweight exit (where we are forced to unload it).
> This reduces unnecessary exits.
> 
> We also defer fpu activation on clts; while clts signals the intent to use the
> fpu, we can't be sure the guest will actually use it.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>

> +static void svm_fpu_deactivate(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +
> +	if (npt_enabled) {
> +		/* hack: npt requires active fpu at this time */
> +		vcpu->fpu_active = 1;
> +		return;
> +	}

Why is that ?

>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
> -	kvm_x86_ops->vcpu_put(vcpu);
>  	kvm_put_guest_fpu(vcpu);
> +	kvm_x86_ops->vcpu_put(vcpu);
>  }

It might be possible to defer host FPU restoration to
user-return-notifier/kernel_fpu_begin time, so you'd keep the guest FPU
loaded across qemukvm->kernel task->qemukvm switches. Not sure if its
worthwhile though.