From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH 4/5] KVM: Lazify fpu activation and deactivation
Date: Tue, 5 Jan 2010 22:25:29 -0200
Message-ID: <20100106002529.GA11233@amt.cnet>
References: <1262190342-18611-1-git-send-email-avi@redhat.com> <1262190342-18611-5-git-send-email-avi@redhat.com>
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Cc: Sheng Yang <sheng@linux.intel.com>,
	Joerg Roedel <joerg.roedel@amd.com>, kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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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.