linux-kernel - Re: [PATCH 09/10] KVM: SVM: Make lazy FPU switching work with nested svm

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Message-ID: <20100218162935.GR22141@amd.com>
Date:	Thu, 18 Feb 2010 17:29:35 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/10] KVM: SVM: Make lazy FPU switching work with
 nested svm

On Thu, Feb 18, 2010 at 04:32:02PM +0200, Avi Kivity wrote:
> On 02/18/2010 01:38 PM, Joerg Roedel wrote:
> >TDB.
> >
> 
> ...
> 
> >@@ -973,6 +973,7 @@ static void svm_decache_cr4_guest_bits(struct kvm_vcpu *vcpu)
> >
> >  static void update_cr0_intercept(struct vcpu_svm *svm)
> >  {
> >+	struct vmcb *vmcb = svm->vmcb;
> >  	ulong gcr0 = svm->vcpu.arch.cr0;
> >  	u64 *hcr0 =&svm->vmcb->save.cr0;
> >
> >@@ -984,11 +985,25 @@ static void update_cr0_intercept(struct vcpu_svm *svm)
> >
> >
> >  	if (gcr0 == *hcr0&&  svm->vcpu.fpu_active) {
> >-		svm->vmcb->control.intercept_cr_read&= ~INTERCEPT_CR0_MASK;
> >-		svm->vmcb->control.intercept_cr_write&= ~INTERCEPT_CR0_MASK;
> >+		vmcb->control.intercept_cr_read&= ~INTERCEPT_CR0_MASK;
> >+		vmcb->control.intercept_cr_write&= ~INTERCEPT_CR0_MASK;
> >+		if (is_nested(svm)) {
> >+			struct vmcb *hsave = svm->nested.hsave;
> >+
> >+			hsave->control.intercept_cr_read&= ~INTERCEPT_CR0_MASK;
> >+			hsave->control.intercept_cr_write&= ~INTERCEPT_CR0_MASK;
> >+			vmcb->control.intercept_cr_read  |= svm->nested.intercept_cr_read;
> >+			vmcb->control.intercept_cr_write |= svm->nested.intercept_cr_write;
> 
> Why are the last two lines needed?

Because we don't know if the l1 hypervisor wants to intercept cr0. In
this case we need this intercept to stay enabled.

> >+		}
> >  	} else {
> >  		svm->vmcb->control.intercept_cr_read |= INTERCEPT_CR0_MASK;
> >  		svm->vmcb->control.intercept_cr_write |= INTERCEPT_CR0_MASK;
> >+		if (is_nested(svm)) {
> >+			struct vmcb *hsave = svm->nested.hsave;
> >+
> >+			hsave->control.intercept_cr_read |= INTERCEPT_CR0_MASK;
> >+			hsave->control.intercept_cr_write |= INTERCEPT_CR0_MASK;
> >+		}
> >  	}
> >  }
> 
> Maybe it's better to call update_cr0_intercept() after a vmexit
> instead, to avoid this repetition, and since the if () may take a
> different branch for the nested guest and guest cr0.

Thinking again about it I am not sure if this is needed at all. At
vmexit emulation we call svm_set_cr0 which itself calls
update_cr0_intercept. I'll try this.

	Joerg


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