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

Joerg Roedel
2010-02-18T16:29:51Z

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