Re: [PATCH v6 4/4] KVM: nSVM: implement on demand allocation of the nested state

[Maxim Levitsky]

On Mon, 2020-09-28 at 22:15 -0700, Sean Christopherson wrote:
> On Wed, Sep 23, 2020 at 12:10:25AM +0300, Maxim Levitsky wrote:
> > This way we don't waste memory on VMs which don't use nesting
> > virtualization even when the host enabled it for them.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/svm/nested.c | 42 ++++++++++++++++++++++++++++++
> >  arch/x86/kvm/svm/svm.c    | 55 ++++++++++++++++++++++-----------------
> >  arch/x86/kvm/svm/svm.h    |  6 +++++
> >  3 files changed, 79 insertions(+), 24 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 09417f5197410..dd13856818a03 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -467,6 +467,9 @@ int nested_svm_vmrun(struct vcpu_svm *svm)
> >  
> >  	vmcb12 = map.hva;
> >  
> > +	if (WARN_ON(!svm->nested.initialized))
> 
> Probably should use WARN_ON_ONCE, if this is somehow it, userspace could
> easily spam the kernel.
Makes sense. 

> 
> Side topic, do we actually need 'initialized'?  Wouldn't checking for a
> valid nested.msrpm or nested.hsave suffice?

It a matter of taste - I prefer to have a single variable controlling this,
rather than two. 
a WARN_ON(svm->nested.initialized && !svm->nested.msrpm || !svm->nested.hsave))
would probably be nice to have. IMHO I rather leave this like it is if you
don't object.

> 
> > +		return 1;
> > +
> >  	if (!nested_vmcb_checks(svm, vmcb12)) {
> >  		vmcb12->control.exit_code    = SVM_EXIT_ERR;
> >  		vmcb12->control.exit_code_hi = 0;
> > @@ -684,6 +687,45 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
> >  	return 0;
> >  }

Best regards,
	Maxim Levitsky