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

[Sean Christopherson]

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.

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

> +		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;
>  }