Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the nested state

[Sean Christopherson]

On Thu, Sep 17, 2020 at 01:10:48PM +0300, Maxim Levitsky wrote:
> This way we don't waste memory on VMs which don't use
> nesting virtualization even if it is available to them.
> 
> If allocation of nested state fails (which should happen,
> only when host is about to OOM anyway), use new KVM_REQ_OUT_OF_MEMORY
> request to shut down the guest
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/nested.c | 42 ++++++++++++++++++++++++++++++
>  arch/x86/kvm/svm/svm.c    | 54 ++++++++++++++++++++++-----------------
>  arch/x86/kvm/svm/svm.h    |  7 +++++
>  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..fe119da2ef836 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))
> +		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;
>  }
>  
> +int svm_allocate_nested(struct vcpu_svm *svm)
> +{
> +	struct page *hsave_page;
> +
> +	if (svm->nested.initialized)
> +		return 0;
> +
> +	hsave_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +	if (!hsave_page)
> +		goto error;

goto is unnecessary, just do

		return -ENOMEM;

> +
> +	svm->nested.hsave = page_address(hsave_page);
> +
> +	svm->nested.msrpm = svm_vcpu_init_msrpm();
> +	if (!svm->nested.msrpm)
> +		goto err_free_hsave;
> +
> +	svm->nested.initialized = true;
> +	return 0;
> +err_free_hsave:
> +	__free_page(hsave_page);
> +error:
> +	return 1;

As above, -ENOMEM would be preferable.

> +}
> +
> +void svm_free_nested(struct vcpu_svm *svm)
> +{
> +	if (!svm->nested.initialized)
> +		return;
> +
> +	svm_vcpu_free_msrpm(svm->nested.msrpm);
> +	svm->nested.msrpm = NULL;
> +
> +	__free_page(virt_to_page(svm->nested.hsave));
> +	svm->nested.hsave = NULL;
> +
> +	svm->nested.initialized = false;
> +}
> +
>  /*
>   * Forcibly leave nested mode in order to be able to reset the VCPU later on.
>   */
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 3da5b2f1b4a19..57ea4407dcf09 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -266,6 +266,7 @@ static int get_max_npt_level(void)
>  void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	u64 old_efer = vcpu->arch.efer;
>  	vcpu->arch.efer = efer;
>  
>  	if (!npt_enabled) {
> @@ -276,9 +277,26 @@ void svm_set_efer(struct kvm_vcpu *vcpu, u64 efer)
>  			efer &= ~EFER_LME;
>  	}
>  
> -	if (!(efer & EFER_SVME)) {
> -		svm_leave_nested(svm);
> -		svm_set_gif(svm, true);
> +	if ((old_efer & EFER_SVME) != (efer & EFER_SVME)) {
> +		if (!(efer & EFER_SVME)) {
> +			svm_leave_nested(svm);
> +			svm_set_gif(svm, true);
> +
> +			/*
> +			 * Free the nested state unless we are in SMM, in which
> +			 * case the exit from SVM mode is only for duration of the SMI
> +			 * handler
> +			 */
> +			if (!is_smm(&svm->vcpu))
> +				svm_free_nested(svm);
> +
> +		} else {
> +			if (svm_allocate_nested(svm)) {
> +				vcpu->arch.efer = old_efer;
> +				kvm_make_request(KVM_REQ_OUT_OF_MEMORY, vcpu);

I really dislike KVM_REQ_OUT_OF_MEMORY.  It's redundant with -ENOMEM and
creates a huge discrepancy with respect to existing code, e.g. nVMX returns
-ENOMEM in a similar situation.

The deferred error handling creates other issues, e.g. vcpu->arch.efer is
unwound but the guest's RIP is not.

One thought for handling this without opening a can of worms would be to do:

	r = kvm_x86_ops.set_efer(vcpu, efer);
	if (r) {
		WARN_ON(r > 0);
		return r;
	}

I.e. go with the original approach, but only for returning errors that will
go all the way out to userspace.

> +				return;
> +			}
> +		}
>  	}
>  
>  	svm->vmcb->save.efer = efer | EFER_SVME;