Re: [PATCH 3/7] KVM: nVMX: Ignore 'hv_clean_fields' data when eVMCS data is copied in vmx_get_nested_state()

[Sean Christopherson <seanjc@google.com>]

On Tue, May 11, 2021, Vitaly Kuznetsov wrote:
> 'Clean fields' data from enlightened VMCS is only valid upon vmentry: L1
> hypervisor is not obliged to keep it up-to-date while it is mangling L2's
> state, KVM_GET_NESTED_STATE request may come at a wrong moment when actual
> eVMCS changes are unsynchronized with 'hv_clean_fields'. As upon migration
> VMCS12 is used as a source of ultimate truce, we must make sure we pick all
> the changes to eVMCS and thus 'clean fields' data must be ignored.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 43 +++++++++++++++++++++++----------------
>  1 file changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ea2869d8b823..7970a16ee6b1 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1607,16 +1607,23 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
>  	vmcs_load(vmx->loaded_vmcs->vmcs);
>  }
>  
> -static int copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx)
> +static int copy_enlightened_to_vmcs12(struct vcpu_vmx *vmx, bool from_vmentry)
>  {
>  	struct vmcs12 *vmcs12 = vmx->nested.cached_vmcs12;
>  	struct hv_enlightened_vmcs *evmcs = vmx->nested.hv_evmcs;
> +	u32 hv_clean_fields;
>  
>  	/* HV_VMX_ENLIGHTENED_CLEAN_FIELD_NONE */
>  	vmcs12->tpr_threshold = evmcs->tpr_threshold;
>  	vmcs12->guest_rip = evmcs->guest_rip;
>  
> -	if (unlikely(!(evmcs->hv_clean_fields &
> +	/* Clean fields data can only be trusted upon vmentry */
> +	if (likely(from_vmentry))
> +		hv_clean_fields = evmcs->hv_clean_fields;
> +	else
> +		hv_clean_fields = 0;

...

> @@ -3503,7 +3510,7 @@ static int nested_vmx_run(struct kvm_vcpu *vcpu, bool launch)
>  		return nested_vmx_failInvalid(vcpu);
>  
>  	if (vmx->nested.hv_evmcs) {
> -		copy_enlightened_to_vmcs12(vmx);
> +		copy_enlightened_to_vmcs12(vmx, true);

Rather than pass a bool, what about having the caller explicitly specify the
clean fields?  Then the migration path can have a comment about needing to
assume all fields are dirty, and the normal path would be self-documenting.
E.g. with evmcs captured in a local var:

	if (evmcs) {
		copy_enlightened_to_vmcs12(vmx, evmcs->hv_clean_fields);
	} else if (...) {
	}

>  		/* Enlightened VMCS doesn't have launch state */
>  		vmcs12->launch_state = !launch;
>  	} else if (enable_shadow_vmcs) {
> @@ -6136,7 +6143,7 @@ static int vmx_get_nested_state(struct kvm_vcpu *vcpu,
>  		copy_vmcs02_to_vmcs12_rare(vcpu, get_vmcs12(vcpu));
>  		if (!vmx->nested.need_vmcs12_to_shadow_sync) {
>  			if (vmx->nested.hv_evmcs)
> -				copy_enlightened_to_vmcs12(vmx);
> +				copy_enlightened_to_vmcs12(vmx, false);
>  			else if (enable_shadow_vmcs)
>  				copy_shadow_to_vmcs12(vmx);
>  		}
> -- 
> 2.30.2
>