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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Sean Christopherson <seanjc@google.com> writes:

> 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 (...) {
> 	}
>

I like the idea, thanks! Will incorporate into v2.

>>  		/* 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
>> 
>

-- 
Vitaly