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

Vitaly Kuznetsov
2021-05-12T07:15:05Z

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