Re: [PATCH] KVM: nVMX: Fix nested bus lock VM exit

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 9/2/2021 2:08 AM, Sean Christopherson wrote:
> On Fri, Aug 27, 2021, Chenyi Qiang wrote:
>> Nested bus lock VM exits are not supported yet. If L2 triggers bus lock
>> VM exit, it will be directed to L1 VMM, which would cause unexpected
>> behavior. Therefore, handle L2's bus lock VM exits in L0 directly.
>>
>> Fixes: fe6b6bc802b4 ("KVM: VMX: Enable bus lock VM exit")
>> Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
>> ---
>>   arch/x86/kvm/vmx/nested.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>> index bc6327950657..754f53cf0f7a 100644
>> --- a/arch/x86/kvm/vmx/nested.c
>> +++ b/arch/x86/kvm/vmx/nested.c
>> @@ -5873,6 +5873,8 @@ static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu,
>>   	case EXIT_REASON_VMFUNC:
>>   		/* VM functions are emulated through L2->L0 vmexits. */
>>   		return true;
>> +	case EXIT_REASON_BUS_LOCK:
>> +		return true;
> 
> Hmm, unless there is zero chance of ever exposing BUS_LOCK_DETECTION to L1, it
> might be better to handle this in nested_vmx_l1_wants_exit(), e.g.
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index b3f77d18eb5a..793534b7eaba 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -6024,6 +6024,8 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu,
>                          SECONDARY_EXEC_ENABLE_USR_WAIT_PAUSE);
>          case EXIT_REASON_ENCLS:
>                  return nested_vmx_exit_handled_encls(vcpu, vmcs12);
> +       case EXIT_REASON_BUS_LOCK:
> +               return nested_cpu_has2(vmcs12, SECONDARY_EXEC_BUS_LOCK_DETECTION);

yes, for now, it equals

                   return false;

because KVM doesn't expose it to L1.

>          default:
>                  return true;
>          }
> 
> It's a rather roundabout way of reaching the same result, but I'd prefer to limit
> nested_vmx_l0_wants_exit() to cases where L0 wants to handle the exit regardless
> of what L1 wants.  This kinda fits that model, but it's not really that L0 "wants"
> the exit, it's that L1 can't want the exit.  Does that make sense?

something like below has to be in nested_vmx_l0_wants_exit()

--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5873,6 +5873,8 @@ static bool nested_vmx_l0_wants_exit(struct 
kvm_vcpu *vcpu,
         case EXIT_REASON_VMFUNC:
                 /* VM functions are emulated through L2->L0 vmexits. */
                 return true;
+       case EXIT_REASON_BUS_LOCK:
+               return vcpu->kvm->arch.bus_lock_detection_enabled;
         default:
                 break;
         }


L0 wants this VM exit because it enables BUS LOCK VM exit, not because 
L1 doesn't enable it.

> 
>>   	default:
>>   		break;
>>   	}
>> -- 
>> 2.17.1
>>