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

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