Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL

Binbin Wu
2025-02-13T05:11:12Z

On 2/13/2025 11:23 AM, Binbin Wu wrote:
>
>
> On 2/13/2025 2:56 AM, Sean Christopherson wrote:
>> On Wed, Feb 12, 2025, Binbin Wu wrote:
>>> On 2/12/2025 8:46 AM, Sean Christopherson wrote:
>>>> I am completely comfortable saying that KVM doesn't care about STI/SS shadows
>>>> outside of the HALTED case, and so unless I'm missing something, I think it makes
>>>> sense for tdx_protected_apic_has_interrupt() to not check RVI outside of the HALTED
>>>> case, because it's impossible to know if the interrupt is actually unmasked, and
>>>> statistically it's far, far more likely that it _is_ masked.
>>> OK. Will update tdx_protected_apic_has_interrupt() in "TDX interrupts" part.
>>> And use kvm_vcpu_has_events() to replace the open code in this patch.
>> Something to keep an eye on: kvm_vcpu_has_events() returns true if pv_unhalted
>> is set, and pv_unhalted is only cleared on transitions KVM_MP_STATE_RUNNABLE.
>> If the guest initiates a spurious wakeup, pv_unhalted could be left set in
>> perpetuity.
>
> Oh, yes.
> KVM_HC_KICK_CPU is allowed in TDX guests.
>
> The change below looks good to me.
>
> One minor issue is when guest initiates a spurious wakeup, pv_unhalted is
> left set, then later when the guest want to halt the vcpu, in
> __kvm_emulate_halt(), since pv_unhalted is still set and the state will not
> transit to KVM_MP_STATE_HALTED.
> But I guess it's guests' responsibility to not initiate spurious wakeup,
> guests need to bear the fact that HLT could fail due to a previous
> spurious wakeup?

Just found a patch set for fixing the issue.
https://lore.kernel.org/kvm/20250113200150.487409-1-jmattson@google.com/

>
>>
>> I _think_ this would work and is generally desirable?
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 8e77e61d4fbd..435ca2782c3c 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -11114,9 +11114,6 @@ static bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
>>              kvm_apic_init_sipi_allowed(vcpu))
>>                  return true;
>>   -       if (vcpu->arch.pv.pv_unhalted)
>> -               return true;
>> -
>>          if (kvm_is_exception_pending(vcpu))
>>                  return true;
>>   @@ -11157,7 +11154,8 @@ static bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
>>     int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
>>   {
>> -       return kvm_vcpu_running(vcpu) || kvm_vcpu_has_events(vcpu);
>> +       return kvm_vcpu_running(vcpu) || vcpu->arch.pv.pv_unhalted ||
>> +              kvm_vcpu_has_events(vcpu);
>>   }
>>     /* Called within kvm->srcu read side.  */
>> @@ -11293,7 +11291,7 @@ static int __kvm_emulate_halt(struct kvm_vcpu *vcpu, int state, int reason)
>>           */
>>          ++vcpu->stat.halt_exits;
>>          if (lapic_in_kernel(vcpu)) {
>> -               if (kvm_vcpu_has_events(vcpu))
>> +               if (kvm_vcpu_has_events(vcpu) || vcpu->arch.pv.pv_unhalted)
>>                          vcpu->arch.pv.pv_unhalted = false;
>>                  else
>>                          vcpu->arch.mp_state = state;
>>
>>
>
>