linux-kernel - Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>

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Message-ID: <f12e1c06-d38d-4ed0-b471-7f016057f604@linux.intel.com>
Date: Thu, 13 Feb 2025 11:23:00 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Chao Gao <chao.gao@...el.com>, Yan Zhao <yan.y.zhao@...el.com>,
 pbonzini@...hat.com, kvm@...r.kernel.org, rick.p.edgecombe@...el.com,
 kai.huang@...el.com, adrian.hunter@...el.com, reinette.chatre@...el.com,
 xiaoyao.li@...el.com, tony.lindgren@...el.com, isaku.yamahata@...el.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>



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?

>
> 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;
>
>