Re: [PATCH 6/8] KVM: SVM: Re-inject INTn instead of retrying the insn on "failure"

[Sean Christopherson]

On Sat, Apr 02, 2022, Sean Christopherson wrote:
> Re-inject INTn software interrupts instead of retrying the instruction if
> the CPU encountered an intercepted exception while vectoring the INTn,
> e.g. if KVM intercepted a #PF when utilizing shadow paging.  Retrying the
> instruction is architecturally wrong e.g. will result in a spurious #DB
> if there's a code breakpoint on the INT3/O, and lack of re-injection also
> breaks nested virtualization, e.g. if L1 injects a software interrupt and
> vectoring the injected interrupt encounters an exception that is
> intercepted by L0 but not L1.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/svm.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index ecc828d6921e..00b1399681d1 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3425,14 +3425,24 @@ static void svm_inject_nmi(struct kvm_vcpu *vcpu)
>  static void svm_inject_irq(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> +	u32 type;
>  
>  	WARN_ON(!gif_set(svm));
>  
> +	if (vcpu->arch.interrupt.soft) {
> +		if (svm_update_soft_interrupt_rip(vcpu))
> +			return;
> +
> +		type = SVM_EVTINJ_TYPE_SOFT;
> +	} else {
> +		type = SVM_EVTINJ_TYPE_INTR;
> +	}
> +
>  	trace_kvm_inj_virq(vcpu->arch.interrupt.nr);
>  	++vcpu->stat.irq_injections;
>  
>  	svm->vmcb->control.event_inj = vcpu->arch.interrupt.nr |
> -		SVM_EVTINJ_VALID | SVM_EVTINJ_TYPE_INTR;
> +				       SVM_EVTINJ_VALID | type;
>  }
>  
>  void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode,
> @@ -3787,9 +3797,13 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>  	case SVM_EXITINTINFO_TYPE_INTR:
>  		kvm_queue_interrupt(vcpu, vector, false);
>  		break;
> +	case SVM_EXITINTINFO_TYPE_SOFT:
> +		kvm_queue_interrupt(vcpu, vector, true);

I believe this patch is wrong, it needs to also tweak the soft_int_injected logic
to look for SVM_EXITINTINFO_TYPE_EXEPT _or_ SVM_EXITINTINFO_TYPE_SOFT, e.g. if the
SOFT-type interrupt doesn't complete due to a non-legacy-exception exit, e.g. #NPF.

> +		break;
>  	default:
>  		break;
>  	}
> +
>  }