Re: [PATCH v3 1/2] KVM: PPC: New capability to control MCE behaviour

[Paul Mackerras <paulus@ozlabs.org>]

On Wed, Jan 13, 2016 at 12:37:59PM +0530, Aravinda Prasad wrote:
> This patch introduces a new KVM capability to control
> how KVM behaves on machine check exception (MCE).
> Without this capability, KVM redirects machine check
> exceptions to guest's 0x200 vector if the address in
> error belongs to the guest. With this capability KVM
> causes a guest exit with NMI exit reason.
> 
> This is required to avoid problems if a new kernel/KVM
> is used with an old QEMU for guests that don't issue
> "ibm,nmi-register". As old QEMU does not understand the
> NMI exit type, it treats it as a fatal error. However,
> the guest could have handled the machine check error
> if the exception was delivered to guest's 0x200 interrupt
> vector instead of NMI exit in case of old QEMU.

[snip]

> @@ -1132,6 +1135,10 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
>  		break;
>  	}
>  #endif /* CONFIG_KVM_XICS */
> +	case KVM_CAP_PPC_FWNMI:
> +		r = 0;
> +		vcpu->kvm->arch.fwnmi_enabled = true;
> +		break;

Might we ever want to set this flag back to false after setting it to
true?  If so perhaps we should do vcpu->kvm->arch.fwnmi_enabled =
!!cap->args[0].  However, I admit I can't actually think of a
situation where we would need to reset it. :)

Paul.