Re: [PATCH v3] KVM: PPC: Book3S HV: Fix L2 guest reboot failure due to empty 'arch_compat'

[Michael Ellerman]

Hi Amit,

One comment below ...

Amit Machhiwal <amachhiw@xxxxxxxxxxxxx> writes:
> Currently, rebooting a pseries nested qemu-kvm guest (L2) results in
> below error as L1 qemu sends PVR value 'arch_compat' == 0 via
> ppc_set_compat ioctl. This triggers a condition failure in
> kvmppc_set_arch_compat() resulting in an EINVAL.
..
>  	
> diff --git a/arch/powerpc/kvm/book3s_hv_nestedv2.c b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> index 5378eb40b162..6042bdc70230 100644
> --- a/arch/powerpc/kvm/book3s_hv_nestedv2.c
> +++ b/arch/powerpc/kvm/book3s_hv_nestedv2.c
> @@ -347,8 +348,26 @@ static int gs_msg_ops_vcpu_fill_info(struct kvmppc_gs_buff *gsb,
>  			break;
>  		}
>  		case KVMPPC_GSID_LOGICAL_PVR:
> -			rc = kvmppc_gse_put_u32(gsb, iden,
> -						vcpu->arch.vcore->arch_compat);
> +			/*
> +			 * Though 'arch_compat == 0' would mean the default
> +			 * compatibility, arch_compat, being a Guest Wide
> +			 * Element, cannot be filled with a value of 0 in GSB
> +			 * as this would result into a kernel trap.
> +			 * Hence, when `arch_compat == 0`, arch_compat should
> +			 * default to L1's PVR.
> +			 *
> +			 * Rework this when PowerVM supports a value of 0
> +			 * for arch_compat for KVM API v2.
> +			 */

Is there an actual plan that PowerVM will support this in future?

If so, how will a future kernel know that it's running on a version of
PowerVM that does support arch_compat == 0?

Similarly how will we know when it's OK to drop support for this
workaround?

cheers