Re: [PATCH 1/6] KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode

[Harsh Prateek Bora <harshpb@linux.ibm.com>]

On 30/04/26 11:19 am, Amit Machhiwal wrote:
> On IBM POWER systems, newer processor generations can operate in
> compatibility modes corresponding to earlier generations. This becomes
> relevant for nested virtualization, where nested KVM guests may need to
> run with a specific processor compatibility level.
> 
> Currently, when running a nested KVM guest (L2) inside a Power11 pSeries
> logical partition (L1) booted in Power10 compatibility mode, the guest
> fails to boot while setting 'arch_compat'. This happens because the CPU
> class is derived from the hardware PVR (via mfspr()), which reflects the
> physical processor generation (Power11), rather than the effective
> compatibility mode (Power10).
> 
> As a result, userspace may request a Power11 arch_compat for the L2
> guest. However, the L1 partition, running in Power10 compatibility, has
> only negotiated support up to Power10 with the Power Hypervisor (L0).
> When H_SET_STATE is invoked with a Power11 Logical PVR, the hypervisor
> rejects the request, leading to a late guest boot failure:
> 
>    KVM-NESTEDv2: couldn't set guest wide elements
>    [..KVM reg dump..]
> 
> This situation should be detected earlier. Rejecting unsupported
> 'arch_compat' values in 'kvmppc_set_arch_compat()' avoids issuing an
> invalid H_SET_STATE hcall and provides a clearer failure mode.
> 
> Add a check to reject Power11 'arch_compat' requests when the host is
> running in Power10 compatibility mode, returning -EINVAL early instead
> of deferring the failure to the hypervisor.
> 
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
>   arch/powerpc/kvm/book3s_hv.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 61dbeea317f3..948c6b099a29 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -446,7 +446,13 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
>   			guest_pcr_bit = PCR_ARCH_300;
>   			break;
>   		case PVR_ARCH_31:
> +			guest_pcr_bit = PCR_ARCH_31;
> +			break;
>   		case PVR_ARCH_31_P11:
> +			if ((PVR_ARCH_31 & cur_cpu_spec->pvr_mask) ==
> +				cur_cpu_spec->pvr_value) {
> +				return -EINVAL;
> +			}

Is it possible to keep the check generic for applicable ISA versions 
(ISA 3.1 onwards?) instead of keeping it specific for P11 ?
Also need to add a comment, why it's not applicable for older ISAs.

>   			guest_pcr_bit = PCR_ARCH_31;
>   			break;
>   		default: