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

[Gautam Menghani]

On Wed, Jun 03, 2026 at 07:45:39PM +0530, 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_GUEST_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_GUEST_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.
> 
> Suggested-by: Vaibhav Jain <vaibhav@xxxxxxxxxxxxx>
> Tested-by: Anushree Mathur <anushree.mathur@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.13+
> Signed-off-by: Amit Machhiwal <amachhiw@xxxxxxxxxxxxx>
> ---
> Changelog:
> 
> * Moved this patch out of the v3 series [1] as discussed here [2]
> * Addressed below review comments from Ritesh:
>   - Based the PVR validation on cpu features
>   - Fixed hcall name typo
>   - Stable backport
> 
> [1] https://lore.kernel.org/all/20260522152744.55251-1-amachhiw@xxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/20260522152744.55251-2-amachhiw@xxxxxxxxxxxxx/
> ---
>  arch/powerpc/kvm/book3s_hv.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 61dbeea317f3..e16dbb199366 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -446,7 +446,17 @@ 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:
> +			/*
> +			 * Need to check this for ISA 3.1, as Power10 and
> +			 * Power11 share the same PCR. For any subsequent ISA
> +			 * versions, this will be taken care of by the guest vs
> +			 * host PCR comparison below.
> +			 */
> +			if (!cpu_has_feature(CPU_FTR_P11_PVR))
> +				return -EINVAL;
>  			guest_pcr_bit = PCR_ARCH_31;
>  			break;
>  		default:
> 
> base-commit: ba3e43a9e601636f5edb54e259a74f96ca3b8fd8
> -- 
> 2.50.1 (Apple Git-155)
> 

I booted a KVM guest on LPAR with this patch in the following scenarios:
1. P10 guest on P10 host: No error observed
2. P11 guest on P11 host: No error observed
3. P11 guest on P11 host booted in P10 compat mode: No error observed

Tested-by: Gautam Menghani <gautam@xxxxxxxxxxxxx>