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

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Harsh,

Thanks for looking at the series and your review. Please find my comments
inline.

On 2026/05/05 12:14 PM, Harsh Prateek Bora wrote:
> 
> 
> 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.

I believe the check is only required for ISA 3.1, the reason being that Power10
and Power11 share the same PCR value. For newer ISA versions, that might not
necessarily be the case. For any subsequent ISA versions, this check would
already be taken care of by the below check in kvmppc_set_arch_compat():

	if (guest_pcr_bit > host_pcr_bit)
		return -EINVAL;

Its mostly implicit but I'm okay to include a comment mentioning the same.

Thanks,
Amit

> 
> >   			guest_pcr_bit = PCR_ARCH_31;
> >   			break;
> >   		default:
>