Re: [PATCH v3 4/5] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV

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

On 2026/06/03 09:47 AM, Vaibhav Jain wrote:
> Hi Amit,
> 
> Thanks for the patch. My review comments inline:
> 
> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 
> > Currently, when booting a compatibility-mode KVM guest (L1) on a PowerNV
> > hypervisor (L0), the guest runs with the expected processor
> > compatibility level. However, when booting a nested KVM guest (L2)
> > inside the L1, QEMU derives the CPU model from the raw host PVR and
> > attempts to run the nested guest at that level, instead of honoring the
> > compatibility mode of the L1.
> >
> > Extend host CPU compatibility capability reporting to support nested
> > virtualization on PowerNV systems (PAPR nested API v1).
> >
> > For nested API v2 (PowerVM), compatibility capabilities are obtained
> > from the hypervisor via the H_GUEST_GET_CAPABILITIES hcall. This
> > information is not available on PowerNV systems.
> >
> > For nested API v1, derive the compatibility capabilities from the L1
> > guest by reading the "cpu-version" property from the device tree, which
> > reflects the effective (logical) processor compatibility level. Map this
> > value to the corresponding compatibility capability bitmap.
> >
> > Introduce a helper to translate CPU version values into compatibility
> > capability bits and integrate it into kvmppc_get_compat_cpu_caps().
> >
> > This allows userspace to query host CPU compatibility modes on both
> > PowerVM and PowerNV platforms via the KVM_PPC_GET_COMPAT_CAPS ioctl.
> >
> > Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> > Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
> > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > ---
> >  arch/powerpc/kvm/book3s_hv.c | 37 +++++++++++++++++++++++++++++++++++-
> >  1 file changed, 36 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index 38de7040e2b7..18774c49af85 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -6522,15 +6522,50 @@ static bool kvmppc_hash_v3_possible(void)
> >  	return true;
> >  }
> >  
> > +static int kvmppc_map_compat_capabilities(const __be32 cpu_version,
> > +				      unsigned long *capabilities)
> > +{
> > +	switch (cpu_version) {
> > +	case PVR_ARCH_31_P11:
> > +		*capabilities |= H_GUEST_CAP_POWER11;
> > +		break;
> > +	case PVR_ARCH_31:
> > +		*capabilities |= H_GUEST_CAP_POWER10;
> > +		break;
> > +	case PVR_ARCH_300:
> > +		*capabilities |= H_GUEST_CAP_POWER9;
> > +		break;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> >  
> >  static int kvmppc_get_compat_cpu_caps(struct kvm_ppc_compat_caps *host_caps)
> >  {
> > +	struct device_node *np;
> >  	unsigned long capabilities = 0;
> > +	const __be32 *prop = NULL;
> >  	long rc = -EINVAL;
> > +	u32 cpu_version;
> >  
> >  	if (kvmhv_on_pseries()) {
> > -		if (kvmhv_is_nestedv2())
> > +		if (kvmhv_is_nestedv2()) {
> >  			rc = plpar_guest_get_capabilities(0,
> >  	&capabilities);
> Need to mask capabilities as mentioned in the review comments for
> previous patch. I would suggest creating a helper that performs the
> hcall and applies the mask which can then be used at
> plpar_guest_get_capabilities() call sites.

Sure, will do.

Thanks,
Amit

> 
> > +		} else {
> > +			for_each_node_by_type(np, "cpu") {
> > +				prop = of_get_property(np, "cpu-version", NULL);
> > +				if (prop) {
> > +					cpu_version = be32_to_cpup(prop);
> > +					break;
> > +				}
> > +			}
> > +			if (!prop)
> > +				return -EINVAL;
> > +			rc = kvmppc_map_compat_capabilities(cpu_version,
> > +								&capabilities);
> > +		}
> >  		host_caps->compat_capabilities = capabilities;
> >  	}
> >  
> > -- 
> > 2.50.1 (Apple Git-155)
> >
> 
> -- 
> Cheers
> ~ Vaibhav