Re: [PATCH v7 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

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

On 2026/08/07 10:01 AM, Ritesh Harjani wrote:
> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 
> > On POWER systems, the host CPU may run in a compatibility mode (e.g., a
> > Power11 processor operating in Power10 compatibility mode). In such
> > cases, the effective CPU level exposed to guests differs from the
> > physical processor generation.
> >
> > When running nested KVM guests, QEMU derives the host CPU type using
> > mfpvr(), which reflects the physical processor version. This can result
> > in a mismatch between the CPU model selected by QEMU and the
> > compatibility mode enforced by the host, leading to guest boot failures.
> >
> > For example, booting a nested guest on a Power11 LPAR configured in
> > Power10 compatibility mode fails with:
> >
> >   KVM-NESTEDv2: couldn't set guest wide elements
> >   [..KVM reg dump..]
> >
> > This occurs because QEMU selects a CPU model corresponding to the
> > physical processor (via mfpvr()), while the host operates in a lower
> > compatibility mode. As a result, KVM rejects the requested compatibility
> > level during guest initialization.
> >
> > On pseries nestedv2 systems, add support for retrieving host CPU
> > compatibility capabilities for nested guests on PowerVM. The capability
> > bitmap reflects the processor modes negotiated between the Power
> > hypervisor (L0) and the host partition (L1) via the
> > H_GUEST_GET_CAPABILITIES hcall, but is retrieved from the cached
> > nested_capabilities value populated during module initialization,
> > avoiding repeated hypervisor calls. A WARN_ON_ONCE() flags the
> > unexpected case where nested_capabilities is zero on a nestedv2 system.
> > The implementation defines KVM-specific capability constants
> > (KVM_PPC_COMPAT_CAP_POWER9/10/11), masks unsupported bits, and exposes
> > the result through the KVM_PPC_GET_COMPAT_CAPS ioctl.
> >
> > Hook the implementation into the Book3S HV kvmppc_ops so that it can be
> > invoked by the generic KVM ioctl handling code.
> >
> > Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> > Tested-by: Gautam Menghani <gautam@linux.ibm.com>
> > Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
> > Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
> > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > ---
> >  arch/powerpc/include/uapi/asm/kvm.h | 10 ++++++++++
> >  arch/powerpc/kvm/book3s_hv.c        | 20 ++++++++++++++++++++
> >  2 files changed, 30 insertions(+)
> 
> Looks good to me. Please feel free to add:
> 
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

Thanks for the review, Ritesh!

~Amit