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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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>