Re: [PATCH v2 0/5] KVM: PPC: Handle CPU compatibility mode for nested guests

[Anushree Mathur <anushree.mathur@linux.ibm.com>]

On 13/05/26 3:37 PM, Amit Machhiwal wrote:
> On POWER systems, newer processor generations can operate in compatibility
> modes corresponding to earlier generations (e.g., a Power11 system running
> in Power10 compatibility mode). In such cases, the effective CPU level
> exposed to guests differs from the physical processor generation.
>
> This creates a problem for nested virtualization. When booting a nested KVM
> guest (L2) inside a host KVM guest (L1) running in a compatibility mode,
> userspace (e.g., QEMU) may derive the CPU model from the raw hardware PVR
> and attempt to configure the nested guest accordingly. However, the L1
> partition is constrained by the compatibility level negotiated with the
> hypervisor (L0), and requests exceeding that level are rejected, leading to
> guest boot failures such as:
>
>    KVM-NESTEDv2: couldn't set guest wide elements
>
> This series addresses the issue in two steps:
>
> 1. Detect and reject invalid compatibility requests early in KVM to avoid
>     late failures.
>
> 2. Provide a mechanism for userspace to query the effective CPU
>     compatibility modes supported by the host, so it can select an
>     appropriate CPU model for nested guests.
>
> To achieve this, the series introduces a new KVM capability and ioctl
> (KVM_CAP_PPC_COMPAT_CAPS / KVM_PPC_GET_COMPAT_CAPS) that expose the
> compatibility modes supported by the host.
>
> The implementation supports both:
>
>    - PowerVM (nested API v2), where compatibility information is obtained
>      via the H_GUEST_GET_CAPABILITIES hypercall.
>    - PowerNV (nested API v1), where compatibility is derived from the device
>      tree ("cpu-version") representing the effective processor compatibility
>      level.
>
> This allows userspace (e.g., QEMU) to select a CPU model consistent with
> the host compatibility mode, avoiding mismatches and enabling successful
> nested guest boot.
>
> Changes in v2:
>    - Squashed patches 2 and 3 from v1 (capability introduction and ioctl
>      wiring) into a single patch for better logical grouping
>    - Changed kvm_ppc_compat_caps.flags from __u32 to __u64 for consistency
>      and future extensibility
>    - Addressed other review comments
>    - Improved commit messages with clearer explanations of the changes
>
> Patch summary:
>    [1/5] Validate arch_compat against host compatibility mode
>    [2/5] Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
>    [3/5] Implement capability retrieval for PowerVM (API v2)
>    [4/5] Add PowerNV support (API v1)
>    [5/5] Document the new ioctl
>
> Tested on:
>    - Power11 pSeries LPAR in Power10 compatibility mode (nested API v2)
>    - Power10 PowerNV system (and QEMU TCG PowerNV 11) with nested
>      virtualization (API v1) with various combinations of KVM L1/L2 guests
>      in various supported compatibility modes.
>
> With this series, nested guests boot successfully in configurations where
> they previously failed due to compatibility mismatches.
>
> Related QEMU series:
>    A corresponding QEMU series adds support for querying and using these
>    compatibility capabilities when configuring nested KVM guests:
>    https://lore.kernel.org/all/20260502140021.69712-1-amachhiw@linux.ibm.com/
>
> v1: https://lore.kernel.org/linuxppc-dev/20260430054906.94431-1-amachhiw@linux.ibm.com/
>
> Amit Machhiwal (5):
>    KVM: PPC: Book3S HV: Validate arch_compat against host compatibility
>      mode
>    KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
>    KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM
>      on PowerVM
>    KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM
>      on PowerNV
>    KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl
>
>   Documentation/virt/kvm/api.rst      | 35 ++++++++++++++++
>   arch/powerpc/include/asm/kvm_ppc.h  |  1 +
>   arch/powerpc/include/uapi/asm/kvm.h |  6 +++
>   arch/powerpc/kvm/book3s_hv.c        | 63 +++++++++++++++++++++++++++++
>   arch/powerpc/kvm/powerpc.c          | 21 ++++++++++
>   include/uapi/linux/kvm.h            |  4 ++
>   6 files changed, 130 insertions(+)
>
>
> base-commit: 1d5dcaa3bd65f2e8c9baa14a393d3a2dc5db7524

Hi Amit,
I tried booting up a guest on P11 lpar booted with P10 compat mode 
applying your patch along with the qemu patch series and it has been 
working perfectly fine.

Host lscpu:

lscpu
Architecture:                ppc64le
   Byte Order:                Little Endian
CPU(s):                      80
   On-line CPU(s) list:       0-79
Model name:                  POWER10 (architected), altivec supported


Guest lscpu:

lscpu
Architecture:                ppc64le
   Byte Order:                Little Endian
CPU(s):                      10
   On-line CPU(s) list:       0-9
Model name:                  POWER10 (architected), altivec supported

Feel free to add :

Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>

Thank you!
Anushree Mathur