Re: [PATCH v2 0/3] ppc/kvm: Handle CPU compatibility mode correctly for nested guests

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

On 02/05/26 7:30 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 issues for nested virtualization. When booting a nested KVM
> guest, QEMU may derive the CPU model from the raw hardware PVR and attempt
> to configure the guest accordingly. However, the host is constrained by the
> compatibility level negotiated with the hypervisor, and requests exceeding
> that level are rejected by KVM, leading to guest boot failures such as:
>
>    KVM-NESTEDv2: couldn't set guest wide elements
>
> This series addresses the issue in two ways:
>
> 1. Do not silently fall back to raw mode when KVM rejects a requested
>     compatibility level during CAS. Instead, propagate the error so invalid
>     configurations are visible and fail early.
>
> 2. Query the effective CPU compatibility modes supported by the host via
>     KVM and use this information to select an appropriate CPU model for
>     nested guests.
>
> With these changes, QEMU avoids masking KVM errors and ensures that nested
> guests are configured with CPU models consistent with the host
> compatibility mode, allowing them to boot correctly.
>
> Patch summary:
>    [1/3] hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
>    [2/3] [DO_NOT_MERGE] linux-headers: Add uapi header changes
>    [3/3] target/ppc/kvm: Use host compatibility mode for nested guests
>
> Changes in v2:
> - Patch 3: Guard compatibility mode code with #if defined(TARGET_PPC64)
>    to fix compilation for ppc32 targets. The POWER9/10/11 PVR constants
>    are only defined for 64-bit builds, and compatibility modes are only
>    relevant for 64-bit systems.
>
> Tested on:
>    - Power11 pSeries LPAR in Power10 compatibility mode
>    - Power10 PowerNV and QEMU PowerNV 11 TCG L0 host
>
> CI test results: https://gitlab.com/amachhiw/qemu/-/pipelines/2494987253
>
> Note: Patch 2 is marked DO_NOT_MERGE as it contains linux-headers updates
> that will be synced separately once the corresponding kernel patches are
> merged.
>
> The corresponding Linux patches have been posted [1]
>
> [1] https://lore.kernel.org/all/20260430054906.94431-1-amachhiw@linux.ibm.com/
>
> Amit Machhiwal (3):
>    hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
>    [DO_NOT_MERGE] linux-headers: Add uapi header changes
>    target/ppc/kvm: Use host compatibility mode for nested guests
>
>   hw/ppc/spapr_hcall.c            |  9 ++++++
>   linux-headers/asm-powerpc/kvm.h |  7 ++++
>   linux-headers/linux/kvm.h       |  3 ++
>   target/ppc/kvm.c                | 57 +++++++++++++++++++++++++++++++++
>   4 files changed, 76 insertions(+)
>
>
> base-commit: 3d626609ccae61a2e552bccd59c7a0931bab8261



Hi Amit,
I tried booting up a guest on P11 lpar booted with P10 compat mode 
applying your patch along with the linux 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