[PATCH 0/2] ppc/kvm: Handle CPU compatibility mode correctly for nested guests

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

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:
  [1/2] hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
  [2/2] target/ppc/kvm: Use host compatibility mode for nested guests

Tested on:
  - Power11 pSeries LPAR in Power10 compatibility mode
  - Power10 PowerNV and QEMU PowerNV 11 TCG L0 host

Note: The corresponding Linux patches have been posted [1]

[1] https://lore.kernel.org/all/20260430054906.94431-1-amachhiw@linux.ibm.com/

Amit Machhiwal (2):
  hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
  target/ppc/kvm: Use host compatibility mode for nested guests

 hw/ppc/spapr_hcall.c |  9 ++++++++
 target/ppc/kvm.c     | 52 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)


base-commit: 759c456b1d22fe4083c8b384da27d3f56fd53f82
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2.50.1