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 by preventing fallback to raw mode when the
host itself is booted in a compatibility mode, and by querying the effective
CPU compatibility modes supported by the host via KVM. The implementation
includes size field validation for ABI compatibility between QEMU and the
kernel. With these changes, QEMU ensures that nested guests are configured
with CPU models consistent with the host compatibility mode, allowing them
to boot correctly.

Patch summary:
  [1/3] [DO_NOT_MERGE] linux-headers: Add uapi header changes
  [2/3] target/ppc/kvm: Add support for querying host compatibility mode
  [3/3] target/ppc/kvm: Use host compatibility mode for nested guests

Changes in v3 (based on review from Vaibhav):
- Patch 1: Moved compatibility mode check from do_client_architecture_support()
  to cas_check_pvr(). Instead of error handling when KVM rejects compat mode,
  now prevents raw mode fallback when host is booted in compatibility mode.
  This ensures guests cannot exceed the host's compatibility level.

- Patch 2: Added size field to kvm_ppc_compat_caps structure for ABI
  versioning. Changed flags field from __u32 to __u64. Added capability bit
  definitions (KVM_PPC_COMPAT_CAP_POWER9/10/11) and KVM_PPC_COMPAT_BITMASK.

- Patch 3: Added size field initialization and validation in
  kvmppc_get_compat_caps() to ensure ABI compatibility between QEMU and
  kernel. Changed from H_GUEST_CAP_* constants to KVM_PPC_COMPAT_CAP_*
  constants. Added capability masking with KVM_PPC_COMPAT_BITMASK.

- Dropped Tested-by from Anushree due to code changes in v3.

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.

Testing (with kernel v4 patches):

KVM APIv1 Testing
=================
  On P10 PowerNV machine (L0)
  ---------------------------
    - P10 L1 KVM guest -> works
      - P10 nested L2 KVM guest -> works
      - P9 compat nested L2 KVM guest -> works
    - P9 compat L1 KVM guest -> works
      - P9 nested L2 KVM guest -> works
  
  On Powernv11 TCG Guest (L0)
  ---------------------------
    - P11 L1 KVM guest -> works
      - P11 L2 KVM guest -> works
    - P10 compat L1 KVM guest -> works
      - P10 L2 KVM guest -> works
    - P9  compat L1 KVM guest -> works
      - P9 L2 KVM guest -> works

KVM APIv2 Testing
=================
  On P11 PowerVM LPAR (L1)
  ------------------------
    - P11 L2 KVM guest -> works
    - P10 compat L2 KVM guest -> works
  
  On P11 LPAR in P10 compat (L1)
  ------------------------------
    - P10 (host compat) L2 KVM guest -> works

  On P10 PowerVM LPAR (L1)
  ------------------------
    - P10 L2 KVM guest -> works

CI test results: https://gitlab.com/amachhiw/qemu/-/pipelines/2604638350

Note: Patch 1 is marked DO_NOT_MERGE as it contains linux-headers updates
that will be synced separately once the corresponding kernel patches are
merged.

v2: https://lore.kernel.org/all/20260502140021.69712-1-amachhiw@linux.ibm.com/
v1: https://lore.kernel.org/all/20260430061333.37905-1-amachhiw@linux.ibm.com/

The corresponding Linux kernel patches (v4) are being posted concurrently.

Previous kernel patch versions:
v3: https://lore.kernel.org/all/20260522152744.55251-1-amachhiw@linux.ibm.com/
v2: https://lore.kernel.org/all/20260513100755.83215-1-amachhiw@linux.ibm.com/
v1: https://lore.kernel.org/all/20260430054906.94431-1-amachhiw@linux.ibm.com/

Amit Machhiwal (3):
  linux-headers: Add uapi header changes
  target/ppc/kvm: Add support for querying host compatibility mode
  target/ppc/kvm: Use host compatibility mode for nested guests

 hw/ppc/spapr_hcall.c            | 14 +++++++
 linux-headers/asm-powerpc/kvm.h | 18 ++++++++
 linux-headers/linux/kvm.h       |  3 ++
 target/ppc/kvm.c                | 74 +++++++++++++++++++++++++++++++++
 target/ppc/kvm_ppc.h            | 16 +++++++
 5 files changed, 125 insertions(+)


base-commit: 2f28d34ea0aead9830478cd1d3d0dd9d9191d82e
-- 
2.50.1 (Apple Git-155)