[PATCH v6 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests

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 provides 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.

Why a new UAPI?
===============
While cpu-version is available in /proc/device-tree/cpus/<cpu#>/cpu-version
on both L1 booted on PowerNV and PowerVM LPARs, the UAPI approach is
preferable for several reasons:

1. pHYP (L0) capabilities: On PowerVM, we need to rely on capabilities
   negotiated with pHYP in KVM, not just device tree properties. The
   cpu-version property depicts the current compat mode but doesn't point
   to what all compat modes are supported for the nested guest.

2. procfs dependency: Not all systems run with procfs enabled (CONFIG_PROC_FS
   is optional). Minimal configurations like buildroot might disable it, but
   KVM ioctl works regardless since it accesses kernel data structures
   directly.

3. Kernel validation: The kernel validates and normalizes the compatibility
   information, ensuring userspace gets validated, consistent data.

4. Abstraction & stability: /proc/device-tree is an implementation detail.
   The UAPI provides a stable interface that won't break if the underlying
   mechanism changes.

5. Semantic clarity: KVM_PPC_GET_COMPAT_CAPS clearly expresses what
   compatibility modes can be used for KVM guests, vs. parsing device tree
   which requires understanding the semantic meaning of cpu-version.

The implementation supports both:

  - KVM on PowerVM (nested API v2), where compatibility information is
    served from the cached nested_capabilities value, originally obtained
    via the H_GUEST_GET_CAPABILITIES hypercall at module init.
  - KVM on 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.

Note: This series is built on top of patch [1] which must be applied first.
Patch [1] ensures arch_compat is validated against the host compatibility
mode before this series adds the capability query mechanism.
Commit e4de1b9cb3b5 ("powerpc/dt_cpu_ftrs: Set CPU_FTR_P11_PVR for Power11
and later processors") which was also a prerequisite has been merged upstream.

Changes in v6:
  - Changed KVM_PPC_GET_COMPAT_CAPS ioctl number from 0xe4 to 0xb8 to
    avoid placing it in the KVM_CREATE_DEVICE fd ioctl range (0xe0-0xe3);
    relocated definition to sit alongside other PPC vm ioctls (patch 1)
    - [Gautam]
  - kvmppc_map_compat_capabilities(): changed parameter type from
    'const __be32' to 'u32' to fix Sparse type annotation warning (patch 3)
    - [Sashiko]
  - kvmppc_get_compat_caps(): replaced of_get_property() + be32_to_cpup()
    with of_property_read_u32() for implicit length validation and cleaner
    endianness handling (patch 3) - [Sashiko]
  - Documentation: corrected :Parameters: from (out) to (in/out) since
    userspace must set size and flags before calling (patch 4) - [Sashiko]

Patch summary:
  [1/4] Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
  [2/4] Implement capability retrieval for KVM on PowerVM (API v2)
  [3/4] Add KVM on PowerNV support (API v1)
  [4/4] Document the new ioctl

Testing (with QEMU v4 patches and on top of patch [1]):

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 PowerNV TCG L0 guest -> works
    - 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
    - P9 compat L2 KVM guest fails to boot as expected
    - Without QEMU patches but Linux patches
      - P11 L2 KVM guest -> works
      - P10 compat L2 KVM guest -> works
      - P9 compat L2 KVM guest fails to boot as expected
    - Without Linux patches but QEMU patches
      - 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
    - Without QEMU patch but Linux patches
      - P10 guest fails to boot as expected (error: kvm run failed Invalid argument)
    - Without Linux patch but QEMU patches
      - P10 guest fails to boot as expected (KVM: unknown exit, hardware reason ffffffffffffffea)

  On P10 PowerVM LPAR (L1)
  ------------------------
    - P10 L2 KVM guest -> works
    - P9 compat L2 KVM guest fails to boot as expected

TCG pSeries Guest
=================
    - P11 (default) pSeries guest boots fine

ABI Extensibility Testing (struct size 32, extra member)
=========================================================
    - Newer struct on QEMU, older kernel -> works (kernel returns -E2BIG,
      QEMU retries with correct size)
    - New struct on Linux kernel, older QEMU -> works (kernel zero-pads
      trailing fields, QEMU gets correct data)

With this series, nested guests boot successfully in configurations where
they previously failed due to compatibility mismatches.

Related QEMU series:
====================
A corresponding QEMU v5 series will be sent soon.

Previous QEMU versions:
v4: https://lore.kernel.org/all/[email protected]/
v3: https://lore.kernel.org/all/[email protected]/
v2: https://lore.kernel.org/all/[email protected]/
v1: https://lore.kernel.org/all/[email protected]/

Previous versions:
==================
v5: https://lore.kernel.org/linuxppc-dev/[email protected]/
v4: https://lore.kernel.org/linuxppc-dev/[email protected]/
v3: https://lore.kernel.org/linuxppc-dev/[email protected]/
v2: https://lore.kernel.org/linuxppc-dev/[email protected]/
v1: https://lore.kernel.org/linuxppc-dev/[email protected]/

References:
===========
[1] https://lore.kernel.org/all/[email protected]/

Amit Machhiwal (4):
  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      | 79 +++++++++++++++++++++++++++++
 arch/powerpc/include/asm/kvm_ppc.h  |  1 +
 arch/powerpc/include/uapi/asm/kvm.h | 18 +++++++
 arch/powerpc/kvm/book3s_hv.c        | 56 ++++++++++++++++++++
 arch/powerpc/kvm/powerpc.c          | 71 ++++++++++++++++++++++++++
 include/uapi/linux/kvm.h            |  3 ++
 6 files changed, 228 insertions(+)


base-commit: 848acc8ffe1b7cd5f1bf427b93069becfebc2c9d
prerequisite-patch-id: 7755786f0e4f415e47065ff1972765008727fe10
-- 
2.50.1 (Apple Git-155)