[PATCH v5 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests

[PATCH v5 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests

[Amit Machhiwal]

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 patches [1] and [2] 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.
Patch [2] sets CPU_FTR_P11_PVR for Power11 and later processors, which is
needed for proper CPU feature detection in dt-cpu-ftrs environments.

Changes in v5:
  - Moved 'size' to be the first member of struct kvm_ppc_compat_caps;
    replaced strict size equality with copy_struct_from_user/to_user for
    proper forward and backward ABI compatibility; added
    KVM_PPC_COMPAT_CAPS_SIZE_VER0 as a frozen version floor constant and
    flags == 0 enforcement to prevent ABI ambiguity (patch 1) - [Vaibhav,
    Amit]
  - Updated PowerVM implementation to use cached nested_capabilities
    instead of a live H_GUEST_GET_CAPABILITIES hcall; added a
    WARN_ON_ONCE(!nested_capabilities) sanity check (patch 2) - [Vaibhav,
    Amit]
  - Converted switch in kvmppc_map_compat_capabilities() to use fallthrough
    for cumulative compat mode reporting; added of_node_put() in
    for_each_node_by_type() to fix OF node reference leak; check 'rc'
    error before assigning capabilities (patch 3) - [Vaibhav, Harsh]
  - Updated documentation to reflect extensibility model, added E2BIG
    error (patch 4) - [Amit]

Changes in v4:
  - Added 'size' field to struct kvm_ppc_compat_caps for forward
    compatibility and ABI extensibility
  - Implemented size validation in ioctl handler to ensure correct structure
    size from userspace
  - Introduced KVM-specific capability constants (KVM_PPC_COMPAT_CAP_POWER9/
    10/11) instead of exposing hypervisor-internal H_GUEST_CAP_* constants
  - Added capability masking using KVM_PPC_COMPAT_BITMASK to ensure only
    supported processor modes are exposed
  - Enhanced error handling with comprehensive error codes (EINVAL, EFAULT,
    ENOTTY) and detailed documentation
  - Removed Tested-by tags pending re-testing with v4 changes
  - Separated validation patch (patch 1 from v3) and sent independently [1]

Changes in v3:
  - Added "Why a new UAPI?" section to cover letter addressing questions
    about the need for a new UAPI vs. using existing mechanisms like
    /proc/device-tree
  - Fixed initialization of 'r' in KVM_PPC_GET_COMPAT_CAPS ioctl handler
    from 0 to -ENOTTY for proper error handling when the operation is not
    supported
  - Added Vaibhav's "Suggested-by" tags
  - Have retained Anushree's "Tested-by" tags as no major code changes
  - Fixed documentation build warning reported by kernel test robot and
    added "Reported-by" and "Closes" tags to patch 5

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:
  [1/4] 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 patches [1] and [2]):

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 v4 series will be sent soon.

Previous QEMU versions:
v3: https://lore.kernel.org/all/20260616113915.25589-1-amachhiw@linux.ibm.com/
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/

Previous versions:
==================
v4: https://lore.kernel.org/linuxppc-dev/20260616123314.82721-1-amachhiw@linux.ibm.com/
v3: https://lore.kernel.org/linuxppc-dev/20260522152744.55251-1-amachhiw@linux.ibm.com/
v2: https://lore.kernel.org/linuxppc-dev/20260513100755.83195-1-amachhiw@linux.ibm.com/
v1: https://lore.kernel.org/linuxppc-dev/20260430054906.94431-1-amachhiw@linux.ibm.com/

References:
===========
[1] https://lore.kernel.org/all/20260609053327.61563-1-amachhiw@linux.ibm.com/
[2] https://lore.kernel.org/all/20260614173437.26352-1-amachhiw@linux.ibm.com/

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        | 58 +++++++++++++++++++++
 arch/powerpc/kvm/powerpc.c          | 71 ++++++++++++++++++++++++++
 include/uapi/linux/kvm.h            |  4 ++
 6 files changed, 231 insertions(+)


base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
prerequisite-patch-id: e328a3183c9e9499436c666c30f3659c44e6f3a2
prerequisite-patch-id: 4662f01d2101cfae8502f04290658deed60eec26
-- 
2.50.1 (Apple Git-155)

[PATCH v5 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

[Amit Machhiwal]

Introduce a new capability and ioctl to expose CPU compatibility modes
supported by the host processor for nested guests.

On IBM POWER systems, newer processor generations (N) can operate in
compatibility modes corresponding to earlier generations, like (N-1) and
(N-2). This is particularly relevant for nested virtualization, where
nested KVM guests may need to run with a specific processor compatibility
level.

Introduce KVM_CAP_PPC_COMPAT_CAPS capability and the corresponding
KVM_PPC_GET_COMPAT_CAPS vm ioctl. The ioctl returns a bitmap describing
the compatibility modes supported by the host in respective bit numbers,
allowing userspace (e.g., QEMU) to select an appropriate compatibility
level when configuring nested KVM guests.

The ioctl handling is added in kvm_arch_vm_ioctl() and retrieves host
CPU compatibility capabilities via a PowerPC-specific backend
implementation when available.

The struct kvm_ppc_compat_caps places the 'size' field first so it can
be read alone via get_user() before copy_struct_from_user() is called,
avoiding pointer arithmetic to locate the size field.

The ioctl is defined using _IO so the ioctl number remains stable even if
the struct grows in future versions. It uses copy_struct_from_user() and
copy_struct_to_user() to provide forward- and backward-compatible
extensibility: older userspace passing a smaller struct to a newer kernel
gets zero-padded trailing fields, while newer userspace passing a larger
struct to an older kernel (usize > ksize) gets sizeof(struct
kvm_ppc_compat_caps)written back to host_caps.size so it can retry with the
older kernel-supported size, after which the kernel returns -E2BIG.

KVM_PPC_COMPAT_CAPS_SIZE_VER0 is defined as a frozen integer constant
(24) marking the size of the initial struct version, used as the
minimum floor for size field validation, similar to other versioned
struct interfaces in the kernel.

The 'flags' field is reserved for future use. The kernel rejects any
call where flags is non-zero with -EINVAL, preventing garbage values
from being baked into ABI permanently.

The ioctl returns appropriate error codes: EINVAL for an invalid size
or non-zero reserved fields, E2BIG if new userspace provides a larger
struct than the kernel knows about (with ksize written back into
host_caps.size for the retry), EFAULT for failed copy operations, and
ENOTTY if the backend doesn't implement get_compat_caps.

Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Moved size as the first member of the struct
  - Replaced strict size equality check with copy_struct_from_user() and
    copy_struct_to_user() for proper forward and backward ABI compatibility
  - Added KVM_PPC_COMPAT_CAPS_SIZE_VER0 (24) as a frozen version floor
    constant, following the convention used by similar interfaces in the kernel
  - Added flags == 0 enforcement to prevent uninitialized stack values from
    being baked into ABI permanently

 arch/powerpc/include/asm/kvm_ppc.h  |  1 +
 arch/powerpc/include/uapi/asm/kvm.h |  8 ++++
 arch/powerpc/kvm/powerpc.c          | 71 +++++++++++++++++++++++++++++
 include/uapi/linux/kvm.h            |  4 ++
 4 files changed, 84 insertions(+)

diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
index 0953f2daa466..169ea6a7fbad 100644
--- a/arch/powerpc/include/asm/kvm_ppc.h
+++ b/arch/powerpc/include/asm/kvm_ppc.h
@@ -319,6 +319,7 @@ struct kvmppc_ops {
 	bool (*hash_v3_possible)(void);
 	int (*create_vm_debugfs)(struct kvm *kvm);
 	int (*create_vcpu_debugfs)(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry);
+	int (*get_compat_caps)(struct kvm_ppc_compat_caps *host_caps);
 };
 
 extern struct kvmppc_ops *kvmppc_hv_ops;
diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
index 077c5437f521..19e53d5ae540 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h
@@ -437,6 +437,14 @@ struct kvm_ppc_cpu_char {
 	__u64	behaviour_mask;		/* valid bits in behaviour */
 };
 
+/* For KVM_PPC_GET_COMPAT_CAPS */
+struct kvm_ppc_compat_caps {
+	__u64	size;			/* Size of this structure */
+	__u64	flags;			/* Reserved for future use */
+	__u64	compat_capabilities;	/* Capabilities supported by the host */
+};
+#define KVM_PPC_COMPAT_CAPS_SIZE_VER0	24 /* sizeof first published struct */
+
 /*
  * Values for character and character_mask.
  * These are identical to the values used by H_GET_CPU_CHARACTERISTICS.
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 98de68379b18..a2919b8b31c0 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -701,6 +701,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 			}
 		}
 		break;
+#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE)
+	case KVM_CAP_PPC_COMPAT_CAPS:
+		r = 0;
+		if (kvmhv_on_pseries())
+			r = 1;
+		break;
+#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */
 	default:
 		r = 0;
 		break;
@@ -2467,6 +2474,70 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		r = kvm->arch.kvm_ops->svm_off(kvm);
 		break;
 	}
+	case KVM_PPC_GET_COMPAT_CAPS: {
+		struct kvm_ppc_compat_caps host_caps = {};
+		u64 usize;
+
+		/*
+		 * Read the size field first to drive copy_struct_from_user.
+		 * size must be the first field of the struct.
+		 */
+		r = -EFAULT;
+		if (get_user(usize, (__u64 __user *)argp))
+			goto out;
+
+		/*
+		 * Enforce a minimum: reject buffers smaller than the initial
+		 * struct version (VER0). This allows old userspace compiled
+		 * against the original struct to still work on a newer kernel
+		 * that has grown the struct with appended fields.
+		 */
+		r = -EINVAL;
+		if (usize < KVM_PPC_COMPAT_CAPS_SIZE_VER0)
+			goto out;
+
+		/*
+		 * New userspace with a larger struct called an older kernel.
+		 * Write back ksize in host_caps.size so userspace knows which
+		 * older struct to retry with, then fail with -E2BIG.
+		 */
+		if (usize > sizeof(host_caps)) {
+			host_caps.size = sizeof(host_caps);
+			r = -EFAULT;
+			if (put_user(host_caps.size, (__u64 __user *)argp))
+				goto out;
+			r = -E2BIG;
+			goto out;
+		}
+
+		/*
+		 * copy_struct_from_user() handles forward/backward compat:
+		 *   usize == ksize: verbatim copy
+		 *   usize <  ksize: zero-pad trailing (old userspace, new kernel)
+		 */
+		r = copy_struct_from_user(&host_caps, sizeof(host_caps),
+					  argp, usize);
+		if (r)
+			goto out;
+
+		/* Reserved fields must be zero */
+		r = -EINVAL;
+		if (host_caps.flags)
+			goto out;
+
+		r = -ENOTTY;
+		if (!kvm->arch.kvm_ops->get_compat_caps)
+			goto out;
+
+		r = kvm->arch.kvm_ops->get_compat_caps(&host_caps);
+		if (r)
+			goto out;
+
+		host_caps.size = sizeof(host_caps);
+		r = copy_struct_to_user(argp, usize, &host_caps,
+					sizeof(host_caps), NULL);
+		break;
+	}
 	default: {
 		struct kvm *kvm = filp->private_data;
 		r = kvm->arch.kvm_ops->arch_vm_ioctl(filp, ioctl, arg);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 419011097fa8..1cf9a959669e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -997,6 +997,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_S390_KEYOP 247
 #define KVM_CAP_S390_VSIE_ESAMODE 248
 #define KVM_CAP_S390_HPAGE_2G 249
+#define KVM_CAP_PPC_COMPAT_CAPS 250
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
@@ -1350,6 +1351,9 @@ struct kvm_s390_keyop {
 #define KVM_GET_DEVICE_ATTR	  _IOW(KVMIO,  0xe2, struct kvm_device_attr)
 #define KVM_HAS_DEVICE_ATTR	  _IOW(KVMIO,  0xe3, struct kvm_device_attr)
 
+/* Available with KVM_CAP_PPC_COMPAT_CAPS */
+#define KVM_PPC_GET_COMPAT_CAPS	_IO(KVMIO,  0xe4)
+
 /*
  * ioctls for vcpu fds
  */
-- 
2.50.1 (Apple Git-155)

[PATCH v5 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM

[Amit Machhiwal]

On POWER systems, the host CPU may run in a compatibility mode (e.g., a
Power11 processor operating in Power10 compatibility mode). In such
cases, the effective CPU level exposed to guests differs from the
physical processor generation.

When running nested KVM guests, QEMU derives the host CPU type using
mfpvr(), which reflects the physical processor version. This can result
in a mismatch between the CPU model selected by QEMU and the
compatibility mode enforced by the host, leading to guest boot failures.

For example, booting a nested guest on a Power11 LPAR configured in
Power10 compatibility mode fails with:

  KVM-NESTEDv2: couldn't set guest wide elements
  [..KVM reg dump..]

This occurs because QEMU selects a CPU model corresponding to the
physical processor (via mfpvr()), while the host operates in a lower
compatibility mode. As a result, KVM rejects the requested compatibility
level during guest initialization.

On pseries nestedv2 systems, add support for retrieving host CPU
compatibility capabilities for nested guests on PowerVM. The capability
bitmap reflects the processor modes negotiated between the Power
hypervisor (L0) and the host partition (L1) via the
H_GUEST_GET_CAPABILITIES hcall, but is retrieved from the cached
nested_capabilities value populated during module initialization,
avoiding repeated hypervisor calls. A WARN_ON_ONCE() flags the
unexpected case where nested_capabilities is zero on a nestedv2 system.
The implementation defines KVM-specific capability constants
(KVM_PPC_COMPAT_CAP_POWER9/10/11), masks unsupported bits, and exposes
the result through the KVM_PPC_GET_COMPAT_CAPS ioctl.

Hook the implementation into the Book3S HV kvmppc_ops so that it can be
invoked by the generic KVM ioctl handling code.

Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Updated PowerVM implementation to use cached nested_capabilities instead
    of making a live H_GUEST_GET_CAPABILITIES hcall on every ioctl call
  - Added WARN_ON_ONCE(!nested_capabilities); sanity check when
    nested_capabilities is unexpectedly zero on a nestedv2 system

 arch/powerpc/include/uapi/asm/kvm.h | 10 ++++++++++
 arch/powerpc/kvm/book3s_hv.c        | 20 ++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
index 19e53d5ae540..913a64b901a3 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h
@@ -445,6 +445,16 @@ struct kvm_ppc_compat_caps {
 };
 #define KVM_PPC_COMPAT_CAPS_SIZE_VER0	24 /* sizeof first published struct */
 
+/*
+ * Capability bits for compat_capabilities field in kvm_ppc_compat_caps.
+ * These bits indicate which processor compatibility modes are supported.
+ */
+#define KVM_PPC_COMPAT_CAP_POWER9	(1ULL << 62)
+#define KVM_PPC_COMPAT_CAP_POWER10	(1ULL << 61)
+#define KVM_PPC_COMPAT_CAP_POWER11	(1ULL << 60)
+#define KVM_PPC_COMPAT_BITMASK		(KVM_PPC_COMPAT_CAP_POWER9 | \
+					 KVM_PPC_COMPAT_CAP_POWER10 | \
+					 KVM_PPC_COMPAT_CAP_POWER11)
 /*
  * Values for character and character_mask.
  * These are identical to the values used by H_GET_CPU_CHARACTERISTICS.
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index f9380ef65750..152cd08a5b38 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6523,6 +6523,25 @@ static bool kvmppc_hash_v3_possible(void)
 	return true;
 }
 
+
+static int kvmppc_get_compat_caps(struct kvm_ppc_compat_caps *host_caps)
+{
+	unsigned long capabilities = 0;
+	long rc = -EINVAL;
+
+	if (kvmhv_on_pseries()) {
+		if (kvmhv_is_nestedv2()) {
+			WARN_ON_ONCE(!nested_capabilities);
+			capabilities = nested_capabilities;
+			rc = 0;
+		}
+	}
+
+	host_caps->compat_capabilities = capabilities & KVM_PPC_COMPAT_BITMASK;
+
+	return rc;
+}
+
 static struct kvmppc_ops kvm_ops_hv = {
 	.get_sregs = kvm_arch_vcpu_ioctl_get_sregs_hv,
 	.set_sregs = kvm_arch_vcpu_ioctl_set_sregs_hv,
@@ -6565,6 +6584,7 @@ static struct kvmppc_ops kvm_ops_hv = {
 	.hash_v3_possible = kvmppc_hash_v3_possible,
 	.create_vcpu_debugfs = kvmppc_arch_create_vcpu_debugfs_hv,
 	.create_vm_debugfs = kvmppc_arch_create_vm_debugfs_hv,
+	.get_compat_caps = kvmppc_get_compat_caps,
 };
 
 static int kvm_init_subcore_bitmap(void)
-- 
2.50.1 (Apple Git-155)

[PATCH v5 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV

[Amit Machhiwal]

Currently, when booting a compatibility-mode KVM guest (L1) on a PowerNV
hypervisor (L0), the guest runs with the expected processor
compatibility level. However, when booting a nested KVM guest (L2)
inside the L1, QEMU derives the CPU model from the raw host PVR and
attempts to run the nested guest at that level, instead of honoring the
compatibility mode of the L1.

Extend host CPU compatibility capability reporting to support nested
virtualization on PowerNV systems (PAPR nested API v1).

For nested API v2 (PowerVM), compatibility capabilities are served from
the cached nested_capabilities value (populated at module init via
kvmhv_nested_init() using the H_GUEST_GET_CAPABILITIES hcall). This
information is not available on PowerNV systems.

For nested API v1, derive the compatibility capabilities from the L1
guest by reading the "cpu-version" property from the device tree, which
reflects the effective (logical) processor compatibility level. Map this
value to the corresponding compatibility capability bitmap using
KVM-specific constants.

The mapping is cumulative: a system running at a given compatibility
level is assumed to also support older generations down the supported
chain. Note that unlike KVM on PowerVM (nested API v2), KVM on PowerNV
currently does not strictly enforce older generation compatibility modes
for nested guests - the reported capabilities reflect what the host CPU
can present, not what the hypervisor independently validates.

Introduce a helper kvmppc_map_compat_capabilities() to translate CPU
version values into KVM_PPC_COMPAT_CAP bits using a fallthrough switch,
and integrate it into kvmppc_get_compat_caps(). The implementation
applies masking to ensure only supported processor modes are exposed.

This allows userspace to query host CPU compatibility modes on both
KVM on PowerVM and on PowerNV platforms via the KVM_PPC_GET_COMPAT_CAPS
ioctl.

Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Converted switch in kvmppc_map_compat_capabilities() to use fallthrough
    for cumulative compat mode reporting
  - Check for 'rc' error before assigning 'capabilities' to
    'host_caps->compat_capabilities'
  - Call of_node_put(np) before break in for_each_node_by_type() loop to
    avoid leaking the OF node reference

 arch/powerpc/kvm/book3s_hv.c | 38 ++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 152cd08a5b38..ba4b2b3aaf4e 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6523,20 +6523,58 @@ static bool kvmppc_hash_v3_possible(void)
 	return true;
 }
 
+static int kvmppc_map_compat_capabilities(const __be32 cpu_version,
+				      unsigned long *capabilities)
+{
+	switch (cpu_version) {
+	case PVR_ARCH_31_P11:
+		*capabilities |= KVM_PPC_COMPAT_CAP_POWER11;
+		fallthrough;
+	case PVR_ARCH_31:
+		*capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
+		fallthrough;
+	case PVR_ARCH_300:
+		*capabilities |= KVM_PPC_COMPAT_CAP_POWER9;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return 0;
+}
 
 static int kvmppc_get_compat_caps(struct kvm_ppc_compat_caps *host_caps)
 {
+	struct device_node *np;
 	unsigned long capabilities = 0;
+	const __be32 *prop = NULL;
 	long rc = -EINVAL;
+	u32 cpu_version;
 
 	if (kvmhv_on_pseries()) {
 		if (kvmhv_is_nestedv2()) {
 			WARN_ON_ONCE(!nested_capabilities);
 			capabilities = nested_capabilities;
 			rc = 0;
+		} else {
+			for_each_node_by_type(np, "cpu") {
+				prop = of_get_property(np, "cpu-version", NULL);
+				if (prop) {
+					cpu_version = be32_to_cpup(prop);
+					of_node_put(np);
+					break;
+				}
+			}
+			if (!prop)
+				return -EINVAL;
+			rc = kvmppc_map_compat_capabilities(cpu_version,
+							    &capabilities);
 		}
 	}
 
+	if (rc < 0)
+		return rc;
+
 	host_caps->compat_capabilities = capabilities & KVM_PPC_COMPAT_BITMASK;
 
 	return rc;
-- 
2.50.1 (Apple Git-155)

[PATCH v5 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl

[Amit Machhiwal]

Add documentation for the KVM_PPC_GET_COMPAT_CAPS ioctl to the KVM API
documentation.

The ioctl exposes host processor compatibility modes supported for
nested KVM guests on PowerPC systems. The documentation covers error
code descriptions including E2BIG for forward compatibility, the
extensible size-based versioning contract using
KVM_PPC_COMPAT_CAPS_SIZE_VER0, the rationale for rejecting non-zero
reserved fields to prevent ABI ambiguity, bit numbering clarification
for IBM MSB-0 convention, and KVM-specific capability bit constants.

Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Updated error table: EINVAL now reflects size < VER0 or flags != 0;
    added E2BIG for new userspace on old kernel
  - Replaced stale strict-size-validation paragraph with description of
    the copy_struct_from_user/to_user extensibility model and
    KVM_PPC_COMPAT_CAPS_SIZE_VER0 versioning contract
  - Added rationale for flags == 0 enforcement to prevent ABI ambiguity

 Documentation/virt/kvm/api.rst | 79 ++++++++++++++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index a5f9ee92f43e..43810c451317 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6566,6 +6566,85 @@ KVM_S390_KEYOP_SSKE
   Sets the storage key for the guest address ``guest_addr`` to the key
   specified in ``key``, returning the previous value in ``key``.
 
+4.145 KVM_PPC_GET_COMPAT_CAPS
+-----------------------------
+:Capability: KVM_CAP_PPC_COMPAT_CAPS
+:Architectures: powerpc
+:Type: vm ioctl
+:Parameters: struct kvm_ppc_compat_caps (out)
+:Returns: 0 on success, negative value on failure
+
+Errors include:
+
+  ======== ============================================================
+  EFAULT   if ``struct kvm_ppc_compat_caps`` cannot be read from or
+           written to userspace
+  EINVAL   if the ``size`` field is smaller than
+           ``KVM_PPC_COMPAT_CAPS_SIZE_VER0``, if the ``flags`` field
+           is non-zero, or if the backend fails to retrieve or map
+           CPU compatibility capabilities
+  E2BIG    if ``size`` is larger than the kernel's struct size
+           (new userspace on old kernel); the kernel writes back its
+           own struct size into the ``size`` field so userspace can
+           retry with the correct size
+  ENOTTY   if the backend does not implement the ``get_compat_caps``
+           operation (e.g., on non-pseries platforms or when the
+           required KVM operations are not available)
+  ======== ============================================================
+
+IBM POWER system server-based processors provide a compatibility mode feature
+where an Nth generation processor can operate in modes consistent with earlier
+generations such as (N-1) and (N-2).
+
+This ioctl provides userspace with information about the CPU compatibility modes
+supported by the current host processor for booting the nested KVM guests on
+KVM on PowerNV (nested API v1) and KVM on PowerVM (nested API v2) platforms.
+
+::
+
+  struct kvm_ppc_compat_caps {
+	__u64	size;			/* Size of this structure */
+	__u64	flags;			/* Reserved for future use, must be 0 */
+	__u64	compat_capabilities;	/* Capabilities supported by the host */
+  };
+
+Before calling this ioctl, userspace must set the ``size`` field to
+``sizeof(struct kvm_ppc_compat_caps)`` and zero the ``flags`` field.
+The kernel rejects non-zero ``flags`` with ``-EINVAL`` to prevent
+uninitialized stack values from being silently accepted, keeping the
+field available for future use without ABI ambiguity.
+
+The ioctl uses ``copy_struct_from_user()`` and ``copy_struct_to_user()``
+to support extensible versioning: if userspace passes a struct smaller
+than the current kernel version (``size >= KVM_PPC_COMPAT_CAPS_SIZE_VER0``),
+the kernel zero-pads unknown trailing fields. If userspace passes a larger
+struct (``size > sizeof(struct kvm_ppc_compat_caps)``), the kernel writes
+back its own struct size into the ``size`` field and returns ``-E2BIG``,
+allowing userspace to discover the kernel's struct size and retry.
+``KVM_PPC_COMPAT_CAPS_SIZE_VER0`` (24) is a frozen constant marking the
+size of the initial struct version.
+
+The ``compat_capabilities`` bit field describes the processor compatibility
+modes supported by the host. The following bits indicate support for specific
+processor modes (using IBM's MSB-0 convention where bit 0 is the most
+significant bit):
+
+- ``KVM_PPC_COMPAT_CAP_POWER9``  (bit 1) -- KVM guests can run in Power9 processor mode
+- ``KVM_PPC_COMPAT_CAP_POWER10`` (bit 2) -- KVM guests can run in Power10 processor mode
+- ``KVM_PPC_COMPAT_CAP_POWER11`` (bit 3) -- KVM guests can run in Power11 processor mode
+
+.. note::
+
+   The bit numbering above uses IBM's MSB-0 convention (bit 0 is the most
+   significant bit). In the actual implementation, these are defined as:
+
+   - ``KVM_PPC_COMPAT_CAP_POWER9``  = ``(1ULL << 62)``
+   - ``KVM_PPC_COMPAT_CAP_POWER10`` = ``(1ULL << 61)``
+   - ``KVM_PPC_COMPAT_CAP_POWER11`` = ``(1ULL << 60)``
+
+   Userspace should use the defined constants from ``<linux/kvm.h>`` rather
+   than hardcoding bit positions.
+
 .. _kvm_run:
 
 5. The kvm_run structure
-- 
2.50.1 (Apple Git-155)