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

[PATCH v6 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 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/20260701052341.62289-1-amachhiw@linux.ibm.com/
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:
==================
v5: https://lore.kernel.org/linuxppc-dev/20260701051409.51820-1-amachhiw@linux.ibm.com/
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/20260714175432.86388-1-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        | 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)

[PATCH v6 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>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Changed ioctl number from 0xe4 to 0xb8 to avoid the KVM_CREATE_DEVICE
    fd ioctl range (0xe0-0xe3); relocated definition alongside other PPC
    vm ioctls

 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            |  3 ++
 4 files changed, 83 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 b6b83fe3233f..732336092b03 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -703,6 +703,13 @@ 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;
@@ -2469,6 +2476,70 @@ 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..70e36e6a0ad4 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;
@@ -1341,6 +1342,8 @@ struct kvm_s390_keyop {
 /* Available with KVM_CAP_COUNTER_OFFSET */
 #define KVM_ARM_SET_COUNTER_OFFSET _IOW(KVMIO,  0xb5, struct kvm_arm_counter_offset)
 #define KVM_ARM_GET_REG_WRITABLE_MASKS _IOR(KVMIO,  0xb6, struct reg_mask_range)
+/* Available with KVM_CAP_PPC_COMPAT_CAPS */
+#define KVM_PPC_GET_COMPAT_CAPS	_IO(KVMIO,  0xb8)
 
 /* ioctl for vm fd */
 #define KVM_CREATE_DEVICE	  _IOWR(KVMIO,  0xe0, struct kvm_create_device)
-- 
2.50.1 (Apple Git-155)

[PATCH v6 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>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
 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 v6 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>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - kvmppc_map_compat_capabilities(): changed parameter type from
    'const __be32' to 'u32' to fix Sparse type annotation warning
  - kvmppc_get_compat_caps(): replaced of_get_property() + be32_to_cpup()
    with of_property_read_u32() for implicit length validation and cleaner
    endianness handling; dropped 'const __be32 *prop' variable

 arch/powerpc/kvm/book3s_hv.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 152cd08a5b38..4d0307f55e06 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6523,20 +6523,56 @@ static bool kvmppc_hash_v3_possible(void)
 	return true;
 }
 
+static int kvmppc_map_compat_capabilities(u32 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;
 	long rc = -EINVAL;
+	u32 cpu_version = 0;
 
 	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") {
+				if (!of_property_read_u32(np, "cpu-version",
+							  &cpu_version)) {
+					of_node_put(np);
+					break;
+				}
+			}
+			if (!cpu_version)
+				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 v6 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.

Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Reviewed-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Changes in this version:
  - Corrected :Parameters: from (out) to (in/out) since userspace must
    set size and flags before calling

 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 e3003a241d5b..22fedb0aa34b 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 (in/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-HV KVM implementations where 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)

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

[Anushree Mathur]

On 04/08/26 11:37 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 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/20260701052341.62289-1-amachhiw@linux.ibm.com/
> 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:
> ==================
> v5: https://lore.kernel.org/linuxppc-dev/20260701051409.51820-1-amachhiw@linux.ibm.com/
> 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/20260714175432.86388-1-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        | 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

Hi Amit,
I have tested this patch and it works as expected. Here is my analysis :

I booted a host with Power10 compat mode and tried following scenarios -


lscpu on host :

Architecture:                ppc64le
   Byte Order:                Little Endian
CPU(s):                      8
   On-line CPU(s) list:       0-7
Model name:                  POWER10 (architected), altivec supported


Before applying the patch :

When I am trying to bringup the guest on a compat mode host it was 
bringing up a Power11 guest and was failing as

[ 1411.578944] [   T2928] KVM-NESTEDv2: couldn't set guest wide elements
[ 1411.578963] [   T2928] vcpu 000000000b9c4155 (0):
[ 1411.578968] [   T2928] pc  = 000000007daf9790  msr = 
8000000000103000  trap = ffffffea
[ 1411.578973] [   T2928] r 0 = 8000000000003000  r16 = 0000000000000000
[ 1411.578978] [   T2928] r 1 = 000000007e581e20  r17 = 0000000000000000
[ 1411.578982] [   T2928] r 2 = 000000007db26c00  r18 = 0000000000000000
[ 1411.578985] [   T2928] r 3 = 0000000000000000  r19 = 0000000000000000
[ 1411.578989] [   T2928] r 4 = 0000000002e30c80  r20 = 0000000000000000
[ 1411.578993] [   T2928] r 5 = 000000007df80000  r21 = 0000000000000000
[ 1411.578996] [   T2928] r 6 = 0000000000200000  r22 = 00000000018c5fd6
[ 1411.579000] [   T2928] r 7 = 000000007df80000  r23 = 000000007db21cc0
[ 1411.579003] [   T2928] r 8 = 000000007db6e5d8  r24 = 000000007db66000
[ 1411.579006] [   T2928] r 9 = 000000007e6655d8  r25 = 000000007e665508
[ 1411.579010] [   T2928] r10 = 000000007db6e5d0  r26 = 00000000018c5fd6
[ 1411.579013] [   T2928] r11 = 0000000000003000  r27 = 0000000000000003
[ 1411.579017] [   T2928] r12 = 8000000000000001  r28 = 000000007db6e5e0
[ 1411.579020] [   T2928] r13 = 0000000000000000  r29 = 000000007db224b0
[ 1411.579024] [   T2928] r14 = 0000000000000000  r30 = 000000007daf274c
[ 1411.579028] [   T2928] r15 = 0000000000000000  r31 = 000000007db76000
[ 1411.579033] [   T2928] ctr = 000000007daf1b44  lr  = 000000007daf1b7c
[ 1411.579037] [   T2928] srr0 = 000000007daf9790 srr1 = 8000000000102000
[ 1411.579041] [   T2928] sprg0 = 0000000000000000 sprg1 = 000000000000ff10
[ 1411.579045] [   T2928] sprg2 = 0000000000000000 sprg3 = 0000000000000000
[ 1411.579049] [   T2928] cr = 20000402  xer = 0000000020040000 dsisr = 
00000000
[ 1411.579054] [   T2928] dar = 0000000000000000
[ 1411.579057] [   T2928] fault dar = 0000000000000000 dsisr = 00000000
[ 1411.579061] [   T2928] SLB (0 entries):
[ 1411.579064] [   T2928] lpcr = 0040000000020400 sdr1 = 
0000000000000000 last_inst = ffffffffffffffff
[ 1411.579069] [   T2928] trap=0xffffffea | pc=0x7daf9790 | 
msr=0x8000000000103000





After applying this patch along with the qemu built with it's dependent 
patch 
(https://lore.kernel.org/all/20260804182914.83091-1-amachhiw@linux.ibm.com/):

I am able to bringup a guest and it got boot up with Power10 by default:


lscpu on guest -

ltcbonn53-vm2:~ # lscpu
Architecture:                ppc64le
   Byte Order:                Little Endian
CPU(s):                      8
   On-line CPU(s) list:       0-7
Model name:                  POWER10 (architected), altivec supported
   Model:                     2.0 (pvr 0082 0200)
   Thread(s) per core:        2
   Core(s) per socket:        4
   Socket(s):                 1



Please feel free to add my tested-by:
Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>


Thank you,
Anushree Mathur

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

[Ritesh Harjani]

Hi Amit,

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> 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.
>

Sorry, but I am somehow not convinced on whether we need all of this
machinary just to get these 3 bits of information, which we are
returning today.

Since KVM_CHECK_EXTENSION can already return an int, so why can't we use
KVM_CAP_PPC_COMPAT_CAPS itself and return the bitmap of supported compat
modes to the user?
Say if the cap is not supported, we can return 0, otherwise we can
return the bitmap of supported compat modes. This will easily allow us
to use 31-bits which as I see would be hardly a problem in the near
future. In the future if it grows - we can always use KVM_CAP_PPC_COMPAT_CAPS2.

This should reduce the code complexity both in the kernel and
userspace and we don't even need a new ioctl then.

Thoughts?

-ritesh

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

[Amit Machhiwal]

Hi Ritesh,

Thanks for taking a look. Please find my response inline.

On 2026/08/06 12:09 AM, Ritesh Harjani wrote:
> 
> Hi Amit,
> 
> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 
> > 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.
> >
> 
> Sorry, but I am somehow not convinced on whether we need all of this
> machinary just to get these 3 bits of information, which we are
> returning today.
> 
> Since KVM_CHECK_EXTENSION can already return an int, so why can't we use
> KVM_CAP_PPC_COMPAT_CAPS itself and return the bitmap of supported compat
> modes to the user?
> Say if the cap is not supported, we can return 0, otherwise we can
> return the bitmap of supported compat modes. This will easily allow us
> to use 31-bits which as I see would be hardly a problem in the near
> future. In the future if it grows - we can always use KVM_CAP_PPC_COMPAT_CAPS2.
> 
> This should reduce the code complexity both in the kernel and
> userspace and we don't even need a new ioctl then.

Thanks for the suggestion. I considered this approach but would like to
go with a dedicated ioctl for the following reasons:

1. Intended semantics: The KVM API documentation states:

   ..kvm defines extension identifiers and a facility to query
   whether a particular extension identifier is available.  If it is, a
   set of ioctls is available for application use.

   [...]

   KVM defines many constants of the form KVM_CAP_*, each corresponding
   to a set of functionality provided by one or more ioctls. Availability
   of these capabilities can be checked with KVM_CHECK_EXTENSION.

   The intended role of KVM_CAP_* is to signal ioctl availability, not
   to serve as a data retrieval mechanism itself.

   You may take a look at KVM_CAP_PPC_GET_CPU_CHAR for instance.

2. Return type constraint: KVM_CHECK_EXTENSION returns a signed 32-bit int. The
   capability bits are defined as (1ULL << 62), (1ULL << 61), and (1ULL << 60) —
   64-bit values that cannot fit in a 32-bit return. Renumbering them to small
   integers would be a UAPI change and would lose alignment with the
   H_GUEST_CAP_* values from the hypervisor ABI.

3. Extensibility: The struct-based approach with the size field provides clean
   forward and backward ABI versioning via copy_struct_from/to_user(), without
   needing a KVM_CAP_PPC_COMPAT_CAPS2 in the future.

Thanks,
Amit

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

[Ritesh Harjani]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

> Hi Ritesh,
>
> Thanks for taking a look. Please find my response inline.
>
> On 2026/08/06 12:09 AM, Ritesh Harjani wrote:
>> 
>> Hi Amit,
>> 
>> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
>> 
>> > 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.
>> >
>> 
>> Sorry, but I am somehow not convinced on whether we need all of this
>> machinary just to get these 3 bits of information, which we are
>> returning today.
>> 
>> Since KVM_CHECK_EXTENSION can already return an int, so why can't we use
>> KVM_CAP_PPC_COMPAT_CAPS itself and return the bitmap of supported compat
>> modes to the user?
>> Say if the cap is not supported, we can return 0, otherwise we can
>> return the bitmap of supported compat modes. This will easily allow us
>> to use 31-bits which as I see would be hardly a problem in the near
>> future. In the future if it grows - we can always use KVM_CAP_PPC_COMPAT_CAPS2.
>> 
>> This should reduce the code complexity both in the kernel and
>> userspace and we don't even need a new ioctl then.
>
> Thanks for the suggestion. I considered this approach

Then we should have brought that up early on during the design
discussion. But for the sake of discussion let's call this as
approach-2.

> but would like to
> go with a dedicated ioctl for the following reasons:
>

> 1. Intended semantics: The KVM API documentation states:
>
>    ..kvm defines extension identifiers and a facility to query
>    whether a particular extension identifier is available.  If it is, a
>    set of ioctls is available for application use.
>
>    [...]
>
>    KVM defines many constants of the form KVM_CAP_*, each corresponding
>    to a set of functionality provided by one or more ioctls. Availability
>    of these capabilities can be checked with KVM_CHECK_EXTENSION.
>
>    The intended role of KVM_CAP_* is to signal ioctl availability, not
>    to serve as a data retrieval mechanism itself.

That's not entirely true. We do return data as part of check extension
for e.g. for getting the SMT modes check KVM_CAP_PPC_SMT_POSSIBLE.

>
>    You may take a look at KVM_CAP_PPC_GET_CPU_CHAR for instance.
>
> 2. Return type constraint: KVM_CHECK_EXTENSION returns a signed 32-bit int. The
>    capability bits are defined as (1ULL << 62), (1ULL << 61), and (1ULL << 60) —
>    64-bit values that cannot fit in a 32-bit return. Renumbering them to small
>    integers would be a UAPI change and would lose alignment with the

There is no _change_ in the UAPI so far. This is the patch which is
defining that in the first place.

>    H_GUEST_CAP_* values from the hypervisor ABI.

No please. Those are 2 different ABIs and there is no need to set a hard
dependency among the two.

>
> 3. Extensibility: The struct-based approach with the size field provides clean
>    forward and backward ABI versioning via copy_struct_from/to_user(), without
>    needing a KVM_CAP_PPC_COMPAT_CAPS2 in the future.
>

This only make sense if we really have a usecase already in mind which
you are planning to extend it for. Otherwise, IMO, this is a lot of
machinary and I think we should consider the simpler approach.

IMO - I think approach-2 is a much simpler for this usecase. I don't see
any valid reason on why we should not do that instead. We don't need an
extra ioctl and all the struct machinary along with that just for
returning a bitmask. The existing check extension ioctl can be
easily used for this purpose.

Would it be possible for you to give, approach-2 a try? Do you see any
geniunine roadblock or limitation with that?

-ritesh

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

[Amit Machhiwal]

On 2026/08/06 06:35 PM, Ritesh Harjani wrote:
> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 
> > Hi Ritesh,
> >
> > Thanks for taking a look. Please find my response inline.
> >
> > On 2026/08/06 12:09 AM, Ritesh Harjani wrote:
> >> 
> >> Hi Amit,
> >> 
> >> Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> >> 
> >> > 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.
> >> >
> >> 
> >> Sorry, but I am somehow not convinced on whether we need all of this
> >> machinary just to get these 3 bits of information, which we are
> >> returning today.
> >> 
> >> Since KVM_CHECK_EXTENSION can already return an int, so why can't we use
> >> KVM_CAP_PPC_COMPAT_CAPS itself and return the bitmap of supported compat
> >> modes to the user?
> >> Say if the cap is not supported, we can return 0, otherwise we can
> >> return the bitmap of supported compat modes. This will easily allow us
> >> to use 31-bits which as I see would be hardly a problem in the near
> >> future. In the future if it grows - we can always use KVM_CAP_PPC_COMPAT_CAPS2.
> >> 
> >> This should reduce the code complexity both in the kernel and
> >> userspace and we don't even need a new ioctl then.
> >
> > Thanks for the suggestion. I considered this approach
> 
> Then we should have brought that up early on during the design
> discussion. But for the sake of discussion let's call this as
> approach-2.
> 
> > but would like to
> > go with a dedicated ioctl for the following reasons:
> >
> 
> > 1. Intended semantics: The KVM API documentation states:
> >
> >    ..kvm defines extension identifiers and a facility to query
> >    whether a particular extension identifier is available.  If it is, a
> >    set of ioctls is available for application use.
> >
> >    [...]
> >
> >    KVM defines many constants of the form KVM_CAP_*, each corresponding
> >    to a set of functionality provided by one or more ioctls. Availability
> >    of these capabilities can be checked with KVM_CHECK_EXTENSION.
> >
> >    The intended role of KVM_CAP_* is to signal ioctl availability, not
> >    to serve as a data retrieval mechanism itself.
> 
> That's not entirely true. We do return data as part of check extension
> for e.g. for getting the SMT modes check KVM_CAP_PPC_SMT_POSSIBLE.
> 
> >
> >    You may take a look at KVM_CAP_PPC_GET_CPU_CHAR for instance.
> >
> > 2. Return type constraint: KVM_CHECK_EXTENSION returns a signed 32-bit int. The
> >    capability bits are defined as (1ULL << 62), (1ULL << 61), and (1ULL << 60) —
> >    64-bit values that cannot fit in a 32-bit return. Renumbering them to small
> >    integers would be a UAPI change and would lose alignment with the
> 
> There is no _change_ in the UAPI so far. This is the patch which is
> defining that in the first place.
> 
> >    H_GUEST_CAP_* values from the hypervisor ABI.
> 
> No please. Those are 2 different ABIs and there is no need to set a hard
> dependency among the two.
> 
> >
> > 3. Extensibility: The struct-based approach with the size field provides clean
> >    forward and backward ABI versioning via copy_struct_from/to_user(), without
> >    needing a KVM_CAP_PPC_COMPAT_CAPS2 in the future.
> >
> 
> This only make sense if we really have a usecase already in mind which
> you are planning to extend it for. Otherwise, IMO, this is a lot of
> machinary and I think we should consider the simpler approach.
> 
> IMO - I think approach-2 is a much simpler for this usecase. I don't see
> any valid reason on why we should not do that instead. We don't need an
> extra ioctl and all the struct machinary along with that just for
> returning a bitmask. The existing check extension ioctl can be
> easily used for this purpose.
> 
> Would it be possible for you to give, approach-2 a try? Do you see any
> geniunine roadblock or limitation with that?

As discussed, one additional and deeper technical reason is that the
Power Hypervisor (PHYP) returns a 64-bit capabilities mask as part of
the H_GUEST_GET_CAPABILITIES hcall (defined in the PAPR specification).
Our KVM_PPC_COMPAT_CAP_* bits are deliberately aligned to this 64-bit
mask so that the cached nested_capabilities value can be used directly
without any translation.

The PAPR spec reserves bits beyond the current processor modes and
capability flags for future capabilities. As new processor generations
arrive, some of those reserved bits may become relevant for them — at
which point a 32-bit return from KVM_CHECK_EXTENSION would be
insufficient, forcing a KVM_CAP_PPC_COMPAT_CAPS2.

The struct-based ioctl avoids this problem cleanly.

Thanks,
Amit

> 
> -ritesh
>