* [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
2026-06-16 12:33 [PATCH v4 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests Amit Machhiwal
@ 2026-06-16 12:33 ` Amit Machhiwal
2026-06-19 6:14 ` Vaibhav Jain
2026-06-16 12:33 ` [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM Amit Machhiwal
` (2 subsequent siblings)
3 siblings, 1 reply; 9+ messages in thread
From: Amit Machhiwal @ 2026-06-16 12:33 UTC (permalink / raw)
To: linuxppc-dev, Madhavan Srinivasan
Cc: Vaibhav Jain, Amit Machhiwal, Anushree Mathur, Paolo Bonzini,
Nicholas Piggin, Michael Ellerman, Christophe Leroy (CS GROUP),
Jonathan Corbet, Shuah Khan, kvm, linux-kernel, linux-doc, lkp
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 implementation validates the structure
size from userspace to ensure forward compatibility and returns
appropriate error codes (EINVAL for invalid size, EFAULT for copy
failures, ENOTTY if backend is not implemented). The struct
kvm_ppc_compat_caps includes a size field to support future ABI
extensions.
Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
arch/powerpc/include/asm/kvm_ppc.h | 1 +
arch/powerpc/include/uapi/asm/kvm.h | 7 ++++++
arch/powerpc/kvm/powerpc.c | 35 +++++++++++++++++++++++++++++
include/uapi/linux/kvm.h | 4 ++++
4 files changed, 47 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..8a38be6c3b03 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h
@@ -437,6 +437,13 @@ struct kvm_ppc_cpu_char {
__u64 behaviour_mask; /* valid bits in behaviour */
};
+/* For KVM_PPC_GET_COMPAT_CAPS */
+struct kvm_ppc_compat_caps {
+ __u64 flags; /* Reserved for future use */
+ __u64 size; /* Size of this structure */
+ __u64 compat_capabilities; /* Capabilities supported by the host */
+};
+
/*
* 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..9153b0034b45 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 +2474,34 @@ 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 user_size;
+
+ r = -EFAULT;
+ /* First, get the size field from userspace to validate */
+ if (copy_from_user(&user_size, &((struct kvm_ppc_compat_caps
+ __user *)argp)->size, sizeof(user_size))) {
+ goto out;
+ }
+
+ /* Validate size - must be at least the current structure size */
+ r = -EINVAL;
+ if (user_size < sizeof(host_caps))
+ goto out;
+
+ r = -ENOTTY;
+ memset(&host_caps, 0, sizeof(host_caps));
+ if (!kvm->arch.kvm_ops->get_compat_caps)
+ goto out;
+
+ r = kvm->arch.kvm_ops->get_compat_caps(&host_caps);
+ /* Set the actual size of the structure we're returning */
+ host_caps.size = sizeof(host_caps);
+ if (!r && copy_to_user(argp, &host_caps, sizeof(host_caps)))
+ r = -EFAULT;
+ 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 6c8afa2047bf..1788a0068662 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -996,6 +996,7 @@ struct kvm_enable_cap {
#define KVM_CAP_S390_USER_OPEREXEC 246
#define KVM_CAP_S390_KEYOP 247
#define KVM_CAP_S390_VSIE_ESAMODE 248
+#define KVM_CAP_PPC_COMPAT_CAPS 249
struct kvm_irq_routing_irqchip {
__u32 irqchip;
@@ -1349,6 +1350,9 @@ 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 _IOR(KVMIO, 0xe4, struct kvm_ppc_compat_caps)
+
/*
* ioctls for vcpu fds
*/
--
2.50.1 (Apple Git-155)
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
2026-06-16 12:33 ` [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl Amit Machhiwal
@ 2026-06-19 6:14 ` Vaibhav Jain
0 siblings, 0 replies; 9+ messages in thread
From: Vaibhav Jain @ 2026-06-19 6:14 UTC (permalink / raw)
To: Amit Machhiwal, linuxppc-dev, Madhavan Srinivasan
Cc: Amit Machhiwal, Anushree Mathur, Paolo Bonzini, Nicholas Piggin,
Michael Ellerman, Christophe Leroy (CS GROUP), Jonathan Corbet,
Shuah Khan, kvm, linux-kernel, linux-doc, lkp
Hi Amit.
Thanks for the patch and incorporating V3 review comments. Further
review comments inline below:
Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 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 implementation validates the structure
> size from userspace to ensure forward compatibility and returns
> appropriate error codes (EINVAL for invalid size, EFAULT for copy
> failures, ENOTTY if backend is not implemented). The struct
> kvm_ppc_compat_caps includes a size field to support future ABI
> extensions.
>
> Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> arch/powerpc/include/asm/kvm_ppc.h | 1 +
> arch/powerpc/include/uapi/asm/kvm.h | 7 ++++++
> arch/powerpc/kvm/powerpc.c | 35 +++++++++++++++++++++++++++++
> include/uapi/linux/kvm.h | 4 ++++
> 4 files changed, 47 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..8a38be6c3b03 100644
> --- a/arch/powerpc/include/uapi/asm/kvm.h
> +++ b/arch/powerpc/include/uapi/asm/kvm.h
> @@ -437,6 +437,13 @@ struct kvm_ppc_cpu_char {
> __u64 behaviour_mask; /* valid bits in behaviour */
> };
>
> +/* For KVM_PPC_GET_COMPAT_CAPS */
> +struct kvm_ppc_compat_caps {
> + __u64 flags; /* Reserved for future use */
> + __u64 size; /* Size of this structure */
Suggesting moving the 'size' as the first member of the struct. That way
copying the struct from userspace becomes bit easier.
> + __u64 compat_capabilities; /* Capabilities supported by the host */
> +};
> +
> /*
> * 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..9153b0034b45 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 +2474,34 @@ 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 user_size;
> +
> + r = -EFAULT;
> + /* First, get the size field from userspace to validate */
> + if (copy_from_user(&user_size, &((struct kvm_ppc_compat_caps
> + __user *)argp)->size, sizeof(user_size))) {
move the struct size member to the first field. That way
from_from_user() call is simplified and you wont have to do some wired
pointer arithmetic.
> + goto out;
> + }
> +
> + /* Validate size - must be at least the current structure size */
> + r = -EINVAL;
> + if (user_size < sizeof(host_caps))
> + goto out;
Check should be strengthed to
if (user_size != sizeof(host_caps))
So that in case used space sends a struct larger than what kernel knows
abt it will be rejected. This will prevent surprises in future in case
VMM sends a larger struct expecting kernel to know abt it but an older
kernel only knows abt older smaller sized struct. Also look at the
review comment below.
> +
> + r = -ENOTTY;
> + memset(&host_caps, 0, sizeof(host_caps));
> + if (!kvm->arch.kvm_ops->get_compat_caps)
> + goto out;
> +
> + r = kvm->arch.kvm_ops->get_compat_caps(&host_caps);
> + /* Set the actual size of the structure we're returning */
> + host_caps.size = sizeof(host_caps);
> + if (!r && copy_to_user(argp, &host_caps, sizeof(host_caps)))
> + r = -EFAULT;
You are allowing a future userspace VMM to potentially send a larger
'struct kvm_ppc_compat_caps' that what kernel knows about. This makes
error handling in userspace bit involved since there might be some
fields in the 'struct kvm_ppc_compat_caps' given from userspace may
remain un-initialized when userspace sees it. So please mention this
subtle behaviour should be mentioned in patch description and also
update it the doc in the later patch.
> + 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 6c8afa2047bf..1788a0068662 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -996,6 +996,7 @@ struct kvm_enable_cap {
> #define KVM_CAP_S390_USER_OPEREXEC 246
> #define KVM_CAP_S390_KEYOP 247
> #define KVM_CAP_S390_VSIE_ESAMODE 248
> +#define KVM_CAP_PPC_COMPAT_CAPS 249
>
> struct kvm_irq_routing_irqchip {
> __u32 irqchip;
> @@ -1349,6 +1350,9 @@ 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 _IOR(KVMIO, 0xe4, struct kvm_ppc_compat_caps)
> +
> /*
> * ioctls for vcpu fds
> */
> --
> 2.50.1 (Apple Git-155)
>
>
--
Cheers
~ Vaibhav
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM
2026-06-16 12:33 [PATCH v4 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests Amit Machhiwal
2026-06-16 12:33 ` [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl Amit Machhiwal
@ 2026-06-16 12:33 ` Amit Machhiwal
2026-06-19 6:04 ` Vaibhav Jain
2026-06-16 12:33 ` [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV Amit Machhiwal
2026-06-16 12:33 ` [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl Amit Machhiwal
3 siblings, 1 reply; 9+ messages in thread
From: Amit Machhiwal @ 2026-06-16 12:33 UTC (permalink / raw)
To: linuxppc-dev, Madhavan Srinivasan
Cc: Vaibhav Jain, Amit Machhiwal, Anushree Mathur, Paolo Bonzini,
Nicholas Piggin, Michael Ellerman, Christophe Leroy (CS GROUP),
Jonathan Corbet, Shuah Khan, kvm, linux-kernel, linux-doc, lkp
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.
Add support for retrieving host CPU compatibility capabilities for
nested guests on PowerVM (PAPR nested API v2). The hypervisor provides
the effective compatibility levels via the H_GUEST_GET_CAPABILITIES
hcall, which reflects the processor modes negotiated between the Power
hypervisor (L0) and the host partition (L1).
On pseries systems, obtain the capability bitmap using
plpar_guest_get_capabilities() and return it via struct
kvm_ppc_compat_caps. The implementation defines KVM-specific capability
constants (KVM_PPC_COMPAT_CAP_POWER9/10/11) and applies masking to ensure
only supported processor modes are exposed to userspace. This information
is then exposed 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>
---
arch/powerpc/include/uapi/asm/kvm.h | 11 ++++++++++-
arch/powerpc/kvm/book3s_hv.c | 17 +++++++++++++++++
2 files changed, 27 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
index 8a38be6c3b03..730488681443 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h
@@ -443,7 +443,16 @@ struct kvm_ppc_compat_caps {
__u64 size; /* Size of this structure */
__u64 compat_capabilities; /* Capabilities supported by the host */
};
-
+/*
+ * 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..f674386df62c 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6523,6 +6523,22 @@ 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())
+ rc = plpar_guest_get_capabilities(0, &capabilities);
+ 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 +6581,7 @@ 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)
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM
2026-06-16 12:33 ` [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM Amit Machhiwal
@ 2026-06-19 6:04 ` Vaibhav Jain
0 siblings, 0 replies; 9+ messages in thread
From: Vaibhav Jain @ 2026-06-19 6:04 UTC (permalink / raw)
To: Amit Machhiwal, linuxppc-dev, Madhavan Srinivasan
Cc: Amit Machhiwal, Anushree Mathur, Paolo Bonzini, Nicholas Piggin,
Michael Ellerman, Christophe Leroy (CS GROUP), Jonathan Corbet,
Shuah Khan, kvm, linux-kernel, linux-doc, lkp
Hi Amit.
Thanks for the patch and incorporating V3 review comments. Further
review comments inline below:
Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 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.
>
> Add support for retrieving host CPU compatibility capabilities for
> nested guests on PowerVM (PAPR nested API v2). The hypervisor provides
> the effective compatibility levels via the H_GUEST_GET_CAPABILITIES
> hcall, which reflects the processor modes negotiated between the Power
> hypervisor (L0) and the host partition (L1).
>
> On pseries systems, obtain the capability bitmap using
> plpar_guest_get_capabilities() and return it via struct
> kvm_ppc_compat_caps. The implementation defines KVM-specific capability
> constants (KVM_PPC_COMPAT_CAP_POWER9/10/11) and applies masking to ensure
> only supported processor modes are exposed to userspace. This information
> is then exposed 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>
> ---
> arch/powerpc/include/uapi/asm/kvm.h | 11 ++++++++++-
> arch/powerpc/kvm/book3s_hv.c | 17 +++++++++++++++++
> 2 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h
> index 8a38be6c3b03..730488681443 100644
> --- a/arch/powerpc/include/uapi/asm/kvm.h
> +++ b/arch/powerpc/include/uapi/asm/kvm.h
> @@ -443,7 +443,16 @@ struct kvm_ppc_compat_caps {
> __u64 size; /* Size of this structure */
> __u64 compat_capabilities; /* Capabilities supported by the host */
> };
> -
> +/*
> + * 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..f674386df62c 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6523,6 +6523,22 @@ 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())
> + rc = plpar_guest_get_capabilities(0,
> &capabilities);
I think instead of making the hcall you should use the
'nested_capabilities' extern symbol as it would already the same
value. This symbol is already accessible in 'book3s_hv.c'
> + 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 +6581,7 @@ 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)
>
>
--
Cheers
~ Vaibhav
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV
2026-06-16 12:33 [PATCH v4 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests Amit Machhiwal
2026-06-16 12:33 ` [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl Amit Machhiwal
2026-06-16 12:33 ` [PATCH v4 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM Amit Machhiwal
@ 2026-06-16 12:33 ` Amit Machhiwal
2026-06-19 6:12 ` Vaibhav Jain
2026-06-16 12:33 ` [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl Amit Machhiwal
3 siblings, 1 reply; 9+ messages in thread
From: Amit Machhiwal @ 2026-06-16 12:33 UTC (permalink / raw)
To: linuxppc-dev, Madhavan Srinivasan
Cc: Vaibhav Jain, Amit Machhiwal, Anushree Mathur, Paolo Bonzini,
Nicholas Piggin, Michael Ellerman, Christophe Leroy (CS GROUP),
Jonathan Corbet, Shuah Khan, kvm, linux-kernel, linux-doc, lkp
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 obtained
from the hypervisor via 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.
Introduce a helper to translate CPU version values into KVM_PPC_COMPAT_CAP
bits 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
PowerVM and 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>
---
arch/powerpc/kvm/book3s_hv.c | 37 +++++++++++++++++++++++++++++++++++-
1 file changed, 36 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index f674386df62c..375e7a7fa9f8 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -6523,15 +6523,50 @@ 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;
+ break;
+ case PVR_ARCH_31:
+ *capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
+ break;
+ 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())
+ if (kvmhv_is_nestedv2()) {
rc = plpar_guest_get_capabilities(0, &capabilities);
+ } else {
+ for_each_node_by_type(np, "cpu") {
+ prop = of_get_property(np, "cpu-version", NULL);
+ if (prop) {
+ cpu_version = be32_to_cpup(prop);
+ break;
+ }
+ }
+ if (!prop)
+ return -EINVAL;
+ rc = kvmppc_map_compat_capabilities(cpu_version,
+ &capabilities);
+ }
host_caps->compat_capabilities = capabilities &
KVM_PPC_COMPAT_BITMASK;
}
--
2.50.1 (Apple Git-155)
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV
2026-06-16 12:33 ` [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV Amit Machhiwal
@ 2026-06-19 6:12 ` Vaibhav Jain
0 siblings, 0 replies; 9+ messages in thread
From: Vaibhav Jain @ 2026-06-19 6:12 UTC (permalink / raw)
To: Amit Machhiwal, linuxppc-dev, Madhavan Srinivasan
Cc: Amit Machhiwal, Anushree Mathur, Paolo Bonzini, Nicholas Piggin,
Michael Ellerman, Christophe Leroy (CS GROUP), Jonathan Corbet,
Shuah Khan, kvm, linux-kernel, linux-doc, lkp
Hi Amit.
Thanks for the patch and incorporating V3 review comments. Further
review comments inline below:
Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 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 obtained
> from the hypervisor via 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.
>
> Introduce a helper to translate CPU version values into KVM_PPC_COMPAT_CAP
> bits 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
> PowerVM and 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>
> ---
> arch/powerpc/kvm/book3s_hv.c | 37 +++++++++++++++++++++++++++++++++++-
> 1 file changed, 36 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index f674386df62c..375e7a7fa9f8 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -6523,15 +6523,50 @@ 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;
Do you need to do 'break' here instead of falling through. Since P11
host can support P10 and P9 compat modes
> + break;
> + case PVR_ARCH_31:
> + *capabilities |= KVM_PPC_COMPAT_CAP_POWER10;
> + break;
> + 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())
> + if (kvmhv_is_nestedv2()) {
> rc = plpar_guest_get_capabilities(0, &capabilities);
> + } else {
> + for_each_node_by_type(np, "cpu") {
> + prop = of_get_property(np, "cpu-version", NULL);
> + if (prop) {
> + cpu_version = be32_to_cpup(prop);
> + break;
> + }
> + }
> + if (!prop)
> + return -EINVAL;
> + rc = kvmppc_map_compat_capabilities(cpu_version,
> + &capabilities);
> + }
should you check for 'rc' error here before assigning 'capabilities' to
'host_caps->compat_capabilities' . I understand it will be set to '0'
due to its initialization at the top of the function. But would be
better to make it more explicit
> host_caps->compat_capabilities = capabilities &
> KVM_PPC_COMPAT_BITMASK;
> }
> --
> 2.50.1 (Apple Git-155)
>
>
--
Cheers
~ Vaibhav
^ permalink raw reply [flat|nested] 9+ messages in thread
* [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl
2026-06-16 12:33 [PATCH v4 0/4] KVM: PPC: Expose CPU compatibility modes for nested guests Amit Machhiwal
` (2 preceding siblings ...)
2026-06-16 12:33 ` [PATCH v4 3/4] KVM: PPC: Book3S HV: Add support for compat CPU capabilities for KVM on PowerNV Amit Machhiwal
@ 2026-06-16 12:33 ` Amit Machhiwal
2026-06-19 6:14 ` Vaibhav Jain
3 siblings, 1 reply; 9+ messages in thread
From: Amit Machhiwal @ 2026-06-16 12:33 UTC (permalink / raw)
To: linuxppc-dev, Madhavan Srinivasan
Cc: Vaibhav Jain, Amit Machhiwal, Anushree Mathur, Paolo Bonzini,
Nicholas Piggin, Michael Ellerman, Christophe Leroy (CS GROUP),
Jonathan Corbet, Shuah Khan, kvm, linux-kernel, linux-doc, lkp
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 includes
comprehensive error code descriptions, structure field definitions
including the size field for forward compatibility, and KVM-specific
capability bit constants.
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
---
Documentation/virt/kvm/api.rst | 47 ++++++++++++++++++++++++++++++++++
1 file changed, 47 insertions(+)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 52bbbb553ce1..ba6feba74d7d 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6553,6 +6553,53 @@ 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 the current structure
+ size, or if the backend implementation fails to retrieve or
+ map CPU compatibility capabilities
+ 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
+PowerNV (KVM nested APIv1) and PowerVM (KVM nested APIv2) platforms.
+
+::
+
+ struct kvm_ppc_compat_caps {
+ __u64 flags; /* Reserved for future use */
+ __u64 size; /* Size of this structure */
+ __u64 compat_capabilities; /* Capabilities supported by the host */
+ };
+
+The ``compat_capabilities`` bit field describes the processor compatibility
+modes supported by the host. For example, the following bits indicate support
+for specific processor modes.
+
+::
+
+ 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
+
.. _kvm_run:
5. The kvm_run structure
--
2.50.1 (Apple Git-155)
^ permalink raw reply related [flat|nested] 9+ messages in thread* Re: [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl
2026-06-16 12:33 ` [PATCH v4 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl Amit Machhiwal
@ 2026-06-19 6:14 ` Vaibhav Jain
0 siblings, 0 replies; 9+ messages in thread
From: Vaibhav Jain @ 2026-06-19 6:14 UTC (permalink / raw)
To: Amit Machhiwal, linuxppc-dev, Madhavan Srinivasan
Cc: Amit Machhiwal, Anushree Mathur, Paolo Bonzini, Nicholas Piggin,
Michael Ellerman, Christophe Leroy (CS GROUP), Jonathan Corbet,
Shuah Khan, kvm, linux-kernel, linux-doc, lkp
Hi Amit,
Thanks for the patch and incorporating V3 review comments. Further
review comments inline below:
Amit Machhiwal <amachhiw@linux.ibm.com> writes:
> 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 includes
> comprehensive error code descriptions, structure field definitions
> including the size field for forward compatibility, and KVM-specific
> capability bit constants.
>
> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> ---
> Documentation/virt/kvm/api.rst | 47 ++++++++++++++++++++++++++++++++++
> 1 file changed, 47 insertions(+)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 52bbbb553ce1..ba6feba74d7d 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6553,6 +6553,53 @@ 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 the current structure
> + size, or if the backend implementation fails to retrieve or
> + map CPU compatibility capabilities
> + 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
> +PowerNV (KVM nested APIv1) and PowerVM (KVM nested APIv2) platforms.
> +
Please add a detail on how returned 'size' field can be less than what
the userspace has sent and how it should be handled.
> +::
> +
> + struct kvm_ppc_compat_caps {
> + __u64 flags; /* Reserved for future use */
> + __u64 size; /* Size of this structure */
> + __u64 compat_capabilities; /* Capabilities supported by the host */
> + };
> +
> +The ``compat_capabilities`` bit field describes the processor compatibility
> +modes supported by the host. For example, the following bits indicate support
> +for specific processor modes.
> +
> +::
> +
> + 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
> +
> .. _kvm_run:
>
> 5. The kvm_run structure
> --
> 2.50.1 (Apple Git-155)
>
--
Cheers
~ Vaibhav
^ permalink raw reply [flat|nested] 9+ messages in thread