[PATCH v3 2/5] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl
From: Amit Machhiwal <hidden>
Date: 2026-05-22 15:28:25
Also in:
kvm, linux-doc, lkml
Subsystem:
kernel virtual machine (kvm), kernel virtual machine for powerpc (kvm/powerpc), linux for powerpc (32-bit and 64-bit), the rest · Maintainers:
Paolo Bonzini, Madhavan Srinivasan, Michael Ellerman, Linus Torvalds
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. If the capability is not supported, the ioctl returns success with no capabilities set, allowing userspace to fall back gracefully. Suggested-by: Vaibhav Jain <redacted> Tested-by: Anushree Mathur <redacted> Signed-off-by: Amit Machhiwal <redacted> --- arch/powerpc/include/asm/kvm_ppc.h | 1 + arch/powerpc/include/uapi/asm/kvm.h | 6 ++++++ arch/powerpc/kvm/powerpc.c | 21 +++++++++++++++++++++ include/uapi/linux/kvm.h | 4 ++++ 4 files changed, 32 insertions(+)
diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h
index 0953f2daa466..cadfb839e836 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_cpu_ver)(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..081d6c7f7f70 100644
--- a/arch/powerpc/include/uapi/asm/kvm.h
+++ b/arch/powerpc/include/uapi/asm/kvm.h@@ -437,6 +437,12 @@ 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 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 00302399fc37..02b834ebd8d3 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c@@ -697,6 +697,13 @@ 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;
@@ -2463,6 +2470,20 @@ 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; + + r = -ENOTTY; + memset(&host_caps, 0, sizeof(host_caps)); + if (!kvm->arch.kvm_ops->get_compat_cpu_ver) + goto out; + + r = kvm->arch.kvm_ops->get_compat_cpu_ver(&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)