On Wed, Jul 01, 2026 at 10:44:06AM +0530, Amit Machhiwal wrote: > 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 <[email protected]> > Signed-off-by: Amit Machhiwal <[email protected]> > --- > Changes in this version: > - Moved size as the first member of the struct > - Replaced strict size equality check with copy_struct_from_user() and > copy_struct_to_user() for proper forward and backward ABI compatibility > - Added KVM_PPC_COMPAT_CAPS_SIZE_VER0 (24) as a frozen version floor > constant, following the convention used by similar interfaces in the > kernel > - Added flags == 0 enforcement to prevent uninitialized stack values from > being baked into ABI permanently > > arch/powerpc/include/asm/kvm_ppc.h | 1 + > arch/powerpc/include/uapi/asm/kvm.h | 8 ++++ > arch/powerpc/kvm/powerpc.c | 71 +++++++++++++++++++++++++++++ > include/uapi/linux/kvm.h | 4 ++ > 4 files changed, 84 insertions(+) > > diff --git a/arch/powerpc/include/asm/kvm_ppc.h > b/arch/powerpc/include/asm/kvm_ppc.h > index 0953f2daa466..169ea6a7fbad 100644 > --- a/arch/powerpc/include/asm/kvm_ppc.h > +++ b/arch/powerpc/include/asm/kvm_ppc.h > @@ -319,6 +319,7 @@ struct kvmppc_ops { > bool (*hash_v3_possible)(void); > int (*create_vm_debugfs)(struct kvm *kvm); > int (*create_vcpu_debugfs)(struct kvm_vcpu *vcpu, struct dentry > *debugfs_dentry); > + int (*get_compat_caps)(struct kvm_ppc_compat_caps *host_caps); > }; > > extern struct kvmppc_ops *kvmppc_hv_ops; > diff --git a/arch/powerpc/include/uapi/asm/kvm.h > b/arch/powerpc/include/uapi/asm/kvm.h > index 077c5437f521..19e53d5ae540 100644 > --- a/arch/powerpc/include/uapi/asm/kvm.h > +++ b/arch/powerpc/include/uapi/asm/kvm.h > @@ -437,6 +437,14 @@ struct kvm_ppc_cpu_char { > __u64 behaviour_mask; /* valid bits in behaviour */ > }; > > +/* For KVM_PPC_GET_COMPAT_CAPS */ > +struct kvm_ppc_compat_caps { > + __u64 size; /* Size of this structure */ > + __u64 flags; /* Reserved for future use */ > + __u64 compat_capabilities; /* Capabilities supported by the host */ > +}; > +#define KVM_PPC_COMPAT_CAPS_SIZE_VER0 24 /* sizeof first published > struct */ > + > /* > * Values for character and character_mask. > * These are identical to the values used by H_GET_CPU_CHARACTERISTICS. > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c > index 98de68379b18..a2919b8b31c0 100644 > --- a/arch/powerpc/kvm/powerpc.c > +++ b/arch/powerpc/kvm/powerpc.c > @@ -701,6 +701,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long > ext) > } > } > break; > +#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE) > + case KVM_CAP_PPC_COMPAT_CAPS: > + r = 0; > + if (kvmhv_on_pseries()) > + r = 1; > + break; > +#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */ > default: > r = 0; > break; > @@ -2467,6 +2474,70 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int > ioctl, unsigned long arg) > r = kvm->arch.kvm_ops->svm_off(kvm); > break; > } > + case KVM_PPC_GET_COMPAT_CAPS: { > + struct kvm_ppc_compat_caps host_caps = {}; > + u64 usize; > + > + /* > + * Read the size field first to drive copy_struct_from_user. > + * size must be the first field of the struct. > + */ > + r = -EFAULT; > + if (get_user(usize, (__u64 __user *)argp)) > + goto out; > + > + /* > + * Enforce a minimum: reject buffers smaller than the initial > + * struct version (VER0). This allows old userspace compiled > + * against the original struct to still work on a newer kernel > + * that has grown the struct with appended fields. > + */ > + r = -EINVAL; > + if (usize < KVM_PPC_COMPAT_CAPS_SIZE_VER0) > + goto out; > + > + /* > + * New userspace with a larger struct called an older kernel. > + * Write back ksize in host_caps.size so userspace knows which > + * older struct to retry with, then fail with -E2BIG. > + */ > + if (usize > sizeof(host_caps)) { > + host_caps.size = sizeof(host_caps); > + r = -EFAULT; > + if (put_user(host_caps.size, (__u64 __user *)argp)) > + goto out; > + r = -E2BIG; > + goto out; > + } > + > + /* > + * copy_struct_from_user() handles forward/backward compat: > + * usize == ksize: verbatim copy > + * usize < ksize: zero-pad trailing (old userspace, new > kernel) > + */ > + r = copy_struct_from_user(&host_caps, sizeof(host_caps), > + argp, usize); > + if (r) > + goto out; > + > + /* Reserved fields must be zero */ > + r = -EINVAL; > + if (host_caps.flags) > + goto out; > + > + r = -ENOTTY; > + if (!kvm->arch.kvm_ops->get_compat_caps) > + goto out; > + > + r = kvm->arch.kvm_ops->get_compat_caps(&host_caps); > + if (r) > + goto out; > + > + host_caps.size = sizeof(host_caps); > + r = copy_struct_to_user(argp, usize, &host_caps, > + sizeof(host_caps), NULL); > + break; > + } > default: { > struct kvm *kvm = filp->private_data; > r = kvm->arch.kvm_ops->arch_vm_ioctl(filp, ioctl, arg); > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h > index 419011097fa8..1cf9a959669e 100644 > --- a/include/uapi/linux/kvm.h > +++ b/include/uapi/linux/kvm.h > @@ -997,6 +997,7 @@ struct kvm_enable_cap { > #define KVM_CAP_S390_KEYOP 247 > #define KVM_CAP_S390_VSIE_ESAMODE 248 > #define KVM_CAP_S390_HPAGE_2G 249 > +#define KVM_CAP_PPC_COMPAT_CAPS 250 > > struct kvm_irq_routing_irqchip { > __u32 irqchip; > @@ -1350,6 +1351,9 @@ struct kvm_s390_keyop { > #define KVM_GET_DEVICE_ATTR _IOW(KVMIO, 0xe2, struct kvm_device_attr) > #define KVM_HAS_DEVICE_ATTR _IOW(KVMIO, 0xe3, struct kvm_device_attr) > > +/* Available with KVM_CAP_PPC_COMPAT_CAPS */ > +#define KVM_PPC_GET_COMPAT_CAPS _IO(KVMIO, 0xe4)
Can you use another ioctl number for this? This group of ioctls is for fds returned by "KVM_CREATE_DEVICE" /* ioctls for fds returned by KVM_CREATE_DEVICE */ #define KVM_SET_DEVICE_ATTR _IOW(KVMIO, 0xe1, struct kvm_device_attr) #define KVM_GET_DEVICE_ATTR _IOW(KVMIO, 0xe2, struct kvm_device_attr) #define KVM_HAS_DEVICE_ATTR _IOW(KVMIO, 0xe3, struct kvm_device_attr) > + > /* > * ioctls for vcpu fds > */ > -- > 2.50.1 (Apple Git-155) > > Thanks, Gautam
