Hi Vaibhav, Thanks for the detailed review. My responses are inline below.
On 2026/06/19 11:44 AM, Vaibhav Jain wrote: > Hi Amit. > > Thanks for the patch and incorporating V3 review comments. Further > review comments inline below: > > Amit Machhiwal <[email protected]> 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 <[email protected]> > > Signed-off-by: Amit Machhiwal <[email protected]> > > --- > > 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. Yeah, I think it would make more sense and will simplify the copy_from_user() call. I will make the change in v5. I will change to: struct kvm_ppc_compat_caps { __u64 size; __u64 flags; __u64 compat_capabilities; }; > > > + __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. Will do as mentioned above. > > > > + 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. Agreed. I'll change the validation to use strict equality. This is simpler and clearer - userspace must provide exactly the size the kernel expects. > > > + > > + 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. With the strict equality check (user_size != sizeof(host_caps)), this concern should be addressed - we won't accept larger structs from userspace. However, I'll still improve the documentation to: 1. In the commit message: - Explain the size field validation - Document that exact size match is required - Clarify error handling behavior 2. In Documentation/virt/kvm/api.rst: - Add improved documentation for KVM_PPC_GET_COMPAT_CAPS - Document the size field requirement and validation Thanks, Amit > > > + 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
