Re: [PATCH v3 2/5] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

[Amit Machhiwal <amachhiw@linux.ibm.com>]

Hi Vaibhav,

Thanks for reviewing the patches. Please find my response inline.

On 2026/06/03 09:16 AM, Vaibhav Jain wrote:
> Hi Amit,
> 
> Thanks for this patch. Few review comments 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. If the capability is not supported, the
> > ioctl returns success with no capabilities set, allowing userspace to
> > fall back gracefully.
> >
> > Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
> > Tested-by: Anushree Mathur <anushree.mathur@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 |  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 */
> Please introduce a size field also for the UAPI so that in this
> structure can evolve in future without breaking kernel ABI.

Sure, adding a size field for validations and future extensibility makes
sense. I'll add it in the next version.

> 
> > +	__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)))
> As mentioned above please introduce a size field in the structure thats
> being copied to the userspace and use the size field to copy the
> apporiate structure to the userspace. Otherwise a future kernel may
> unintentionally overwrite unintended userspace memory if it happens to
> be using a  larger structure size then what VMM knows about.

Sure, I'll add a validation around the structure size.

> 
> > +			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)
> Minor: you may want to align the name of the newly introduced kvmppc_ops
> to KVM CAP you are introducing here.

Will do.

Thanks,
Amit

> 
> > +
> >  /*
> >   * ioctls for vcpu fds
> >   */
> > -- 
> > 2.50.1 (Apple Git-155)
> >
> >
> 
> -- 
> Cheers
> ~ Vaibhav