Re: [PATCH v4 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl

[Vaibhav Jain <vaibhav@linux.ibm.com>]

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