Re: [RFC PATCH v11 06/29] KVM: Introduce KVM_SET_USER_MEMORY_REGION2

[Quentin Perret]

On Tuesday 18 Jul 2023 at 16:44:49 (-0700), Sean Christopherson wrote:
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -95,6 +95,16 @@ struct kvm_userspace_memory_region {
>  	__u64 userspace_addr; /* start of the userspace allocated memory */
>  };
>  
> +/* for KVM_SET_USER_MEMORY_REGION2 */
> +struct kvm_userspace_memory_region2 {
> +	__u32 slot;
> +	__u32 flags;
> +	__u64 guest_phys_addr;
> +	__u64 memory_size;
> +	__u64 userspace_addr;
> +	__u64 pad[16];

Should we replace that pad[16] with:

	__u64 size;

where 'size' is the size of the structure as seen by userspace? This is
used in other UAPIs (see struct sched_attr for example) and is a bit
more robust for future extensions (e.g. an 'old' kernel can correctly
reject a newer version of the struct with additional fields it doesn't
know about if that makes sense, etc).