Re: [PATCH v5 7/9] iommufd: Associate fault object with iommufd_hw_pgtable

[Jason Gunthorpe]

On Tue, Apr 30, 2024 at 10:57:08PM +0800, Lu Baolu wrote:
>  /**
> @@ -412,6 +415,9 @@ enum iommu_hwpt_data_type {
>   * @data_type: One of enum iommu_hwpt_data_type
>   * @data_len: Length of the type specific data
>   * @data_uptr: User pointer to the type specific data
> + * @fault_id: The ID of IOMMUFD_FAULT object. Valid only if flags field of
> + *            IOMMU_HWPT_FAULT_ID_VALID is set.
> + * @__reserved2: Padding to 64-bit alignment. Must be 0.
>   *
>   * Explicitly allocate a hardware page table object. This is the same object
>   * type that is returned by iommufd_device_attach() and represents the
> @@ -442,6 +448,8 @@ struct iommu_hwpt_alloc {
>  	__u32 data_type;
>  	__u32 data_len;
>  	__aligned_u64 data_uptr;
> +	__u32 fault_id;
> +	__u32 __reserved2;
>  };
>  #define IOMMU_HWPT_ALLOC _IO(IOMMUFD_TYPE, IOMMUFD_CMD_HWPT_ALLOC)

[..]

> @@ -359,7 +359,7 @@ static const struct iommufd_ioctl_op iommufd_ioctl_ops[] = {
>  	IOCTL_OP(IOMMU_GET_HW_INFO, iommufd_get_hw_info, struct iommu_hw_info,
>  		 __reserved),
>  	IOCTL_OP(IOMMU_HWPT_ALLOC, iommufd_hwpt_alloc, struct iommu_hwpt_alloc,
> -		 __reserved),
> +		 __reserved2),

This is now how the back compat mechanism works. The value here is the
absolute minimum size, it should never increase. The first __reserved
is always the right value.

If you change it then old userspace that doesn't include the fault_id
will stop working.

Jason