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Date: Tue, 7 May 2024 21:25:08 -0300
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Joerg Roedel <joro@8bytes.org>,
	Will Deacon <will@kernel.org>, Robin Murphy <robin.murphy@arm.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Nicolin Chen <nicolinc@nvidia.com>, Yi Liu <yi.l.liu@intel.com>,
	Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Joel Granados <j.granados@samsung.com>, iommu@lists.linux.dev,
	virtualization@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 7/9] iommufd: Associate fault object with
 iommufd_hw_pgtable
Message-ID: <20240508002508.GQ4718@ziepe.ca>
References: <20240430145710.68112-1-baolu.lu@linux.intel.com>
 <20240430145710.68112-8-baolu.lu@linux.intel.com>
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In-Reply-To: <20240430145710.68112-8-baolu.lu@linux.intel.com>

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