Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 1 Oct 2021 09:43:22 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     David Gibson <david@gibson.dropbear.id.au>
Cc:     Liu Yi L <yi.l.liu@intel.com>, alex.williamson@redhat.com,
        hch@lst.de, jasowang@redhat.com, joro@8bytes.org,
        jean-philippe@linaro.org, kevin.tian@intel.com, parav@mellanox.com,
        lkml@metux.net, pbonzini@redhat.com, lushenming@huawei.com,
        eric.auger@redhat.com, corbet@lwn.net, ashok.raj@intel.com,
        yi.l.liu@linux.intel.com, jun.j.tian@intel.com, hao.wu@intel.com,
        dave.jiang@intel.com, jacob.jun.pan@linux.intel.com,
        kwankhede@nvidia.com, robin.murphy@arm.com, kvm@vger.kernel.org,
        iommu@lists.linux-foundation.org, dwmw2@infradead.org,
        linux-kernel@vger.kernel.org, baolu.lu@linux.intel.com,
        nicolinc@nvidia.com
Subject: Re: [RFC 07/20] iommu/iommufd: Add iommufd_[un]bind_device()
Message-ID: <20211001124322.GN964074@nvidia.com>
References: <20210919063848.1476776-1-yi.l.liu@intel.com>
 <20210919063848.1476776-8-yi.l.liu@intel.com>
 <YVP44v4FVYJBSEEF@yekko>
 <20210929122457.GP964074@nvidia.com>
 <YVUqpff7DUtTLYKx@yekko>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <YVUqpff7DUtTLYKx@yekko>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?UUX5mLGje05Mcjz9YJKtCXrb4objHRb4cqbbr2+7dPgM30rDCgAMQPlRWzQM?=
 =?us-ascii?Q?O+0f5u1iwg2gmgU6hDOukGVfImZmT19RML4ptRPzXcP/v5VQsm4dR96h/Z72?=
 =?us-ascii?Q?z+uTp/Bu4HcPTBeLrYA9BbvChlhuVn/rDnYCskaJYvz2M2KkHE+yrYyV7otP?=
 =?us-ascii?Q?+EU0YPUnhUYalnovqL4YvInK1MXz/jDcKO1mMuFYrlVj2s5SKFjKXMYPmnru?=
 =?us-ascii?Q?37UFdDcXxvMpwV/zCZkynJVC8P/Uup0YC3guK13ZZufyOj87b8IH/FOF3E3w?=
 =?us-ascii?Q?D3+i9qyf9T7XMi2qjuCszYBYp/zR24cRjzuuLuntmqsr8tmyZwu0FKKNUt5Q?=
 =?us-ascii?Q?VBjE+357/j3523JtBdrEZ6f4/D2RbqrwIv3X8giDHo6+DZjwkpQ6KYtJPZa0?=
 =?us-ascii?Q?vpwQH/OgtNwb9iThShjsdVQXxbon/DNW4ptuMFstG9dhqg24JYvx+woRKSoV?=
 =?us-ascii?Q?9LauWEn4W9BvTsN1S87mx9/FK9OB11xDegL/6VpwjeEjwt87XA0zr7Dph28T?=
 =?us-ascii?Q?LiBcVHwhTgaiT02LPVLH00ukXPOWTvM2PawoM4+MXSRU/0IrWfocA3x2NFbI?=
 =?us-ascii?Q?zDtHs0712C3pnk0D2/B0MdvYWlXoI9wSf5ylOISqSUUmAtk5/4LPE0MsZ/yC?=
 =?us-ascii?Q?KjhZkuSTvdtd0EgRdJs728JdZVLk4mXukliynkB4TMmItmcaxGg0wWSOustm?=
 =?us-ascii?Q?tSpqPPgQ3AnE3d5ubo9iqSntzOYO2v7m9F/kb4tDO1axOGz4w+3nQMjO1EzS?=
 =?us-ascii?Q?pR+meNX0gVBXatV3b52uP3TEzIRGL06ux7nwGdTNmc8IAcu35BDrEGLBj0N/?=
 =?us-ascii?Q?OLLcRT+10aeF0kQB3jm8pSdCH9QtyM70T8xkoPLvd6wKrSmcMYN9kpug7/06?=
 =?us-ascii?Q?QnjKnnKJ6YtU3QBZxgSlf/IgCvWkQeZggLa1afVyM5dPoiSjBZQ0AyVcG6NE?=
 =?us-ascii?Q?FqcNjPbVpL7IZ+8IUdJ9NcdzOB9V30ihAK4dEMvHUiNi0ABeawbGl2jcg4pK?=
 =?us-ascii?Q?FumePUyGAQECLV5Qv6DJUdFV64N9QqwJ/F0otYaEDkL4MVRpfVPKPlCUdADs?=
 =?us-ascii?Q?dXg1MAd04WnUq01Muo51t/VADga01ln/F1ErnmLNAKB77XOBHUe7vWDxU7xa?=
 =?us-ascii?Q?k2zA9UB6C/8HxrE7A5Z16V4WZ/s3bnj8PddRFbY2vdSWa6tCZ+nU2M3qk71d?=
 =?us-ascii?Q?mNxh59BlMtl4PBLbtlSviP2B/OkCMVmTandBuuM15Od+MXGb+DjaT5vxmMoD?=
 =?us-ascii?Q?GassRVnyPZ3F8lJa2RidAjjSO+YZRM982yKi3dFambNOpg97ieOIwwC+KXct?=
 =?us-ascii?Q?IZtsShWwWkE79xaJyN+SkEQl1cx8cj6lOpTvkd5NbWMc5Q=3D=3D?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 5ea59dc0-dae7-4485-e743-08d984d90e4b
X-MS-Exchange-CrossTenant-AuthSource: BL0PR12MB5506.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Oct 2021 12:43:23.4939
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: qOkSG1vYDotwECsZH3/UOdgBQEC28NIqILAa97/hMdicIGPw/qG0bZdmuzRYwc44
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BL1PR12MB5256
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Sep 30, 2021 at 01:10:29PM +1000, David Gibson wrote:
> On Wed, Sep 29, 2021 at 09:24:57AM -0300, Jason Gunthorpe wrote:
> 65;6402;1c> On Wed, Sep 29, 2021 at 03:25:54PM +1000, David Gibson wrote:
> > 
> > > > +struct iommufd_device {
> > > > +	unsigned int id;
> > > > +	struct iommufd_ctx *ictx;
> > > > +	struct device *dev; /* always be the physical device */
> > > > +	u64 dev_cookie;
> > > 
> > > Why do you need both an 'id' and a 'dev_cookie'?  Since they're both
> > > unique, couldn't you just use the cookie directly as the index into
> > > the xarray?
> > 
> > ID is the kernel value in the xarray - xarray is much more efficient &
> > safe with small kernel controlled values.
> > 
> > dev_cookie is a user assigned value that may not be unique. It's
> > purpose is to allow userspace to receive and event and go back to its
> > structure. Most likely userspace will store a pointer here, but it is
> > also possible userspace could not use it.
> > 
> > It is a pretty normal pattern
> 
> Hm, ok.  Could you point me at an example?

For instance user_data vs fd in io_uring

RDMA has many similar examples.

More or less anytime you want to allow the kernel to async retun some
information providing a 64 bit user_data lets userspace have an easier
time to deal with it.

Jason