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;
IronPort-SDR: ykFaWqSOqHqjiDOQJezkFHqWShIYxL4HxbNkkEBXrRsLSqFEs9ZbGK8z8V/qN0+gzQXLjnVY14
 o/IBBxF5d2Rw==
IronPort-SDR: FfN5exNlnxjQRMzb1VhsLERGNwda5RDbZwhw7OoRG+oM81TTHd3cmVos34CEni3pUpzAI6wzJy
 sO70kO6KSUvA==
From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jason Gunthorpe <jgg@nvidia.com>,
        David Gibson <david@gibson.dropbear.id.au>
CC:     Alex Williamson <alex.williamson@redhat.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Auger Eric <eric.auger@redhat.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        "Alexey Kardashevskiy" <aik@ozlabs.ru>
Subject: RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation
 APIs
Thread-Topic: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs
Thread-Index: AQHXDZub0CDG82VGXUmLEYgvMuPs4KqKkLIAgACg94CAAC72gIAAD04AgAADogCAAErUgIAETQEAgAOaswCAB7C3gIAAhxQwgADYDACAATCSAIAAVGMAgAELsQCAACiXgIAAVBgAgAAaMwCAAAESAIAABy2AgAAgOYCAFdCAgIAAppUAgADsNgCAACqygIAHsmaAgAAzsYCAAAjdgIAAD/yAgAAcQgCAADqggIABMLWAgAAMMYCAABwXgIAABlIAgAAsEgCAABE9AIAGpToAgADKIACAAOE64A==
Date:   Wed, 28 Apr 2021 06:58:19 +0000
Message-ID: <MWHPR11MB1886F0818D30329172C420758C409@MWHPR11MB1886.namprd11.prod.outlook.com>
References: <20210421175203.GN1370958@nvidia.com>
 <20210421133312.15307c44@redhat.com> <20210421230301.GP1370958@nvidia.com>
 <20210422111337.6ac3624d@redhat.com> <20210422175715.GA1370958@nvidia.com>
 <20210422133747.23322269@redhat.com> <20210422200024.GC1370958@nvidia.com>
 <20210422163808.2d173225@redhat.com> <20210422233950.GD1370958@nvidia.com>
 <YIecXkaEGNgICePO@yekko.fritz.box> <20210427171212.GD1370958@nvidia.com>
In-Reply-To: <20210427171212.GD1370958@nvidia.com>
Accept-Language: en-US
Content-Language: en-US
dlp-version: 11.5.1.3
dlp-product: dlpe-windows
dlp-reaction: no-action
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-MS-Exchange-CrossTenant-Network-Message-Id: cfd68d45-33a6-4045-e59c-08d90a130208
X-MS-Exchange-CrossTenant-originalarrivaltime: 28 Apr 2021 06:58:19.1395
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-CrossTenant-userprincipalname: YRNKKo3TPkH2KEPWcrwaLSFA8VuwK9qXAHPrQrHpyMBGr1SuhfR1tiWKChBIEViEE3ws8ILGdF4vmES1A5uB5g==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR1101MB2224
X-OriginatorOrg: intel.com
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Wednesday, April 28, 2021 1:12 AM
>=20
[...]=20
> > As Alex says, if this line fails because of the group restrictions,
> > that's not great because it's not very obvious what's gone wrong.
>=20
> Okay, that is fair, but let's solve that problem directly. For
> instance netlink has been going in the direction of adding a "extack"
> from the kernel which is a descriptive error string. If the failing
> ioctl returned the string:
>=20
>   "cannot join this device to the IOASID because device XXX in the
>    same group #10 is in use"
>=20
> Would you agree it is now obvious what has gone wrong? In fact would
> you agree this is a lot better user experience than what applications
> do today even though they have the group FD?
>=20

Currently all the discussions are around implicit vs. explicit uAPI semanti=
cs
on the group restriction. However if we look beyond group the implicit=20
semantics might be inevitable when dealing with incompatible iommu
domains. An existing example of iommu incompatibility is IOMMU_
CACHE. In the future there could be other incompatibilities such as=20
whether nested translation is supported. In the end the userspace has=20
to do some due diligence on understanding iommu topology and attributes=20
to decide how many VFIO containers or ioasid fds should be created. It=20
does push some burden to userspace but it's difficult to define a group-
like kernel object to enforce such restriction for iommu compatibility.=20
Then the best that the kernel can do is to return an informational error=20
message in case an incompatible device is attached to the existing domain.=
=20
If this is the perceived way to move forward anyway, I feel that removing=20
explicit group FD from uAPI doesn't make userspace worse...

Thanks
Kevin