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: TrnBh1m77ZkKBKkzpctGIIrehRgv6QViueGKCAHNOFzhgWfeCdNWiS3bYkyP/nRv6x4TCe0D9d
 zbpYKgsM9gyA==
IronPort-SDR: z58GGvusPGbLzpMc6/jU8OeMp9dg1QUcVIrxQxq7iLZV3eirBO4J3m46oJZ6SbB0ae9ibqdO5v
 FfF4l7+X9Skw==
From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     Jean-Philippe Brucker <jean-philippe@linaro.org>
CC:     LKML <linux-kernel@vger.kernel.org>,
        Joerg Roedel <joro@8bytes.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Alex Williamson (alex.williamson@redhat.com)" 
        <alex.williamson@redhat.com>, Jason Wang <jasowang@redhat.com>,
        Eric Auger <eric.auger@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Raj, Ashok" <ashok.raj@intel.com>,
        "Liu, Yi L" <yi.l.liu@intel.com>, "Wu, Hao" <hao.wu@intel.com>,
        "Jiang, Dave" <dave.jiang@intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        David Gibson <david@gibson.dropbear.id.au>,
        Kirti Wankhede <kwankhede@nvidia.com>,
        Robin Murphy <robin.murphy@arm.com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal
Thread-Topic: [RFC] /dev/ioasid uAPI proposal
Thread-Index: AddSzQ970oLnVHLeQca/ysPD8zMJZwBEK5yAALaxj6A=
Date:   Tue, 1 Jun 2021 07:50:40 +0000
Message-ID: <MWHPR11MB18861BC9850B615617DE51948C3E9@MWHPR11MB1886.namprd11.prod.outlook.com>
References: <MWHPR11MB1886422D4839B372C6AB245F8C239@MWHPR11MB1886.namprd11.prod.outlook.com>
 <YLEY6yAF1osdtS3e@myrica>
In-Reply-To: <YLEY6yAF1osdtS3e@myrica>
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: 58a49500-f3ec-47e2-aeef-08d924d1f40f
X-MS-Exchange-CrossTenant-originalarrivaltime: 01 Jun 2021 07:50:40.5731
 (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: 5QUC5dZV0jZeJvbnabNC4JrtrNF5ld8ATcbdCPI0FKQ7M+emREh1TbOXvGlmFDlTbP1YhyerUYsc1M7cQcjo/g==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR1101MB2221
X-OriginatorOrg: intel.com
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

> From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> Sent: Saturday, May 29, 2021 12:23 AM
> >
> > IOASID nesting can be implemented in two ways: hardware nesting and
> > software nesting. With hardware support the child and parent I/O page
> > tables are walked consecutively by the IOMMU to form a nested translati=
on.
> > When it's implemented in software, the ioasid driver is responsible for
> > merging the two-level mappings into a single-level shadow I/O page tabl=
e.
> > Software nesting requires both child/parent page tables operated throug=
h
> > the dma mapping protocol, so any change in either level can be captured
> > by the kernel to update the corresponding shadow mapping.
>=20
> Is there an advantage to moving software nesting into the kernel?
> We could just have the guest do its usual combined map/unmap on the child
> fd
>=20

There are at least two intended usages:

1) From previous discussion looks PPC's window-based scheme can be
better supported with software nesting (a shared IOVA address space
as the parent (shared by all devices) which is nested by multiple windows
as the children (per-device);

2) Some mdev drivers (e.g. kvmgt) may want to do write-protection on=20
guest data structures (base address programmed to mediated MMIO
register). The base address is IOVA while  KVM page-tracking API is=20
based on GPA. nesting allows finding GPA according to IOVA.

Thanks
Kevin